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MikeK · 10-21-2019, 08:43 PM

Original pistons were 0.03125" proud if all design tolerances were in the center of spec and the rod throw clearances were zero. (impossible!) Most blocks left the machining bay with the deck on the high side, sometimes as much as +0.010", reducing the popup to about 0.022".

Replacement pistons do not always have the same pin height. Many were produced somewhat shorter to compensate for block decking. Others are close to original pin height causing popup to exceed 0.040 after decking in some cases.

The relief cut in the head (minus any loss from subsequent planing of the head) plus the compressed gasket thickness should leave you with 0.020"+ clearance. Any less and you may have an issue. Measure! You can use clay or even soft lead (Pb) pellets in a test assembly to check 'squish' clearance if you know what you are doing.

10-21-2019, 08:43 PM	#2
MikeK Senior Member Join Date: May 2010 Location: Windy City Posts: 2,919	Re: Piston Height Original pistons were 0.03125" proud if all design tolerances were in the center of spec and the rod throw clearances were zero. (impossible!) Most blocks left the machining bay with the deck on the high side, sometimes as much as +0.010", reducing the popup to about 0.022". Replacement pistons do not always have the same pin height. Many were produced somewhat shorter to compensate for block decking. Others are close to original pin height causing popup to exceed 0.040 after decking in some cases. The relief cut in the head (minus any loss from subsequent planing of the head) plus the compressed gasket thickness should leave you with 0.020"+ clearance. Any less and you may have an issue. Measure! You can use clay or even soft lead (Pb) pellets in a test assembly to check 'squish' clearance if you know what you are doing.