Ring gap and groove

[pkboxer]

2 questions regarding pistons/rings.

Just gathering information before deicding what to do....

1 - my top and oil ring have a gap, larger than spec, of about .7mm. I know that is large, but the middle ring is about 1mm lager than that????

Would someone do that on purpose? I'm reading in my manual that it should be about the same as the others....


2 - I've read that the lands (is that the right word for the groove in the piston that the ring sits in?) can wear, particularly the top one... If that is the case, all else being well, could I have the groove machined wider and then source a wider ring to fit??

Thanks

[vanzen]

Height of groove is spec'd according to the groove in question.
End gap is spec'd according to the ring in question.
Clearance in groove is spec'd according to the specific ring / groove ...

The BMW Service Manual lists the specs.

e.g. for my 81 R100RS:
1st groove,
height, mm – 1.75 -0.010, -0.022
end gap, mm - 0.40...0.65
clearance in groove, mm – 0.06 ... 0.09
2nd groove,
height, mm – 2.0 _0.010, +0.022
end gap, mm - 0.40 ... 0.65
clearance, mm – 0.04 ...0.07
3rd groove,
height, mm – 4.0 +0.010, +o.022
end gap, mm – 0.25 ... 0.40
clearance, mm – 0.03 ...0.06

I do not have a /6 manual handy ...
check / build to the factory specifications listed for your machine.

Machining grooves and new rings to fit ? Yes – can be done.

[pkboxer]

Thanks Vanzen, my extra large end gap on the 2nd ring (somewhere around 1.5mm) got me to think someone set it like that intentially for some unknown reason. (Both sides about the same).

The man formerly known as Fun Holiday is on Advrider and has warned of worn piston groves (usually the 1st grove) that make a piston trash. First thought I had was could someone machine the groove from 1/75mm to 2.0mm and then source an appropriate ring....

A bodge? Yes? Better than a ring in a worn groove? That was my question...

[Duane Ausherman]

Inspect the piston very carefully for excessive wear.

In general, ignore FH.

Yes, you can machine the groove to fit a ring, but why? The correct ones are available.

That gap is really too large. The general rule is .003" for each inch of piston diameter. That means around .010" to .012" and you have .060" That is 4-6 times too large.

[pkboxer]

Oh sorry, I was using mm, not inches.

So I'm out of spec but my 2nd ring had to be set totally wrong it's 2x larger than the other too.

Unless that ring is prone to excessive wear.

If a grove was worn to larger than spec, I thought it could be machined even larger to match a "thicker" ring.

Not planning on this, just asking...

[Steve in Golden]

The man formerly known as Fun Holiday is on Advrider...

Not anymore he ain't AFAIK. "SuperShaft" has been banned from advrider. They are prone to banning people at advrider apparently...

[melville]

The man formerly known as Fun Holiday is on Advrider...

Not anymore he ain't AFAIK. "SuperShaft" has been banned from advrider. They are prone to banning people at advrider apparently...

'Twas just a tempa-ban. He's mellower lately.

For OP: Possibly appropriate top rings, assuming you have an R90, could be taken from a set for a 1679cc VW 411. Should be 90mm bore, but I can't remember if they are 2.0mm top ring. Most VW are, though.

Note that top rings and second rings usually have different profiles, so it's probably not as simple as using extra second rings in an enlarged top groove.

And oh, the things we'll do to avoid buying $300 (EACH!) pistons. I'm used to getting FOUR pistons AND the barrels for that much, shopping for VW stuff.

[mattcfish]

And oh, the things we'll do to avoid buying $300 (EACH!) pistons. I'm used to getting FOUR pistons AND the barrels for that much, shopping for VW stuff.

Unless you buy them from Gowesty. [img=http://img593.imageshack.us/img593/5382/bent1.th.jpg]

[Roy Gavin]

Some of the VW rings which are the correct bore are thicker than Airhead rings, so sourcing them shouldnt be a problem.

Try a googe on Gates Rings and see what comes up.

A long time ago it was possible to buy replacement ring sets for pistons with worn grooves which included spacers.
The ring grooves were machined square to the new size and the spacer, which looked like one part of a 3 part oil ring, fitted on top of the replacement stock width ring.
Not certain why they did this instead of just supplying a wider ring, but back in those days rings were generally wider than they are now, and the may have already been as wide as advisable.

[Major Softie]

Try a googe on Gates Rings and see what comes up.

Oh, man, are we not even saying the whole word now? Why, I remember, back in the day, when we used to say the whole word. We'd say "Google," and we were happy to!

Damned modern generation...