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Breaking the Engine Code

Wed Jan 16, 2013 10:52 am

Looking at a cub engine with the head removed, there are letters and numbers sometimes stamped on the block and the corresponding piston. Do these stampings respresent whether the engine has been reworked or not? Will a NOS engine have letters stamped?

Is there a standard chart that shows:
A---.010 over
B---.020 over etc.

Help please. :lost:

Re: Breaking the Engine Code

Wed Jan 16, 2013 10:55 am

Ron, Someone explained previously that IH had four sizes of standard pistons (A,B,C,D). After the block was machined, they would measure the bore and then match the piston to the bore to have the correct ring gap.


Re: Breaking the Engine Code

Wed Jan 16, 2013 11:26 am

Bob has it pretty close. Due to the ability to due precision manufacturing and machining at a reasonable cost when these were made, they did not come out perfect, so the pistons and bores were measured, and marked accordingly, and as they were assembled they were matched up by the A,B,C, D stamps. The differences were in the range of .001 or maybe less, but they were matched for the best fit. At least that is the way it was explained to me by an old time IH mechanic several years ago. Pistons that are oversize are normally stamped at the factory with the difference, such as .020, .030, etc. in the top of the piston, and many machine shops will also mark the block by the cylinder when they rebore an engine, but not all

Re: Breaking the Engine Code

Wed Jan 16, 2013 11:39 am

Thanks, that is good info.

So, there is no way to tell for sure by looking, whether an engine has ever been rebuilt with new rings, blocked bored, etc.

Re: Breaking the Engine Code

Wed Jan 16, 2013 11:53 am

Normally the pistons themselves are stamped with the oversize markings, .020, .030, etc. If the pistons have the A,B, C, etc. stamps it is a pretty safe bet it has not been bored.

Re: Breaking the Engine Code

Wed Jan 16, 2013 11:59 am

Ditto what John just posted.

No letters or numbers stamped on the head - anybody's guess.

As far as engine having been re-ringed, or condition of the various parts, no way to tell by looking.

Re: Breaking the Engine Code

Wed Jan 16, 2013 6:45 pm

Several years ago I bought a great running engine from Ralph that he removed from a Cub he parted out. He didn't open it up but suspicioned it had been rebuilt as the block had absolutely no paint and all the bolt on parts had original paint on them. Before I installed it in Rusty, my curiosity got the better of me and I removed the head. The engine had come from a '69 Cub and had letters on the block and 020 on the pop-up pistons.


Re: Breaking the Engine Code

Wed Jan 16, 2013 7:31 pm

There were more letters used than A B C D. My cubs engine has E stamped next to each bore.
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