cub c60 engine piston diameter at top and at skirt

was8478 · Sep 1, 2013

Hello,
I am new here and it was suggested that I might be able to get a question answered since its Sunday and everybody is closed.
I am in the process of rebuilding a 1955 cub c60 engine and the .030 oversize pistons I got from IH Case are tapered. meaning that the ring area is .020 smaller than the bottom of the skirt area. The skirt area is the correct diameter for my rebore. But the skirt is also about .012 oval.
Is this typical of a new piston?? To be out of round and tapered??
Any help would be greatly appreciated.
Thank You
Bill

rustred · Sep 1, 2013

sounds good to me. when fitting pistons to sleeves the measure ment is taken at the skirt. have to factor in expansion.

pete 23 · Sep 1, 2013

All aluminum pistons are "cam" ground, meaning egg shaped. Being smaller across on pin side than thrust side. Also, tapered from bottom to top on thrust sides of piston. You measure just below the bottom ring groove on the skirt, and bottom of skirt to determine how much taper. Usually about .002 to .003 depending on size of piston. The ring lands them selves will be much less diameter and probably vary from one ring land to the next. Reminds me of a point one of IH super salesman brought up at a meeting many years ago. He was pointing out that IH pistons were cam ground ( which he had never heard of before and didn't know what it meant). He was using this as a selling point over the competitors. I told him we cam ground pistons in 1955 at school on any and all aluminum pistons. Then another time when a John Deere mechanic was pointing out to me that the new Deere diesels used a keystone (tapered ring land) and I tried to convince him IH and other mfg's had been using it for several years. Most mfg's use very similar style pistons and rings.

was8478 · Sep 1, 2013

Pete 23,
You nailed it!
Following Your instructions that is how these pistons measure. Taper from bottom of oil ring groove to bottom of skirt is more like .004. But I guess that's what You meant by "depending on size of piston". And they are smaller across pin side than thrust side.
Thank You very much for the explanation. It just seemed like most of the tolerances in my blue ribbon manuals (bearings, tappets, valve guides, pins, rings, etc) were much more stringent. Then I got to the pistons and nothing about taper or ovality is mentioned.
I am now going to proceed and get this engine done.
Thanks again!
Bill

Owen Aaland · Sep 2, 2013

The pistons will change shape as they heat up in use. As they expand they will become more round rather than oval. The cam grind is to help them be the correct shape while the engine is being run.

The same hold true for the smaller diameter at the ring lands. The top of the piston runs hotter than the skirt. If the ring land area was not smaller the piston would rub the cylinder wall rather than the load being carried by the skirt.

cub c60 engine piston diameter at top and at skirt

was8478

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rustred

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pete 23

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was8478

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Owen Aaland

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