Proper Specs for Bored Cylinders and Pistons +.020 and over

This may be a bit premature but I’m going to need my block bored for new pistons. I have Ken Bells service manual but Im not sure how to interpret his specs. for bored cylinders and corresponding pistons. I have not had the block checked yet for what bore I will need but Im hoping to get by with +20. What is the correspondence between sizes and piston grades F, G, through K. Ironically this comes from a kid (me) who in late 68’s, while still in high school, worked in the piston grading room at a Ford engine plant and was planning to import a Primrose 68 E type from Autoport in Frankfort to Toronto!! BTW I still have the price list $4,005.00! $150 dock prep, $150 pollution junk, $150shipping … $4563.00 landed in Toronto. Epic fail …didn’t pull the trigger!!!

The pistons you buy will specify the clearance. Do not do any machine shop work until you have the pistons in hand.
I bought mahle’s and went slightly larger than what they suggested based upon the machine shop conversation. The Pistons are usually stamped with the clearance they require.

The ONLY bore spec to pay ANY attention to is the one provided by whoever manufactured the EXACT pistons you are using. Any other spec is WRONG.


So your saying pick your pistons first and work backwards to the boring size to match them. Excellent advice. Is there any advantage between Mahle’s and Hepolites?

In a word, no.

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The piston gives you the diameter and required clearance. Replacements are not graded.


John! have some info. for the pistons etc. you where
looking for. Maybe its what you need.


Thanks good stuff. What service book did that come from?