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.

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)

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 http://www.jaysmithracing.com

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.

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)

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 do......buy 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.

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)

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)

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)

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.