Thread: What do you engine guys see here?

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Every set of pistons I ever used when checked would be around +/- .002" varience in diameter. For example a set of 4" pistons lets say +.030 over would actually measure in a set of eight from 4.029" to 4.031". Sometimes the varience in diameter was more. Too much more and we would order other pistons and send back the ones too far out of spec. Basically make up our own set of pistons within .002" tolerance. This is why we used to bore the block to fit the pistons and file fit the rings to get the correct gap.

Heres a chart from Sealed Power / Speed Pro giving reccomended end gaps per application.

RING END GAP RECOMMENDATION GUIDE (FOR 4.000 INCH BORE)
Speed Pro Top Rings
(ductile iron, 4" bore) 4.000" Example Minimum Gap Factor
Moderate Performance .016 - .018 (.004 per inch of bore diameter)
Drag Racing, Oval Track .018 - .020 (.0045 per inch of bore diameter)
Nitrous Oxide - Street .020 - .022 (.005 per inch of bore diameter)
Nitrous Oxide - Drag .028 - .030 (.007 per inch of bore diameter)
Supercharged .024 - .026 (.006 per inch of bore diameter)

Speed Pro 2nd Rings
(cast Iron, 4" bore) 4.000" Example Minimum Gap Factor
Moderate Performance .020 - .022 (.005 per inch of bore diameter)
Drag Racing, Oval Track .022 - .024 (.0055 per inch of bore diameter)
Nitrous Oxide - Street .024 - .026 (.006 per inch of bore diameter)
Nitrous Oxide - Drag .028 - .030 (.007 per inch of bore diameter)
Supercharged .024 - .026 (.006 per inch of bore diameter)

Notice: Most of the second ring gap recommendations are larger than the top rings. Recent testing has proven that a
larger second gap increases the top ring's ability to seal combustion. This larger "escape" path prevents inter-ring
pressure from building up and lifting the top ring off the piston allowing combustion to get by. Many engine builders
have reported lower blow-by and horsepower gains at the upper RPM ranges with wider second ring gaps. Also,
almost every new car made is using this inter-ring pressure reduction method to lower blow-by and emissions and to
increase engine output