Piston to bore tolrence?

[steven4141]

Frank, I need some of your expertise here, I finally got hold of some inside mic's and you were correct. the bore on my 90 model 50 H.P. should be 3.3125, checked mic's for zero and I;m getting 3.3125 top bottom to 3.3165 on on the cross on one cylinder and 3.3195 on the cross on the other. looks like I;m going to have to go at least 10 over maybe 20 but I hope not. Question is can I get by with oversize rings and the same piston at 10 over, what about 20 over or do I need to go with over size piston too? My dad said he thought tolerance for bore to piston ratio was only about two - five thousandths but he wasn't sure, but too small of a piston would probably result in broken rings. Any help would be appreciated.

 

[Frank Acampora]

Re: Piston to bore tolrence?

Tolerance on Chryslers and most likely Force engines (I use the same tolerance) is .002 out of round, tapered, or barrel shaped. So, you are out of tolerance on the one cylinder. If the cylinder is not scored, you MAY be able to get away with .010 overbore but if it is scored at all, then .020 will be necessary. Let the machine shop tell you what is necessary.

It is absolutely necessary to have a matched oversized piston and ring set. Trying to use a standard piston with oversized rings will result in decreased support for the rings, increased flexing, and increased likelyhood of breaking them. Additionally, the standard piston will have too much clearance and base crankcase compression will drop, resulting less charge inducted and pumped to the combustion chamber, reducing horsepower. The factory does have specifications for piston skirt clearance but to the best of my knowledge, has never published them.

Normally, I suggest that only one cylinder needs to be bored, but since it is only a two cylinder engine, it will only cost a couple of hundred bucks or less extra to do both. Then you would have, in essence, a brand new engine.

 

[steven4141]

Re: Piston to bore tolrence?

Thanks Frank, you pretty much confirmed what I thought, and since I have it down I might as well do both cylinders, Now I've just gotta figure out how to get the head in at work so one of the machine shop guys can bore for me Actually getting it in the easy part, It's getting it back out of the plant that takes some ingenuity thanks again