Oversize Piston Rings
Oversize Piston Rings
(OP)
I'm rebuilding a v-twin engine with a 3.25" nominal bore. I had the cylinders bored .030" over and bought mating pistons. But they were supplied with rings marked ".040 oversize". I called the parts supplier and was told .040" o/s rings are what they supply for .030" o/s pistons.This doesn't seem right to me, so I thought I'd ask the experts here(this is my first engine re-build). BTW,ring gap is to be .010"-020".





RE: Oversize Piston Rings
Locate another ring supplier?
Franz
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RE: Oversize Piston Rings
RE: Oversize Piston Rings
My engines run from 73.5mm to 83.5mm bores in the same range as yourself. I use an absolute minimum ring gap of 0.004" per inch of bore. I actually have a block that is at 3.5" and I set the ring gap at 0.015" for both the JE pistons (1mm-1.5mm-2mm ring set) and the Venolia pistons (1/16"-1/16"-4mm ring set) even though the piston clearances are somewhat different (JE @ .003" and Venolia @ .005").
Clearances are for a water cooled engine, air cooled will differ significantly. I don't do aircooled so you will need input from some other members on that.
As Franz says---Locate a ring supplier that will do what you want and not give you what he has on hand. I use several suppliers but Deves has always been right on for me.
Rod
RE: Oversize Piston Rings
3.290 in. rings are not correct for an 3.280 in. bore. You should find another ring supplier. Or you should rework your cylinder bores to 3.280 in. with sleeving, Nikasil, etc.
The quick way to check for ring-to-bore fit is to square the ring into the bore and look for light coming through the gap between the ring and bore. A freshly bored cylinder with new rings should have less than a "10% light gap" about the circumference.
Of course the best way to check ring fit is a "leak down" check, but that requires an assembled engine.
Good Luck.
Terry
RE: Oversize Piston Rings
I have seen this done with OK results and terrible results, I stay far away from terrible results. I run air cooled Porsche stuff, and follow the engine spec for ring gap. What is the cylinder bore material? The hone finish is also inportant for ring seal for the specific ring and bore material. Is this a long term rebuild or a race deal, it can change the ring. As Evelrod said find a vendor, for my money I go to Total Seal these guys realy go the extra mile.
PFM
RE: Oversize Piston Rings
RE: Oversize Piston Rings
Every engine I ever built for performance or intended extreme longevity I purchased oversize rings and ground to fit. Every engine spec book that I can remember will show ring end gap for that particular engine. evelrod is right on with the "larger gap for air cooled" statement, just consult the manufacturers spec. and you won't go wrong. Cast iron rings are much softer than chrome plated rings so I would set it to the tight end of the spec given.(moly is a cast iron ring with a moly filling and soft too) Very importantly, don't forget to grind the oil ring wipers if a three piece oil ring was supplied.
I use a valve stem grinder and a small homemade fixture. But you are only grinding 4 compression and 4 oil ring wipers. I suggest that you can get comfortable in a chair in front of a bench grinder with a dense wheel and use the side of the wheel. .....Or you can just buy some .030 rings and that will work too.
RE: Oversize Piston Rings