If you are rebuilding a counter rotation engine, the pistons and rods go in backwards compared to the regular motor. I found this out by bringing the rod and piston assy back to the shop, thinking they put them on wrong. Learn something new everyday... BBB

Oh , BTW all the hi perf. motors are all standard rotation. I don't think they would have made a 370 opposite. but it could be done.

HUH?? I guess I am a bit "backward" but I just can't picture how you can install a piston backwards without having all those funny shapes on the piston dome interfering with valves and head chambers when you crank it over unless you have strictly flattops. Could you explain further? --- Jer

actually the rod is backwards, the piston is installed backwards on the rod. The bearing tangs end up the other way. The valve notches would have to be in the usual place. I should have made this more clear ! Basically the numbered side goes in instead of out. BBB

Not following the rod thing. So rods actually have an in and an out side to them? If so, what is the difference?

One side of the rod is chamfered to clear the crank radius, the rods ALWAYS go on the crank with the chamfer facing the outer ends of the journal. So happens this is with the bearing tangs away from the cam.

If the pistons have the pin offset from the centerline there will be an arrow, notch or some other form of ID to represent the normal front of the engine. I don't remember ever seeing a domed piston with an offset pin. Normally you will see it on a dished OR flat top piston with 4 valve notches so it can be used on both sides of the engine. If it was a domed piston (or single valve relief) you would need 4 rights and 4 lefts.

On a counter rotate you would install the pistons so the notch faced the rear. Assuming a notch piston you would just take the pistons/rods from the right bank and put them on the left & vice versa. If it mattered to you to keep the rod numbering correct, or if they were domed you would have to swap them on the rods.

I had to really strain my memory to remember all this, has been a while since I put an engine together

I can't open the motor up beacuse it's in the boat. I know there definately was a difference in assembly.
Thanks, Marc. You always share great advice on here. And steer people in the right direction. BBB

If I may chime in.

Marc pretty good memory.

I dont need to remember because Im staring at one.
Then again my brains toast anyway.

Rods make no difference in the way they go in a counter
or a standard.

Its all about the chamfer facing the radius on the
crank.

Big block billys pistons had to have been offset on the pins.

From my understanding there were two ways of counter
rotation. Custom Ground cam or a gear drive two spin the Camshaft the original way.

The distributer gear is ground different to keep the distributer and oil pump spinning the same way.
The firing order on this custom grind is opposite of STD chevy. 12756348

All reverse rotation engines have that firing order, and in saying that with a belt drive, timing chain, or three-gear(idler) the cam has to be ground specifically for that application so that the cam and crank both turn clockwise (as viewed from behind the engine). In this case the cam gear and distributor gear are ground to turn the distributor and oil pump in the standard direction.
the two-gear timing setup used in stock Merc counter-rotating engines spins the cam in the standard rotation while the crank spins in the opposite direction, allowing a standard distributor gear to be used.

possible differences
1) custom ground cam shaft
2) Distributer gear
3) three gear Idler (no Timing chain)
4) Rear main Seal
5)pistons if offset pins.. look for notches or arrows

Something to think about
Where the oil hole is placed on the journal
of the crank should be in close proximity of where the piston
actualy fires. This would in theory insure a film of oil in between. In a counter rotation the Oil hole is not at its proper clock position.

Cross drilling the crank or atleast adding a radius (chamfer) to the other side of the oil journal which feeds the rod bearing.

Heres a question Marc.
Are the merc. part #,s different from a merc counter crank
compared to a standard rotation crank?

Gerry

Look at the big brain on Gerry!!!

Merry Christmas Gerry...Did you figure out what to do with your setup?

You guys got pretty bogged down in specifics, but the fact that this set-up is an outdrive engine then it is a standard rotation. Only inboards and v-drives used opposite rotation engines. It's much cheaper and easier to do the reversing in the outdrive.
The newer inboards and v-drives use standard rotation engines with a reversing transmission on one side. Other than Marine Power in Louisiana, I don't know of anybody building a new opposite rotation engine these days.