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Message: <blockquote data-quote="Silver95bird" data-source="post: 1404398" data-attributes="member: 21743">I'm not so much building to a set of rules, as much as I'm trying to apply the theory to what 90% of those reading this are more likely to experience. There aren't that many people blueprinting modular engines in their home garages, but there's bound to be a good number of headswappers or those who buy a set of TF or ported heads who will want to make the most of it. Leaving .025-.030 as a safety margin, once stretch in the rods/pistons/crank is considered becomes essentially zero. There's a similar margin taken into account for checking PTV clearance with a cam change. Unlike PTV though, this changes where the heat is distributed in the chamber.With PI heads, the chambers are very small, so essentially the deck face is the zero point for clearance. With the NPI heads, the swirl fin may be about the only part that would directly contact the piston. If someone had a set of ported NPIs with the swirl fin layed back, would that make much of a difference on how the clearance would be set up, or is the diameter of the chamber on the heads narrower than the pistons such that the pistons would kiss the head anyway? I'm mainly talking about the F5AE castings, since that's the main one Mustang NPI'ers would be dealing with.</blockquote>