50hp piston rings

Can anyone help me with the OEM part no. for the piston rings, for a Force 50HP, model no. 507x90D. Master Tech is showing bore should be 3.375 for 1990 model but my calipers, are showing I,ve got some egging, with the bore running from 3.297 to 3.308 granted I,m only checking the top 3/8", but I don't have any inside mic's handy, so I'm trying to make do with what I've got. Still .067 is a pretty big difference, any ideas, or should I try to round up some inside mic's. I don't really want to put her back together without at least ringing it because it has some minor scoring but more distressing is that I've got some aluminum depositing on the side of no. 2. doesn't that indicate some piston slap? Also no.1 has water detonation marks along the chamber ridge that runs along the top of the piston. Should I just put it back together and hope to get another season or do a total rebuild?

Re: 50hp piston rings

The did make the 50 in two bore sizes at various years: 3.375 (3 3/8) and 3.3125 (3 5/16). Nominally, that is 1/16 or .0625 different. You are getting .067 difference. Seems like you have nominal 3 5/16.

Old Chrysler 50 HP engines were 3.187 bore but with your smallest measurement being 3.297 I don't think you have that bore. I suspect that your calipers are not accurate enough or not zeroed properly.

Additionally, the flat top of the piston baffle on the 3.3125 piston is only about 1/4 inch wide while the 3.375 is almost 1/2 inch wide. So if you have the narrow baffles, then definitely the bore is NOT 3.375.

Final piece of evidence: Wiseco shows the 1990 50 HP engine as 3.3125 bore and 1984-88 as 3.187. So: your master tech is wrong.

Tolerance for wear is usually .002 out of round, barrel shaped, or tapered.

Aluminum rub off is due to either detonation from lean running or galling pistons from overheating. However, you also mentioned melting the piston baffle. This is detonation from lean running.

SO; With the measurements you have gotten, and the damage you have described, you are out of tolerance. An overbore of .010, .020, or .030 is due, depending upon damage to cylinder walls.

After you rebuild, be absolutely certain that the carb is set properly. This has been discussed ad nauseum so search the posts for proper synch and setting.

Re: 50hp piston rings

Thanks, Frank,
I just got this engine, the damage is all before me, the power head had dropped all of it's mounting bolts and was literally about to fall off when I picked it up for cheap. decided that since I had to pull it anyway, I'd go ahead and tear it down, seems to be a good thing I did. I'll double check my readings and my calipers against some feeler gauges. I'm a millwright and make my living with precision tooling and they're minatoyo so I don't think they're off but only takes a second to check. I'll have to look at the baffles in the morning but i think they're the narrow ones, now I'm wondering if the power head's been replaced with an older one, would explain the mounting bolts vibrating out. somebody didn't torque and lock tite 'em. thanks again.
Oh yea I learned about lean running some time ago with an Elgin, wonder how many of the youngsters on here have ever even heard of one of those