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Topic-icon Rings at 30K? R100RS

  • jchussey
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2 weeks 1 day ago #4140 by jchussey
Rings at 30K? R100RS was created by jchussey
Hi All:

I just bought a 1984 R100RS with 30k. It runs well, seems to have nice power and has a decent service history, but has leaking push rod tube seals,. I'm about to replace those and looking for advice on whether I should replace the rings while I'm in there. Would it likely be an unnecessary step on this fairly low mile engine?

Many Thanks,

Jim Hussey

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2 weeks 23 hours ago #4142 by Wobbly
Replied by Wobbly on topic Rings at 30K? R100RS
My thoughts (which aren't worth more than you paid for them)...

• Typically, the rings on a BMW are just getting going good at that low mileage. However, that does not prove they are perfect, just that they Should Be in good shape. The only real reason rings might be bad is rust in the bore. But, I believe your model came with a Nikasil bore, making rust fairly close to impossible.

• The best test to perform to get you the most/best insight is a compression test. Make sure all 4 rocker arms have free-play at the adjustment point. Pull both carbs loose from the intake hoses. With both spark plugs OUT, but free to spark against the cylinder heads, let the electric starter run the compression gauge as high as it can go. Typically that will take 3-4 cycles to get 90%, and it may then continue to climb a bit for the next 5-7 beats. You're not looking for a specific compression (except that it should be over 100psi), but one might expect 130psi or higher. What you're really looking for is nearly the same compression on both cylinders. If you've got matched compression, then the engine will run and perform smoothly... which is really what a street rider wants.


Rings run about $55 a set which will add another $100 to your job.New rings will need to be matched to the cylinder diameter and "gaped" accordingly. Placing new o-rings and gaskets on the top end is completely within the competence realm of the owner-rider home mechanic. Gaping new rings is one step higher on the technical totem pole.

Hope this helps.

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2 weeks 15 hours ago #4147 by jchussey
Replied by jchussey on topic Rings at 30K? R100RS
Much appreciated, Wobbly.

Would a ring set from BMW that is matched to the piston not already have the correct gap?

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1 week 6 days ago #4148 by Wobbly
Replied by Wobbly on topic Rings at 30K? R100RS

jchussey wrote: Would a ring set from BMW that is matched to the piston not already have the correct gap?


Rings are matched to fit the cylinder, not the piston. How would the factory know how large your cylinder has grown in 30K miles ??

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1 week 5 days ago #4154 by jchussey
Replied by jchussey on topic Rings at 30K? R100RS
Thanks Wobbly. It didn't occur to me that my cylinders ("B" Nikasil) could be out of factory spec at 30k. I have had the cylinders off of my '91 RT with double that mileage and they still looked pristine with perfect cross hatching.

It makes it further confusing that there is only one piston ring part number listed for all R100 models from 1981 to 1995 (Nikasil), and it is listed in the piston , not the cylinder section.

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1 week 5 days ago #4155 by 8166
Replied by 8166 on topic Rings at 30K? R100RS
Did you check compression before engine disassembly and compare it to factory spec? That would tell you if you need new rings or if there were valve issues. As you mentioned, the cylinders wear very slowly, and at 30K, it's very likely that the rings don't need changed.

As far as new rings go, I've replaced two sets so far, and checked ring end gaps each time. All were larger than factory max, but I understand from reading several gurus that it is common and does not hurt performance.

Also keep in mind that you should measure piston groove dimensions to ensure that they are also within factory spec. I would be surprised to see one fail, though, as dimensions on mine were just fine at 140,000 miles. I did, however, choose to replace the rings at that time.

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