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By Kevin W - 12 Years Ago
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New to the forum and seeking advice. I have a 55 Ranch Wagon project car. After removing the motor and checking, bearings within tolerance, cylinders look good and heads also good. New standard bearings and rings, hone and reassemble. Set timing, confirm spark and fuel but no start. I discovered low compression. 60psi all cyl. Disassemble and found ring gap of .025 all cylinders. Now the questions. The installed rings were old stock Continental standard size. I am reading that newer rings may be larger for better fit and may require filing. I believe the ring gap to be the problem because I could see slight blow by at the oil cap during cranking. I am looking at new rings so new standard size or oversize? I believe the correct ring gap is .008-.020, so is there a formula to figure the correct ring size?
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By Ted - 12 Years Ago
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The rule of thumb for piston ring gaps is 0.004" per inch of bore for the top rings and 0.003" per inch of bore for the second rings. But your 0.025" gaps should not pose an issue as that's the standard minimum gap for hyperutechtic pistons and other applications where nitrous or boosting is involved. At 60 psi cranking pressure I would first verify the pressure gauge being used for the cylinder testing is accurate. Next on the list to check would be the camshaft phasing as if the timing set is one tooth off (in the retarded direction) then the cranking compression will suffer. A quick check for cam timing is to get the #1 and/or #6 cylinder where the two valves are equally depressed (level) at the overlap cycle and check that the damper TDC is within 5° of being at TDC mark. This all assumes the damper is correct and hasn't slipped. If the camshaft was degreed in during the rebuild process, then you should be comfortable that camshaft phasing is not the issue.
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