Farmall Cub

http://farmallcub.com/phpBB2/viewtopic.php?p=131767

Check the last post in above topic.

Bit of research, appears that AP 137 is a piston style that comes in various diameters.

Any way, the engine has been reworked, cylinders bored. Be sure and check the crankshaft journals for size, they may have been turned.

Thanks for clearing that part of it up. So I would assume that the STD stamp on the head of the piston is irrelevant to the cub engine since it is an aftermarket piston.
Edit: what kind of clearance should I run with an after market piston? The book calls for .001 - .002 clearance. I would be afraid to run an aluminum piston that close.

clodhopper wrote:.......
Edit: what kind of clearance should I run with an after market piston? The book calls for .001 - .002 clearance. I would be afraid to run an aluminum piston that close.

I know the aluminum pistons require a little more clearance due to expanding more when hot, but how much, you would need to ask an automotive machine shop, it is over my head. We have local (about 20 miles) that does a real good job at very reasonable prices, and if I have very serious engine work to do I just take mine to them.

What is the history of this engine? Have you had it running and for how long?

Photo blow up of the piston in the photo shows a burr on the bottom lip and perhaps on the top lip of the upper compression ring groove. Also has a number of nicks and scratches.

Take the pistons to the machine shop and have them inspect and suggest either machine work or replacement.

.001 is pretty close for the piston fit, I like .002-.003.
Your block was bored .040 and the pistons are .030. People do strange things.
George

Usually aluminum pistons require .004 clearence. I had one cub engine years ago that the machine shop gave the pistons .003 clearence. After assembly I test ran the engine for about a hour. it slowly started to labor. I had to take it apart all over to hone the cylinders to .004. never had a problem. Keep in mind that if over boring cylinders the the aftermarket pistons these days wheather they are aluminum or cast iron need to be in hand at the mahine shop to be properly fit.