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Join Date: Aug 2012
Location: Philadelphia Area
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Piston ring end gap dimensions
Hello,
I bought a "rebuilt" 79 930 motor with it's stock Mahle pistons and new rings installed. This motor hasn't been run. I've taken the motor apart, it will have mild upgrades: SC cams, K27/7200, Tial, 36mm intake porting/CIS I want to be sure the new piston rings are gapped correctly and I'm not finding any discussions here on this topic. I know there are rule of thumb charts provided by piston manufacturers but I was hoping to compare those to factory dimensions for the 3,3L if anyone has them and what gapping formula people here may have used for their lightly modified engines. Matt |
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09-10-2020, 03:24 AM
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Join Date: Sep 2015
Location: Costa Rica and Pennsylvania U.S.
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ring gap
If it has new Goetze rings the end gap on the top ring should not be less than 0.15mm or .006 thousandths.Being the cylinders were cleaned up a bit it is probably a little more.Just check the bores for ovality.Fred
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09-10-2020, 04:01 AM
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Quote:
I'm getting .02" top and second ring .025" for the oil ring .006" seems tight?    It looks like my cylinders may be out of spec. Last edited by Fixer; 09-10-2020 at 06:58 AM.. |
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09-10-2020, 04:45 AM
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It looks as if they are well within the wear limit.
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09-10-2020, 07:46 AM
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The ring end gap table guide is ok.But you have an aircooled motor and that is different than a water motor.Factory minimum for 3.3 Turbo is 0.15mm.
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09-10-2020, 08:40 AM
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Quote:
This is specifically for 3,3 liter 930. I'm well below the wear limit. The cylinders are round, spec is to be within .04mm or .0015" measuring in each direction.    The other table is for JE forged aluminum pistons for Porsche but they have thinner rings. Last edited by Fixer; 09-11-2020 at 05:06 PM.. |
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09-10-2020, 03:10 PM
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LN can replate my cylinders but it's a 6-8 week wait. I assume and asked if I went that route would my install ring gap be around .2mm
If not I'd rather deglaze them carefully and run them. Tomorrow I'm going to put a set of rings and a cylinder in the oven at 250F take them out and measure gap warm. Then at 300F. Aluminum expands twice the length of iron or steel at a given temp but there is and explosion and friction at play on the poston and rings also. I'd better read. I'm guessing the rings are much hotter than the cylinder. Matt  So it may be better to leave the cylinder at room temperature and heat up a compression ring to +200F over room temperature (275F)and check the gap. It'll likely close up .15mm 
Last edited by Fixer; 09-11-2020 at 06:58 PM.. |
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09-11-2020, 05:05 PM
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