Piston Ring gapping

Crank, rods, sealing, pistons, block, flywheel etc
Guy Croft
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Piston Ring gapping

Post by Guy Croft » February 19th, 2007, 3:37 pm

When you're fitting piston rings you have GOT to know what the recommended end gaps should be. This may not always be the same as the OE spec. You should check every ring in its bore. If you don't and the engine loses power about 10 minutes after start up there is a good chance the ring ends have 'butted' together and are more-or-less seized onto the piston, ie: not working. If you did not check them, or assumed they were OK because 'someone' said they were 'OK', you've got problems. If you have not written the end gaps down in your engine notebook when you ring me for help - you have not checked them/they have not been checked.

The ring end gaps are measured by aligning them in the bore one at a time and slipping a feeler gauge between them. This in itself is not easy. Rings are very easily damaged/snapped.

Rings should be checked in a bore that has been rebored and honed and cleaned/lightly oiled, with a light chamfer at the top of the bores to prevent damage to the rings when the pistons are fitted. If the crank is fitted you must cover it to prevent debris from dropping on the crankpins.

If the rings look like they will butt together hard in the bores don't force them. Overlap one on the other and mark with a very sharp scribe on one ring the excess material to come off.

If a lot has to come off I start on the grinding wheel (nicely dressed for the job, I add) and then do a first check, then I clamp a fine sharp parallel file in the vice and work the ring up and down for the next bit, then final stoning with a fine small carborundun stone or fine carborundun / aluminium-oxide tape wrapped round the file.

Sometimes you have to adjust the entire ring pack and this should take the best part of 4-5hrs done carefully. Take the material just off one end of the ring only each time you adjust. Keep the ends square to each other. Each time you adjust the ring length you must dress the corners or you will scratch the bore to bits. How much can you grind or file off? Well, I have done 1mm removal, but you need experience to make that judgement, no amount of writing by me is going to make you an expert, so find one and ask him about your engine.

If the rings, as-supplied for your bores, are too loose eg: on a Fiat 8V TC engine of about 84mm bore I'd be looking for 0.014-0.016 thou" end gap - if they are wrong you should not use them, though I have to say that you could get away with top ring say 18-20 thou end gap and take a chance on blowby, won't hurt the power on a turbo unit overmuch, some say it's a good thing (I don't agree) but the breather will be working overtime and they won't last as long because they are already oversize. By 22-25 thou end gap on that motor (in the worn state) they are pretty-well worn out. There is a view among some that turbocharged engines and supercharged ones should have 'big ring gaps' - in my experience this is not true.

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