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Old 01-17-2016, 05:48 PM   #1 (permalink)
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Question help.. piston oversize question

i am rebuilding an 86 samurai. (my first rebuild) i purchased flat top standard pistons. when checking the ring gap it turns out the gap was .032" not what i was expecting ( .008 - .013).

does this mean the block was bored over at one time and i need over sized pistons? or just over sized rings or both.

i'm at a loss here. this is not a daily driver just a toy to drive occasionally.
the compression was extremely low across all of the pistons. thats what prompted the rebuild.

any way, am i good with standard pistons here ( which i already have on the rods now) or do i need to rebuy oversized pistons and rings.


thanks for any help here.
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Old 01-17-2016, 07:05 PM   #2 (permalink)
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You need to first determine the bore size. It may have been bored oversize already OR is simply worn beyond what your current pistons / rings will accommodate.

If found THAT significantly worn, then a re-bore and the appropriate oversize pistons / rings will be required, if not already at the maximum bore from previous machine work.
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Old 01-17-2016, 07:31 PM   #3 (permalink)
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Rule #1 on ANY rebuild is to MEASURE before purchasing - this is how you know if you can "re-ring" or you need to bore, and how much oversize you need to bore, the same goes for main & rod bearings - can you just replace with standard bearings, or do you need to grind & go undersize.

From what you're saying, you're seeing a ring gap, at least 0.024" greater than you expect, that would correspond with a bore size approximately 0.008" greater that expected, and that would suggest a worn standard bore (first oversize is 0.5mm or approx. 0.020").

Have the block measured by a competent machinist and either have him supply pistons & rings and bore the block to suit, or, order pistons based on his recommendations and after you receive the pistons, have the block bored to suit.
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Old 01-17-2016, 07:56 PM   #4 (permalink)
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Thanks for the replies, lesson learned. i'll have a machinist check out the block and go from there.

thanks again for the help and advise.
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