21 Stud Piston Fit

frnkeore

frnkeore

Well-known member
I just received my Simi Finished pistons, for my late '36 engine. It will be .080 over or 3.142 basic bore.

These pistons measure 3.149 and are suppose to be finished by cam grinding the skirts.

What I would like to do, is turn them round, instead. What would be the desired clearance for a round piston vs a cam ground clearance?

I have the specs for ring land and ring depth so, that's not a issue.
 

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I didn't even know you could buy them semi finished. Watching with interest.
Mart.
 
i have used some very worn pistons--- early on I learned they have to have a lot more clearance than new pistons because they were closer to round ---in the range of piston slap cold

perhaps if you are going to turn in a lathe turn the major diameter, then offset to each side a few (.005??) to make the "cam ground" ---and you have to add some taper to the top on the skirt and reduce the diameter of the crown ----seems like it would be easier to find .080 pistons ---or find someone still with a machine to make the piston to size
i once made oversize piston to size on a belt sander freehand---it ran fine longer than I owned it
 
Round pistons will be non round when at work temperature...
That they are tapered and oval shaped is for compensating that they expand more at top and that you have a pistonpin in them affecting expansion.
As long as you keep them tapered...straigt they will either seize at top or make a heck of a noise being loose at the skirt...
Measure a finished piston that will give you the ratio between top/skirt...that will give you a starting point...
 
Thank you, all, for your comments. I would love to have more input though.

My lathe chucks, are Buck, adjustable 3 jaws, so I can make soft jaws and do as Kurt, suggests.
 
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