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Barrett .50
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Detonics Pistols
M96 Expeditionary
Mako Boat Project

Mercury V6 and Gearcase



General Notes
(to be categorized)

Powerhead interchange The powerheads are about the same. The 2, 2.4 and 2.5 liters all have the same bolt pattern at the bottom and will techically interchange, but with differences in exhaust size other changes would be needed, specifically the plates at the top of the mid, or in the case of the high perf stuff, the one piece plate. Again, the 3 liter and 3.4's are unique to themselves. (4)
Powerhead Torque Specs 2.4 Fish pg1 - 2.4 Fish pg2 (8)
2.5 Fish pg1
2.5 Hi-Po pg1 - 2.5 Hi-Po pg2 - 2.5 Hi-Po pg3 - 2.5 Assembly Lubes
Cooling system flow diagram 1989 and newer 2.4 & 2.5 - 2.5 260 (10)
Note: Temperatures for Merc. outboards should be in the 140-160 degree range.
Gearcase interchange All 2 liter, 2.4 liter, and 2.5 liter gearcases interchange. They won't interchange onto a 3 liter or the old 3.4 liter, both of which are unique to their size engine. (4)
Electrical System Wiring diagram for 92up fishing motor

Powerhead Comparison
Early and Late Model 2.5
Merc Manual 90-849240

Powerhead Comparison
(Early and Late 2.5)

Early style blocks have 14 bolts in the crankcase cover. later style blocks incorporate four extra bolts, bringing the number of crankcase cover bolts to 18. This is considered the 4 bolt block and crankshaft area.


IMPORTANT: Never mix early and late style pistons in the same engine. Because of weight differences, excessive vibration and possible engine damage could occur.

a Early Style Piston - Lighter forging, Shorter/Lighter Wrist Pin (one end closed), and no Piston Skirt Coating
b Late Style Piston - Heavier forging, Longer/Heavier Open End Wrist Pin, and Coating on Piston Skirts

Early style 2.5 (a) Hi-Performance pistons are a lighter forging, have no anti-scuff coating, and use a shorter wrist pin (with one closed end). Some of the very early pistons used the non-tanged piston retaining clip, but the tanged style can used with these pistons.

Late style 2.5 (b) Hi-Performance pistons are heavier forging, have an anti-scuff coating on the piston skirts, and use a long wrist pin (open both ends). On rebuilds, expect some of the anti-scuffing coating to be worn away.

New pistons should be used if the following criteria are not within tolerance (See engine rebuilding specifications within this section).

  • Piston to cylinder wall clearance
  • Piston ring stake pin height
  • Piston out-of-round

Connecting Rods

Two different methods of aligning the connecting rod to the piston and/or the crankshaft are possible. These two methods of alignment must not be intermixed in any engine.

  1. Bottom aligned, early style. The connecting rod bottem end is guided by the inside surfaces of the crankshaft and the top end is able to move horizontally on the piston wrist pin. Top end washers which are stepped on one side are used with this rod.
  2. Top aligned, later style. The connecting rod top end is guided by the inside surfaces of the piston and the bottom end is able to move horizontally on the crankshaft. Top end washers which are flat on both sides are used for this rod.

IMPORTANT: A different type top end washer is used for each type of connecting rod and cannot be mixed between the connecting rod types.

a Bottom Aligned Rod - .812 in. (20.59mm)
b Washer - stepped one side (stepped down side against connecting rod)
c Top Aligned Rod - .712 in. (18.08mm)
d Washer (flat on both sides)




Part #
Model & Year
Fuel Pressure
Rev. Limits
Supercede To
Supercede From
11350 A27
Early 2.5L Analog ECU used on engines with external TPI and external Speed-Limiter (Rev-Limiter) modules. (6)
A29 2.0 / 1994 N/A 39 11500
A45 2.0 / 1995 2615 56 11150
A46 2.0 / 1995 2615 56 11150
A53 2.0 / 1995 Service Only 2613 / 2650 56 9250
A40 2.4 2514 39 7700
A32 2.5 Offshore Race / 1994, N/A 39 8200
A36 2.5 Offshore Race / 1995 2509 39 8200
A48 2.5 Drag / 1995 2511 39 11500
A63 2.5 Drag / 1995 + Up 2719 / 2796 56 11500
A33 S3000 / 1994 N/A 39 9250
A47 S3000 / 1995 2508 / 2522 56 9250
A52 S3000 / 1995 2508 / 2522 56 9250
A61 S3000 / 1996 2717 / 2776 56 9250
A65 2.5 PROP / 1997 2622 56 8600
A49 2.5 EFI / 1996 2510 39 7750
A62 2.5 / 1996 7750
Digital ECU with internal TPI. Similar to the A49. (6)
849849 A2 2.0 Mod U 7115 56 none / 11500
The 849849 series superceded the 11350 series, and features a thinner ECU, different tester connector, enrichment moved from front to back of box, ECU chips are potted and cannot be removed.
A3 2.0 PROP 7113 56 9300 / 9400
A4 2.4 7114 39 7500 / 7800
Not original equipment on any 2.4L - replacement part only.
A1 2.5 EFI 7110 39 7600 / 7900
A5 2.5 ROS 7109 39 none / 8200
A6 2.5 DRAG 7119 56 none / 11500
A7 S3000 7108 56 9350 / 9650
A8 S3000 7117 56 9300 / 9400
A10 S3000 PROP 7323 56 8650 / 9100
A13 DRAG 2001 56 11500      
A14 S3000 S001 56 10100      
824003 -1, A1 200 2.5 XRi / 1994-1995 E683 / E556
-2, A2 175 2.5 XRi / 1994-1995 A7CO
-4, A4 150 2.5 XRi / 1994-1995 E57A
-5, A5 200 Pro Max / 1995 8714
-6, A6 225 Pro Max / 1995 C9C2 / AD49
-7, A7 150 Pro Max / 1995 A470
-8, A8 200 2.5 XRi / 1996 5AA0 / 4227
-9, A9 175 2.5 XRi / 1996 436F / C2A0
-10, A10 150 2.5 XRi / 1996 1BB6 / 24C5
-11, A11 200 Pro Max / 1996 A287
-13, A13 150 Pro Max / 1996 3E97
-14, A14 175 XRi, 1991 67E4
-15, A15 200 Offshore / Service 1831
A16 200 XRi / 1991 C5D9
A17 225 Pro Max / 1996 5E62
The A12 was recalled for recallibration
A23 225 Pro Max 15DSH / 1998, 225 Pro Max SS 2E88


HP Year Serial Number Range Model Pictures
150 C239553 - Up WMH-1, 2, 3B, 5, 7, 8, 8A, 11A, 12, 12B, 13, 13B, 14A, 15, 16, 18A, 21, 22, 23, 24, 25, 28, 90, 31, 32, 33, 34
150 1978-1990 WH-2, 12, 21, 23, 27, 29, 35, 38, 40, 48 with 2 needles and seats
150 1980-1982 WH-7A
150 1994-1995 WMH-31
150 1996-1997 WMV-2
XR6 1994-1995 WMH-32
XR6 1996-1997 WMV-3
175 C239553- Up WMH - -1, 2, 3B, 5, 7, 8, 8A, 11A, 12, 12B, 13, 13B, 14A, 15, 16, 18A, 21, 22, 23, 24, 25, 28, 90, 31, 32, 33, 34
175 1976-1990 WH-1, 4, 6, 7, 13, 17, 30, 34 with 2 needles and seats
175 1980-1982 WH-7A
175 1994-1997 WMH-33
175 1996-1997 WMV-4
200 C239553- Up WMH - -1, 2, 3B, 5, 7, 8, 8A, 11A, 12, 12B, 13, 13B, 14A, 15, 16, 18A, 21, 22, 23, 24, 25, 28, 90, 31, 32, 33, 34
200 1978-1990 WH-3, 14, 18, 20, 22, 26, 28, 31, 39, 46 with 2 needles and seats
200 1994-1995 WMH-34/39
200 1996-1997 WMV-5
225 1980-1981 WH-15 / 20 L WH-20 , R WH-20, F WH-20 , T WH-20
225 3L 1994 WMH-19A
225 3L 1994 1/2 WMH-46
225 3L 1995 WMH-47
225 3L 1996 WMV-7
225 3L 1997 WMV-13
245 1996 WMH-32/36 (20)


Jet sizing and part numbers for Mercury (9)

Jet location on carburetor (18)

Jay Smith on the subject of correctly jetting a motor

The plugs should be a "paper bag" color to be the ultimate in correct jetting. I don't read plugs as there are too many varibles involved, I have had good luck only reading the piston itself. I make a high speed pass and if the hull will allow it without back washing over the splash well kill it clean. Then I roll # 5 piston ( as generally # 5 is the hot hole on a Mercury and prone to burn if the mixture is too lean ) to bottom dead center and look at the piston crown on the intake side ( that will be the side toward the outside of the block ) with a bend o lite , the intake side of the piston at the intake port should be wet and look moist about the size of a quarter out toward the middle from the port. If the wettness is over across half of the crown your too rich and can lean down a bit. If it is dry and ashy in color your too lean and you need to richen up in a hurry. Maybe other people have different methods but this has always been the ticket for me. Some rely on pyros , I have found that when a motor is too lean a pyro will be too late in its report and the piston is already smoked ( a 2.5 Merc with stock stroke comes to top dead center@ 10,000 rpms at a rate of 120 times a second I'm an old man and I can't react that fast maybe some of the younger ones can but my incounters with a lean motor things just happen too fast ) also on MANY occations I have had customers motors be STUPID rich and the pryos are reading flame not egt and they continue to richen even more chasing thier tails. Also if a pyro is "coked" up" it will also read incorrectly . This is why I haven't alot of faith in the pyro system of jetting. I always tell my customers when I build modified motors to start off very rich and SLOWLY work your way to dialing in your jetting. And by ALL means don't listen to JOE BLOW as to where his motor runs best on jetting as EVERY MOTOR is different and what might run great on one jetting curve might cost you another trip to the machine shop and bank ! - Jay Smith of Jay Smith Racing Engines (11)

Always be conservative and build a cushion as you NEVER know what is in the gas pump at the local gas station. They have maybe just a load of 87 octane in the 91 tank.The cushion of 1 to 2 jet size may pull off a couple of HP BUT may save you a piston if the above mentioned thing occures. (39)


Timing a 2.5

Jay Smith gives the following instructions for setting proper timing on a 2.5

"Insert a dial indicator in # 1 plug hole ( V-6 is located Starbord side top )and rotate the crankshaft CLOCKWISE (with all plugs out) till you establish 0 mark the flywheel adjacent to a stationary pointer . After you've establish O turn the crankshaft COUNTER CLOCK WISE till the dial indicator reads .150" then mark the flywheel at the point where the pointer is dead nuts adjacent to the stationary pointer again that is 25 degrees. ( . 170" is 27 degrees ) Remove your dial indicator then reincert #1 plug and attach plug plug wire then ground all the rest of the plug wires ( I use a coil tester instead of grounding to the block ) attach your timing light on # 1 plug which is screwed in the block, rotate with the starter WITH THE TIMING ARM IN THE LOCKED WIDE OPEN POSITION AT TOTAL TIMING till the spark plug fires and indicates. If timing needs to be adjusted loosen the 7/16 jam nut in the top side of the timing arm and adjust philips headed screw till itswhere you want at WIDE OPEN TIMING( fully advanced ) repeat till it's dead nuts. Also make sure if you have an advancer type timing light it is on 0." (28)

Jay makes a great dial indicator for performing this procedure - it is available at


Cylinder Heads

Pending Research

You can drop the cc's in the 200 heads to 31 cc's and still run 93 pump gas. It will do a lot for all around performance. (14)


  Difference between 2.4 and 2.5 head gaskets: They are different. The cylinder holes are bigger on the 2.5 and the 2.5 has dowel pins. I would get the right gasket. There will be a very poor seal around the cylinders. (15)

I have cut 2.4 as tight as 28CC and folks are happy on pump gas.....Our suggestion on 91 octane on the 2.5 is no tighter than 34CC with thin head gaskets. (16)


The rule of thumb is anything over 145 cranking PSI with a good strong battery is beyond pump gas. (17)
Stock heads on a 200 fishing motor and hi-po motor, where typically 38cc. The first 245/260 motors were even as generous as 42cc, but this was later reduced. (19)

Performance heads and stock heads share the same casting. Primary difference is CC's and the fact that stock heads have black paint, while performance heads are "natural".


As far as just taking the time and added expence to convert to a thin head gaskets VS thick, it is not measurable, some of the heads we cut have to have A LOT of material removed to achieve the desired pump gas top compatable compression. ( 145 ) This amount as I have atested to MANY times vary from head to head , and combustion chamber configuration to combustion chamber configurations. Going from the absolute thickest ( .061 ) to absolute thinnest ( .041 ) ( Hi Per gaskets ) would NOT raise compression enough to even fool with. Maybe 3 to 6 lbs at best and when one is trying to get the max out of a pump gas motor this amount is not measurable. (29)  

"MOST loss foam embrelled 1 piece heads that are stock and NOT A DRAG or F1 motor the formula for cutting is as follows :

For every .010" that is removed from the head surface the cc 's will be deminished by .7 cc's
Example: .040" removed will diminish the CC's by 2.8

This is ballpark and not exact but I have found after cutting a zillion heads this to be the norm ! My max that I will cut a pump gas motor with the thin head gaskets is 35 cc's , I will go 34 cc's with the thicker ( drag motor gaskets) and 33 cc's with the very thick fishing motor gaskest! My target is 145 max lbs anything higher your asking for a problem and will end up at the sleeving shop with your wallet a bunch lighter ! " (37)

2.5L Pro Max : "32 cc's is to tight for pump gas especially on a Pro max. I have found 35.5 cc's will live but that as tight as I have the experience that will live on pump gas on that motor !" (36)   When more compression is added more BTU's are created, which along with more power comes more heat - with more compression being generated more fuel must be added to slow or stop detonation and make a safe HP gain..(39)

To CC your heads, follow the following procedure from Jay Smith

"I don't know but one way to CC a head and that is with a 100cc buret , a flat pulg ( to screw into the plug hole ), a piece of 1/4"thick plexiglas ( with a filler hole drilled in it ) some heavy grease to smear on the sealing surface to prevent the liquid from OZZING out the edges and I use methonol with some red food color mixed in so my blind ass can see the levels. The rest aint rocket science."

There is also a pictorial on how to CC the motor at



Cooling Modifications

From John Marles (These are for one piece mid plates, not two piece)

You MUST throw away the popet and thermostats. You can not run them with a one-piece plate , they will never open thus never pee. The water temp has to reach 140 to open the stat. It not going to happen, reading cylinder head temp. is where you get the 140 degree number. The water temp is 100 degrees when the heads are at 140. If you have a 2.5 exhaust plate and not a 2.4 you will need to plug the dump holes in it and re drill two 19/64 holes, install two 1/8 fender washers in the heads and run a 5/16 hose off the popet for a Nicasil block and no hose for a steel sleeve motor. The cylinder head temp will be about 140 All the mods I am talking about are for a 260 as well as a steel sleeve motor, With a one-piece plate the popet valve does nothing. If it runs too cold just plug the popet hose to get more heat, Merc 2.5 plates dump way too much water. There are four holes in the Merc plate totaling a larger surface area than the L and S plate and 2.4 plate. Calculate the surface area with 3.14 time the radius squared and you will see what I mean Water must come from the thermostat housing when at an idle or the top of the motor will overheat. Only run 1/8 fender washers in the heads, they have a 3/16 hole

Just remember if you rebuild it, to open all cylinders to .008 clearance Wisecos and .007 for Mercury pistons. With the one-piece exhaust plate you don’t have a choice to run a popet and thermostats you are stuck with the low pressure and cold motor.

Set you idle up to 1100 in neutral. Put in diverters The one-piece plate should run 15 lbs. at WOT and 0 lbs. at Idle. Some mercury one piece plates come with dump holes that are too big to accomplish the 15 lbs. minimum to cool the motor at WOT. Yes they are adjustable. I like to plug the stock dump hole in the one piece and redrill 2 new holes that are 19/64. Then run 1/8 fender washers in the heads and fine tune the water pressure with the popet dump hoes size. The 0 lbs. pressure at idle is normal as long as the block stays full of water and continuously dump from the heads. This type of modification is terrible for a motor that turns 6500-RPM or less because it will run way too cold. Unfortunately this is the only way to cool a HIGH RPM motor. The blocks are all the same it is the exhaust plate that has the different water passages in it. The one-piece plate allows water to flow in to both sides of the popet of it rendering it completely useless With a one-piece plate the thermostats will not affect the running temp much. Too much water has to dump through the plate to cool the block at high RPM. I only get 5 or 6 lbs at 6800 I have water temperature gages in the top of the heads and I run 100 degrees at 6800 Nothing does more damage that running a motor ice cold. I do every thing I can to talk people out of converting their stock motors to 260 cooling. Unfortunately an 8000-RPM motor needs the 260 cooling on the top. I would rather see the stockers just fix the stock cooling systems so the motors can develop some heat while running and have the motors last for a few years. I think converting to a 260 cooling system to fix a overheating 200 in not a good thing to do.


Intakes and Reed Cages

Earlier merkz had 5 peddles in thier fish'in motors, 14 in the hi-po'z...newer motors now have the rubbercoated 4 peddles(that replaced the 5 peddle) in fishing motors...U kan replace the 14 peddle Hi-po'z wit the rubber coated Jet drive units (5 petal)...On a river motor(HP bassbote etc) the 4 peddle units are better than any...they flow a lil better than the 5 peddles, but seel better for better bottom/idle/midrange, and may even add a lil top end rpm...Reed material???? Opens a new can ov wormz...I have tried them all, even cut my own the way I figgered would be better for my application...On a 200 2.5, stock, 93' fishing motor, the best performing reed I run on it wuz the original steel ones!!!...I run dual stage glass, single stage glass, carbon fiber(worst ov all), and 1 solid glass reed...bottom end and idle would change a lil bit from reed to reed, but the steel had the best all round performance ov all tried...Now, for safety, glass iz better...I turned the stock 200 in excess of 8000rpm wit the steel reeds(1:87 w/20"yamaha)...I have also changed from a 5 peddle to a 14 peddle front, and lost performance(fishing motor)...On a highly modified motor, a 14 peddle will give that extra flo on top(over 8000-8500) rpm...If your operating range iz below 8000rpm, the 5 peddle(4 rubber) will out perform the bigger cages, because of the enhanced low/midrange punch(higher air velosity)...EFI motors seem to utilize the bigger cages a lil better, but the smaller ones are allround better for the river/bass botes...I also find the single stage glass reeds to be better than the dual, and the 2 finger reeds on a 4 peddle cage to be better than the seperate finger ones...OK, Let the argument begin!!!...REX (13)


Name Original Application Part Number General Notes
4 petal rubberrized 855952A1 Late model 2.5 fishing motors. Replaced 5 petal sets.
4 petal sst120 Vulcanized with dual stage black Boyesen reeds
5 petal Early fishing motors
5 petal rubberized Jet drive units 840425T 1 Replaced 7 petal cages. Comes with single stage reeds. Can use as a replacement for the 14 pedal 2.5 Hi Performance 260,280,Drag, and Champ.
7 petal Good for applications that might go to the 10k range
10 petal
14 petal Original equipment on first 2.5 hi-po motors.

Boyesen Parts Number List for 150hp + Up

Note from Jay Smith on the Boyesen Reeds

Most of the Boysen reeds come with an ultimate thin reed clamp that is screwed down on top of the reed to hold pressure and the reed in place. These are entirely too thin . I have found the stock thicker Mercury clamp will hold the reed down better and allow for a better seal ! Going into competition with a race motor I would not allow ANY gap or leak that is visable to the eye holding up to the light it simply leaves possibly HP at the starting line. (41)



Nikasil Blocks can be rechromed by US Chrome of Fon du Lac, Wi - - Cost is around $650 for all 6 holes.


Crankcase Halves

There are two basic flavors of crankcase halves, 4/5 petal and 7 petal i.e. if you have a 7 front half, and wish to run the new 4 petal rubbizered cages, you will need to put a front half on, from a 4/5 petal motor. Earlier motors also had vertical petal arrangements, which are often swapped over to horizontal 7 or 4/5 petal configuration.

In terms of fitting of case halves from another motor Talon2.5 had the following to say "one thing i know for sure is that you "can" switch front halves, I've done a bunch no sweat, some may say not to but thats cool too, all i know is it always worked for me, anyway what the manual says is ( page 4-24 crank installation special information).

check the crankshaft sealing ring mating surfaces in the cylinder block and crankcase cover for wear grooves that were caused by the crankshaft sealing rings from the previous crankshaft. if wear grooves are present, the sealing rings on the new crankshaft will have to fit into the grooves without binding the the crankshaft before installing crankshaft, remove any burrs that may exist on groove edges. lubricate sealing rings with light oil and install new crankshaft as instructed. install upper and lower end caps and then inspect fit between sealing rings and grooves. temporarily install crankcase cover and rotate crankshaft several timesto check if sealing rings are binding against crankshaft. (you will feel a drag on the crankshaft.) if sealing rings are binding, recheck grooves for burrs. if this does not correct the problem, it is recomended that the cylinder block be replaced.

Well now in my opinion replacing the block just for sealing ring bind is ridiculous when you can smooth out the groove and then the groove issue when switching cranks or front halves is no longer an issue at all when doing a front half swap always check the main bearing surfaces with the case halves together and if they match up, just smooth the ring grooves and rock on, so far I have never come into the scenerio of the bearing surfaces not matching but thats not to say i never will either say you have a good block and front half and ya need a new crank for one reason or another, then you put in your nice new crank and the darn sealing rings dont match the original cranks pattern, what do ya a new block and front half? hell no, smooth the grooves install the crank and rock on" (24) Later on in his posts, Talon2.5 notes that he uses a 3" diameter, 120 grit sanding wheel.

When installing non-matching front halves, it is also suggested that the dowel pins be removed. To quote US1, "I do switch Mercury 2.5 front 1/2s and some 2.4 (with close inspection for mismatch). I always remove the dowel pins because there is always some sort of error in any line-boring machine no matter how expensive it is. I am talking 1/10ths of a .001. Removing the dowel pins allows the case halves to align on the bore and not the dowels. Even though some do work if you leave the dowels in it does cause friction, bearing wear and some loose of horsepower." (25)



Cranks can be interchanged from earlier motors to later motors, as long as the main center bearings are addressed (the main bearing area is shaped differently on the various years) .

In performance applications it is generally considered desireable to run a non-oiler crank, due to increased strength at the area that would be relieved for the oiler gear.

Novalves said the following about small vs. large top bearings. (34)

V-6 Cranks (pre heavy metal ball bearing top journals)

The large / small main diameter refers to the top journal only - 1.375 on small and 1.500 on large. The two center mains are all 1.375 diameter on all versions.

The 1.5 top cranks were used on 2.0 135s, 150s and on hi-performance models. The 1.375 tops were used on most all 2.4 and 2.5 production models. However there are several important differences worth noting

1.5 tops have a narrow center journal with thrust faces that require plastic caged mains. This crank also has the thrust faces required for bottom guided rods. (top or bottom guided rods can be used in these cranks)

1.375 2.4 cranks have a wider center main journals for use with the wire caged mains and also have the thrust surfaces for the bottom guided rods (top or bottom guided rods can be used in these cranks)

1991 and later 2.0 & 2.5 cranks have the 1.375 on all three main journals with thrust faces for plastic cages with the large rollers but lack thrust faces on the rod journals (top guided rods must be used)

The heavy metal cranks exhibit the following

The later cranks have malory metal(?) (which is heavier than steel ) inserts in the counter weights. Hi-performance also went to a two row steel caged C1 spec. ball bearing on top a few years ago.

The C1 spec bearings are the absolute worst posible type of ball bearing (as far as Im concerned) as the C1 spec= tight clearances ( little allowance for misallignment, flex or growth) in a continuus state of binding. The steel cage is too heavy to allow balls to revolve at 1/2 crank speed (alwaise slipping), which explains all the fret rust on and around the top bearing dosn't it?

Beyond that when installed as intended by hi-performance, there is no provision for crank growth (when heated or flexed cranks get longer) I have been told that there is clearence and not to worry, however the seven powerheads that are in process at my shop DO NOT have room for A .005 feeler gauge when mocked up.


V6 Small top, non-oiler 80-90/200-225
V6 Small top oiler (1.375) 1 piece gear 85-91/135-225
V6 Small top oiler (1.375) 2 piece gear 85-91/135-225
V6 Large top non-oiler 76-84/150-200
V6 Large top oiler (1.498) 85 & UP/150
V6 Small Top (1.375) oiler, with 2 piece gear - 8 splines 90 & UP/135-225
V6 Small Top (1.375) oiler, with 2 piece gear - 13 splines 90 & UP/135-225
V6 3.0 Split main 92 & UP/3.0L (23)



Top Guided 2.5L. Refering to appearance, the top guided rods are designed differently and are more rounded. (21) In terms of RPM, they have been successfully run over 10k rpm after replacement of the rod bolts with Merc. Hi-Po SPS units, but are considered unreliable after 7500 with stock bolts. (30)
Bottom Guided Normally from 2.4L motors, however in 91 and 92 there were some bottom guided 2.5. In terms of appearance, the bottom guided rods both large and small are more of an I-beam design. (21)
Side Pinned Side pinned pistons are stock. The pin they are talking about is the piston ring retaining pin that keeps the ring from spinning around the piston. If the ring is allowed to spin then it will hook in the ports. In the side pinned pistons the pins are installed from the side and at high RPM's can work them selves out. (21)
Top Pinned In the top pinned pistons the side pins are removed then there is a small hole drilled from the top twords the bottom at that location. Then a new pin is installed and then the hole can be welded closed so that it can never fall out. (21)
Big or Small ?

Under 7500 RPM the small rods are acceptable, above that the big rods are the desireable setup. Some folks prefer the small rods, due to a lighter rotating mass, which makes spinning up to various RPMs more responsive, while others like the added "stuffing" of the large rods.

Quoting Jay Smith: "Either rod will work the "larger" rod has a "stuffing" advantage but the added weight is a minus. The bottom guide system due to the friction of the rod to the crank (approx 2" surface area) is the lesser HP choice as the more surface there is dragging , rubbing , touching and or "guiding" common sence would tell you there is more drag no matter how small that may be and robs power. Top guided systen drags, rubs , and or guides a MUCH small area (approx 1") to keep the piston and bore alignment straight that would be the small end of the rod to the piston boss this is a quite a smaller friction area verses the larger area of the bottom of the rod to the crank ! Also an 8000 rpm cast piston is a short lived motorsome say they turn em that high and they live "till it come unwound" if your gonna use a cast piston and turn 8000 rpm very often I would opt for the smaller rod cause at that rpm when the piston fails the smaller rod knocks a smaller hole in the block and is easier to weld up !" (22) .... The 50 series rods are awfully heavy (490 grams) verses 350 for the 41 series Hi Performance rod. But the lighter rod will show a BIG improvement in acceleration but not a hugh amount on extreme top rpm ( no more than 500 or so ) (42)

Fish vs. Hi-Po "The Hi Po Rods are heat treated on both ends and have 2 oiling hole in the little end and are also cut for and come stock with SPS Rod bolts that are a ton stronger... The 44 series Hi Po Rods do not have slots in the caps. The older 50 series rods did but they were superseded to the 44 series in 1994 ish (sic motors)" (38)
Rod Bolts

Jay Smith Wrote on the Subject of Rod Bolts - Had a customer bring a 260 EFI in yesterday and when I tore it down it had 41 series rods in it with 50 series SPS rod bolts in it.

For the unknowing this rod bolt is at least 1/4" too long for this rod now put together like this a 1/4" of thread is purtruding out of the rod boss leaving only three threads holding the rod and the cap together. The bolts for the 50 series rod has the same length of thread but the unthreaded shank in the middle is the only thing longer. The 41 series rod takes a shorter (SPS Mercury Pt # 10-848475 ) and the 50 series a longer( SPS Mercury part #10- 91995 ) this the third time I have found this in Hi Per Motors that had reciently been rebuilt in the last 3 months the bolts were new as if someone had either ordered the wrong part number or some parts guy, do it yourselfer, or builder was unaware of the differences. Three threads ain't gonna hold long and when it fails it will cost someone a crank , rod , piston, head , and even worse..........

Make sure when you assemble your rods that the thread ends @ the rod bolt boss and does'nt stick thread through as the thread that is sticking through is suppose to be holding that cap on. (40)


Flywheels & Stators

There are three flavors of flywheel / stator combinations - 9, 16, and 40amp. The 9 amp fishing stator or 16amp amp performance stator with work with either the aluminum lightweight flywheel or the the cast flex-plate style flywheel. The only stator that won't work with a lightweight wheel is the 40amp (31). 9 amp stators are approx 1" in depth, while the 16amp stators are approx 1 1/8" in depth. Besides the advantage in quicker acceleration that is achieved from less spinning mass of a lighter flywheel, the lighter flywheels cause less damage to the upper main bearing (32).

Jeff_G goes into the following details on magnets and different flywheels.

The 40 amp charging system uses heavier magnets in the flywheel as well as larger alternator coils. This creates a larger magnetic drag. No matter what the output of the charging system is. A bigger coil lamination = more steel and a bigger magnet to generate the additional amp output and match the coil laminations.

Try to pull a larger magnet away from a large piece of metal. Now try the same thing with a smaller magnet and smaller piece of metal. See what I mean?

In addition on a charging system the stator is outputting, or trying to at all times. Under load a charging system is putting out about 18 to 20 volts from the stator, (voltage flowing out of the regulator. With the regulator taking away the load (no output from the regulator) the voltage can reach 200+ volts at high rpm at the stator. The amp is very low at this time however due to the no load condition. The voltage "sits" there until the regulator outputs the voltage again and the voltage drops.

Less magnetic drag on a flywheel/ stator = more HP. For instance you can take a 9 amp racing stator and cut off all the poles except 2 and the charge coils and pick up around 2 hp on the dyno. Imagine if you cut off the poles on a 40 amp system.

A rectifier charging system is a constant voltage system. All the rectifier does is change the ac voltage from the stator into DC voltage. It charges the battery at all times until the battery is fully charged. Once charged the battery will "boil" this will eventually destroy the battery. With a conventional wet cell battery this "boiling" of the electrolite will lower the voltage somewhat. The system continues to output full voltage and amperage. On a maintance free battery it can not do this (boil the electrolite) and will destroy the battery in short order. That is why you don't use a maintance free battery in a non regulated system.

On a regulated charging system the voltage under a no load system will as previously stated continue to grow. There is no mechanism to bleed off voltage in a non regulated (rectifier) system. The excess voltage DOES NOT go to ground.

Lets look at some possible consequences. Using a 16 amp stator with a 40 amp flywheel the system can produce over 500 volts due to the increased magnet size. This will destroy the stator. If you were to put a 40 amp stator with a 16 amp flywheel (which will not fit by the way) you could not generate enough voltage to fire the charge coils. The magnetic field is not strong enough.

If you look at the charge coils (ignition) on a 40 amp system you will see the laminations are very small to reduce the voltage produced from the heavier magnets. Also on the front of the low speed charge coils is a shunt plate. This acts as a dead shorted winding and to "shield" the coil from the magnets and there fore producing excess voltage. If you did not have this the low speed charge coils could produce 2,000 volts at 6,000 RPM!

This is why the coils are exposed and not encapsulated. The encapsulated coil can not dissipate the heat.

So the mass of the flywheel is not the only limiting factor, but the drag produced by the magnets passing the heavier laminations of the coils will also drag down the system. So this drag along with the load can create the loss of as much as 5 to 6 HP at full throttle.

A charging system for a 16 amp system will have less drag on the motor than a 40 amp system at the same amp output .

The heavier flywheel also pays a higher price in being harder to balance and putting more strain on the motor itselt. Yes a lighter flywheel is easier to balance, will have better acceleration and top end and will rev quicker. (33)




Oil Injection System Removal

The mercury oil injection system is a popular item to remove, especially given its reputation for failure, and inability to deliver reliably after 6500 rpm.

Remove oil resevoir, oil pump, mag pickup, pump drive shaft, then install plug in shaft hole. Remove oil hose off bottom of vapor seperator, plug hose, remove fitting on starboard side of the block where oil tank pressure hose is coming from, plug hole with pipe fitting either 1/8 or 1/4., Remove oil alarm box, and i think that's all. the plug for the pump shaft hole you will have to get from dealer. (12)

Mercury part number for the plastic block off plate is 43453 and the O-ring for that kit is 32509. Jay Smith Racing Engines also has a billet kit for doing this.

Finally it is recommended by several sources, that you keep the old oil pump gear on the crankshaft, as it provides crankcase volume, and if removed will cause that cylinder to be lean.



Alien cowlings are the latest style that Mercury has produced for the Offshore series, and are available on serial number 0G857000 and up (image). The rest of their line have retained a cowling very similar in appearance to those from the mid 90s (image). Due to the high costs of the cowlings, several companies such as Bob's Machine and Eagle One Performance produce aftermarket replacements.


Motor Installation Drawings

115-150 Optimax
200-225 Optimax
135-200 2.5L
2.5L EFI
150-225 Promax
300 Promax
90-115 Optimax
135-175 Optimax
225-250 Optimax
150-200 2.5L
2.5L Race Drag
2.5L Race Offshore
225 4-Stroke


Gearcase Cross Reference

Unit Name Gear Ratio Original Application Part Number Shiftable Notes:
2.4 Bridgeport
2.5 Offshore
“Crescent Leading Edge” lower unit. An older-generation Mercury high-performance lower unit with integral nosecone and low water pickups. There were two designs of the CLE, both offering the same performance. Designed for high-speed surfacing applications, and originally equipped on 2.4 Bridgeport engines. No longer in production. 2.0/2.4/2.5L applications only. (5) Painted black. Two versions - holes are either below or above the bullet. Late model CLE 4 hole unit - images 1 and 2 (courtesy eagleone). CLE from a 1994 2.5 carb (courtesy gtrain). Swept CLE with factory torque tab (courtesy tufish).
FleetMaster 25" Promax 300
Fleet Master: Designed for use on 25" Promax 300s. Internal components are built to withstand the saltwater environment. All 3.0 Litre gearcases have a radial discharge stainless steel waterpump. These best suited for Offshore Center Console Vee-Bottom type boats. This one is the "workhorse" gearcase. (1)
Lower units used on Mercury 2.0 SST120/S2000 and 2.5L S3000 and Champ race outboards. Available in 14:15 and 15:17 direct drive ratios. Designed exclusively for racing applications. (5) Later models had thicker skeg and can be used on V6 motors. (3) Here are two (1, 2) comparison photos of the 1/1, 14/15, and MC-1 drives (images courtesy Crazy Horse). The latest model of the speedmaster is the SSM6, which is pictured here with a studs-up configuration (images courtesy Brendan Power). Finally a different shot of a stock Speedmaster 6 (courtesy Tabara Racing).
SportMaster 1.87 1687-849780A 2
Current-generation Mercury high-performance lower unit. Includes a nosecone and low water pickups into the design, as well as stronger than stock internals. Designed for high-speed surfacing applications. Available for 2.0/ 2.4/ 2.5, and 3.0 applications. (5) The Sportmaster replaced the CLE.(1) and intruduced a longer bullet than the CLE. Sportmaster features a crescent-nose bullet intended for surfacing applications with gears, bearings and bearings carrier that are heavy-duty components, and the drive shaft and prop-shaft are one-piece. Available in polished or bead blast surface. Sportmaster 15" (courtesy bkp). Sportmaster for 3.0 (courtesy propman). Sportmaster 20" from stock 2002 (courtesy grep4929). Sportmaster 20" w/small shaft - left and right (courtesy red alert).
2.00 135/150
1.87 150XRI, 175-200
1.75 200DFI
1.64 94 225-250
1.75 95-97 225-250
1.64 275
1647 - 9148A47 - 1996
1647 - 9148A92 1997 + Up / Ratcheting Gearcase
859399T18 3.0 Gearcase

TorqueMaster Optimax
A heavy-duty high performance Mercury lower unit that does not have a nosecone. Designed for boats with heavier loads that require power trim for bow lift. Current models include low water pickups, however, earlier models did not. Available for 2.5 and 3.0L applications. (5) Designed to run at elevated transom heights of 25 to 27 inches. Generally stock on Optimax. Torquemaster is the all around replacement for the old V-6 Gearcase as we know it. (1)
The Torquemaster has lower (fewer) water inlet holes (for that higher transom mounting) (2)
1.78 Small Gearcase
1.87 Large Gearcase
Smaller diameter bullet than other shiftable lowers 4.25" vs. 4.75"

Commonly Used 2.5L Performance Parts (26)

Description Part # Aftermarket Notes Qty
Stator (16 amp) 398-5454A66 Rapair/CDI
Stator 398-5454A36 Rapair/CDI
92-94 260 MPP
Updated to A66
Voltage Regulator 88825A 7 1
Rectifier 816770   1
Solenoid-Start 89-76528A 1   1
Starter 50-79472 Arco 5380 8 tooth drive gear 1
Fuel Injector (EFI) 98818 98818 1 for later models 6
Fuel Injector Grommet 25-99123 6
Fuel Regulator (39 lb) 94820 Gold 1
Fuel Regulator (56 lb) 12026 2 Black 1
Fuel Filter - Final - (Carb & EFI) 35-818700 1
EFI harness (digital) 84-98866A26 1
EFI harness (analog) 84-98866A19 Rapari/CDI
Air temp sensor (EFI) 13221A 1 1
Switchbox 332-7778A14 Rapair/CDI
Coil 339-7370A19 6
Plug Wire Set 84-815358A 1 Blue 1
5 Petal Rubberized Cages 840425T 1 240 EFI Jet 6
4 Petal Rubberized Cages 855952A1 6
Head gasket (1.2mm) 27-822844 1 Pro Marine 844-1 Originally on 2 Bolt Main Blocks 2
Head gasket (1mm) Pro Marine 844R1 2
Head gasket (.75mm) 27-814658 1 Pro Marine 844R75 Originally on 4 Bolt Main Blocks 2
Seal, Head Gasket 26-849616 Outer seal for O-Ringed blocks - 2.5L Offshore 2
O Ring, Head Gasket 25-817574 O-Ring for individual cylinders - 2.5L Offshore 6
Seal, Head Gasket 858452 1 Outer seal for O-Ringed blocks - 200 EFI 2
O Ring, Head Gasket 859772 1 O-Ring for individual cylinders - 200 EFI 6
Gasket Set (2 Bolt main block) 27 - 814658A 1 1
Gasket Set (4 Bolt main block) 27 - 814658A 3 1
Gasket Set (Carb) 27 - 814658A 4 1
Intake / Reed Block Gasket 27-12775 4 Pro Marine 12775 1
Air Inlet gasket (EFI) 27-99316 3 Pro Marine 99316 1
Base gasket 27-823142 Pro Marine 2693A 2
Carburetor gasket 27-90430 Pro Marine 2509 Stock and Mod-VP / Formula 1 3
Carburetor gasket 27-67751 (1) 2.5L 3
Rod Bolt by SPS, Bottom Guided 10-93886 Pro Marine RB7000 84-91 12
Rod Bolt by SPS, Top Guided 10-76349-1 Pro Marine RB8000 92-up 12
Rod Bolt by SPS, Hi-Po 10 - 848475 2.5 EFI, F1, S3000 12
2 Liter Champ Adapter 1159-832012T2 Allows installation of a small top bearing crank, into a large top bearing block
Exhaust blockoff plate - Offshore 20" 818261 1
Exhaust blockoff plate - Offshore 15" 818909 1
Thermostat - 143 degree 75692 Pro Marine 403 2
Thermostat - 120 degree 14586 Sierra 18-3549 Originally on 225 Pro Max 2
Thermostat - Substitute washer 12 - 47533 2 2.5 Offshore - for use instead of a thermostat 2
Thermostat gasket 27-62386 Pro Marine 424 2
Outboard Service Manual 90-849240     1


3.0L EFI Family of Motors

Pending Research

Head CC Figures

  • 225/250 73cc
  • 250xb 68cc
  • 300pm 63cc
  • 300x 61cc (43)

Computer Differences

The difference as I understand is the ECU's. The 250 has 4 more degrees of max timing, a rev limiter that's 250 rpm higher and a slightly different fuel curve. (47)

300 Promax ECUs have a rev limiter range of 6200 to 6400 (2000 and later).

Crank Differences between the years.

Best way to explain crank differences is that older cranks have loose bearings with a two piece sleeve that fits over roller bearings and new style has a caged bearing assembly which is way easier to deal with. Crank with cages is found in 2002 250XB. (44)

CRANKSHAFTS (225/250):
Serial Number 0T325116 and BELOW use #9784A4 Crankshaft (or one of several part numbers superceeding to 9784A4).

Serial Number 0T325117 and UP use #9784A11 (also used in the 250 XB). (45)

Engine Block Differences

General consensus is that 2000 and later blocks did not have the relief cuts in them, but around 2001 the newer electronics were introduced, which resulted in some mounting boss changes. (48)

Prior to Serial Number 0G960499, the 225/250 engines use the same BLOCK.

Serial Number 0G960500 and UP: 225, 250, and 250 XB use DIFFERENT BLOCK PART NUMBERS (with different porting).

300 Series Engines: These are different of and unto themselves. (45)





Performance enhancements for the masses - There is a great writeup by kllr bee at on 3.0L High Performance board at I will summarize some of the key points below, but I recommend reading the original.

  • Must implement more compression. You are shooting for a MAX of 140 lbs.
  • Get a 300 Promax ignition module and 250XB fuel computer.
  • Have intake manifold modified by cutting center runner between cylinders
  • Stick to stock or 250XB / 250XS exhaust tuner. Performance gains from other tuners are negatively offset by the law enforemcement attention getting of noise
  • Install Boyesen 2 stage carbon fiber reeds
  • Stick with the factory recommended spark plugs
  • Get a lightened flywheel
  • Get your TPS(throttle position sensor) set richer. Mine is at .102 at idle and .432 at wide open.This keeps the lean sneeze from happening at idle.
  • On a stock 225 EFI you will want to get 250XB/300 PRO MAX ignition ECU's. They transfer the ignition current at a higher voltage.
  • lder 225/250 EFI motors that do not have the 4 butterfly intake will need to have the 4 butterfly upgrade to have adequate air to feed the increase in fuel added by the computer upgrades. (46)








Footnote Source
1 MTCM - 10/21/02 -
2 JW - 10/21/02 -
3 crazy horse - 4/9/03 -
4 Raceman - 4/9/03 -
5 Scream and Glossary
6 MTCM - 4/10/03 -
7 MTCM - 1/29/03 -
8 MTCM - 6/15/02 -
9 Talon2.5 - 3/14/03 -
10 Blizzard - 3/14/03 -
11 Talon2.5 - 4/16/03 - - quoting Jay Smith
12 pirogue - 2/22/03 -
13 T-REX - 3/9/03 -
14 Eagle One - 3/12/03 -
15 us1 - 10/3/02 -
16 Jay Smith - 3/13/03 -
17 Rickracer - 1/2/03 -
18 Rickracer - 4/21/03 -
19 MTCM - 4/24/03 -
20 D.Allison - 3/22/03 -
21 TTriton - 11/9/02 -
22 Jay Smith - 3/4/03 -
24 Talon2.5 - 2/12/03 -
25 us1 - 2/13/03 -
26 Based on idea from
27 Jay Smith - 4/14/02 -
28 Jay Smith - 3/21/02 -
29 Jay Smith - 1/27/02 -
30 us1 - 12/06/01 -
31 T-Rex - 4/12/03 -
32 Jay Smith - 3/11/03 -
33 Jeff_G - 1/28/03 -
34 Novalves - 5/10/03 -
35 US1 - 4/03 -
36 Jay Smith - 6/27/03 -
37 Jay Smith - 6/28/03 -
38 Jay Smith - 5/30/03 -
39 Jay Smith - 5/25/03 -
40 Jay Smith - 1/1/04 -
41 Jay Smith - 2/26/04 -
42 Jay Smith - 2/13/04 -
43 Todd D - - 7/17/2010 -
44 kllr bee - 12/7/2010 -
45 European AM - 12/6/2010 -
46 killr bee - 12/9/2009 -
47 Stich King - 7/26/2008 -
48 mrcrsr - 6/23/2010 -

Last Modified on December 11, 2010