Cast Aluminum Piston to Bore Clearance

[Guest JohnArthurSpinks]

I have a set of Rolloy Cast Aluminum pistons that are tagged 018 A / 986916 / 40 on the head of the piston. They are 3.125"diameter + 040" over size.

From what I can tell the pistons have a relief horizontal slot in behind the lower  oil ring and and a half skirt length vertical slot. The lower half of the skirt is as per a plain skirt piston.

Can anyone advise a rule of thumb dimension for the piston skirt to cylinder wall clearance.

Regards

John Spinks

Australia

email is johnspinks48@gmail .com

[cahartley]

If they are cam ground they probably will spec to .002" under the bore size or even less in your case.

When I bought a new piston set for my '25 Dodge I got them from EGGE >>> https://egge.com/part/egg-t100-e116-4/

When they arrived I mic'd them to find them cam ground with only .002" clearance the "big" way.

I had a notion to turn them for more clearance but wound up leaving them alone figuring if EGGE doesn't know what they are doing nobody does.

Well, EGGE DOES know what they are doing....... 

[Guest JohnArthurSpinks]

hi cahartley

I checked the Egge web site and they seem to mention only forged pistons and unfortunately that is not what I have.

I have cast pistons and the clearance is different.

Thank you all the same.

Regards

JS

[JACK M]

I had an instructor tell me one time that a cam ground piston gets round when it gets hot. Is this true?

[cahartley]

Yes Jack.....totally true.

I couldn't believe the difference between the "big" and "small" side across the aluminum piston.

That's the reason I trusted EGGE's finished clearance.

Mind you there is a significant difference in expansion ratios between cast iron and aluminum.

[JACK M]

Thanks, The Evinrude outboards are referred to as cam ground pistons. All are aluminum.

Something else interesting, not necessary to bore all cylinders if one or more are scored. (this is warrantee information).

Engines from one to eight cylinders, two stroke.