Oversize Piston Rings

[swall]

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".

 

[franzh]

Looks like its "file to fit" time. When you get that done, put the ring in the bore and make sure it fits concentrically, that there is contact around the full circumference. OS rings are formed for a larger bore and when pressed into a smaller bore may not fit as well. You may also encounter higher ring friction. You could also put the ring in the bore and overlap the ends to check for concentric sealing, before filing.

Locate another ring supplier?
Franz

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[swall]

Thanks, franzh. That is exactly what I suspected. I have several catalogs from other parts suppliers and would rather spend an hour on the phone tracking down the correct rings than spend 8 hours filing.

 

[evelrod]

Swall, I won't give a "how to" here as if you don't already know how to do this stuff you need to get some expert help. Too tight ring gaps, too much ring tension can and will radically shorten the life of the engine, often catastrophically! Many speed shops in CA have hand cranked ring "filers" that are pretty cheap and some of them have the good electrically operated to rent out. A lot of engine builders like to set their own gaps. I personally have never found that to be a great help.
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

 

[tbuelna]

swall,

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

 

[PFM]

Swall,

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

 

[swall]

Thanks for all of the help guys. I do have another parts supplier to work with for the correct rings.The cylinders are cast iron and this is an 80 cu. in. flathead engine.HP is around 40 @5400 rpm. It is a motorcycle restoration project rather than a race job.

 

[bmunse]

I'm new to this group but I am an old engine builder.
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.

 

[TOHCan]

The typical oversize file-fit ring that I've used was .005" on top of the overbore size, or .035" for .030" over. If you were to use the larger size mentioned in your motorcycle restoration, I'm fairly certain you'd have problems with overheating from the excess friction. Don't forget to lightly "stone" the freshly filed ring ends each time to break the edge before sliding them in to check the end gap or you'll scratch those nice shiny bores.