over size piston rings

[billy192]

I have STD pistons on my R80 and after measuring the rings in the barrel /jug they are over the tolerance. can i put first over size rings on to STD pistons and get the barrels honed to fit new rings?
Billy

[melville]

Yes, you'll be filing the rings to proper spec for end gaps. 1st OS rings will overlap in a standard bore.

[Rob Frankham]

You might get away with it but it certainly isn't good practice.

If you hone (rebore in fact) the barrels to match the rings there will be way too much piston slap. If you just hone the barrels, it is highly likely that you will break the cast iron piston rings on insertion and in any case, they will be to tight in the bore (even if you adjust the ring gap) causing excessive wear. The only real way to deal with an issue like this is to rebore and fit new pistons/rings.

Rob

[billy192]

if i could get oversize pistons i would. but i can only find the kits at £1000. too much for me
Billy

[Kurt in S.A.]

Have you measured just where your cylinders are? In other words, just what was the value of "out of tolerance"? Could be that a new set of standard rings would fit OK since they won't be as worn as the original.

BTW...what year R80 are we talking about? Iron cylinders (pre 1981) or Nikasil (1981-on)? Older post says 1980??

Kurt

[billy192]

hi,
1980 R80/7. i measured 3 places on each jug all three were over the tolerance. i have the numbers in the garage but i think the end gap was 90 bottom 100 mid and 110 near the top (metric).
Billy

[Kurt in S.A.]

Would be nice to know...what does 100 metric mean? 0.100mm?

I had work done on my R69S and the mechanic suggested just using fresh standard size rings. Has been working fine.

Kurt

[barryh]

Isn't it the case that pretty much all new rings can have end gaps that are bigger than BMW's own spec. If that's true new standard size rings may not help.

Using rings the next size up after using a file to bring the gap into spec has been done many times, even as a matter of course by Airhead professional mechanics. But while it must work I would have thought the slightly different curvature would increase friction. On the other side of the debate, slightly wider gaps than spec don't seem to cause any problems so it depends on what the OP means by over size. 0.25" while over spec wouldn't be anything to worry about. Bear in mind that the gap measured on new rings would quickly increase due to initial running in while that wouldn't be the case for the old rings unless the cylinders were honed.

The 100 metric makes no sense to me as BMW's spec for ring gaps is 0.012" to 0.020" which in metric is 0.3mm to 0.5mm

[billy192]

Would be nice to know...what does 100 metric mean? 0.100mm?

I had work done on my R69S and the mechanic suggested just using fresh standard size rings. Has been working fine.

Kurt

sorry yes 0.100mm

[jackonz]

Putting oversize rings in a standard bore is a no no, first thing to remember is that most of the bore wear is in the top third of the bore and you will see a lip at the top of the bore that will prodonently be on the thrust side of the piston.
If fitting std rings in old bores you need to check the gap with the rings as close as you can to the bottom of the bore and gap them accordingly.
You should also check that the ring groves in the pistons are not worn as well, if they are you are wasting your time.

Oversize pistons CW rings and pins available here.

https://www.boxer2valve.com/piston-r75- ... rsize.html