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Iron alloys will expand about .00125" per inch. The top rings of a 4.000" bore Chevy engine have a circumference of about 12.566" so .00125" X 12.566" = .0157" or rounded off to .016". That is the bare minimum top ring gap because anything smaller would cause the ring to quickly seize in its bore.

The old "rule of thumb" recommends a .004" top ring gap per inch of bore. During the 20 to 30 minute break-in the ring gaps will enlarge several thousands of an inch as a tiny bit of material is worn away from the faces of the rings and bores of the cylinders.

"File To Fit" piston ring sets are available for those who want their ring gaps to be the absolute minimum to limit the amount of blow-by but they are a lot more expensive and not always available in every bore size. Plus they require patience, a special jig, and thin file to do the job accurately. As there is very little gained for the time and money involved very few people choose to go that route.
 
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