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Old 07-13-2014, 02:18 PM   #1
SickASeeinHarleys OP
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Question Question about new piston ring gaps

Getting close to firing up my 5 year rebuild project and I'm starting to second guess my self. Just neurotic I suppose, but this far into my build, I don't want to screw up.

Anyway, I have a '78 R100S that I swapped the original cylinders out for nikisil (or galnikil?) lined ones. I used the original pistons, but purchased rings from Max BMW that are compatible with the new jugs. The pistons are stock- not oversized and the new jugs have not been bored out either.

The engine is all assembled and ready to go, but I didn't measure the ring gaps before installing the pistons in the cylinders. Guess I made the "assume" mistake, thinking that if they were airhead parts from Max BMW, that I wouldn't need to check them.

What's the consensus? Do I need to pull the cylinders off and check the gaps before running the engine or trust that the gaps are OK?

Has anyone had to file the gaps on new airhead piston rings?

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Old 07-13-2014, 02:29 PM   #2
oldroadie
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I have no experience with BMW rings but I have always checked the gaps on other engines. Using the bottom of the piston as a leveling device I would push them into the top bore an inch and then measure with the appropriate feeler gauge. I used the top because some motors have tapered bores, i dont know if this is true for airheads. I have had more that needed a slight adjustment than did not; rings are pretty fragile so the filing process is a delicate dance.
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Old 07-13-2014, 03:09 PM   #3
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Generally, when you get a set of rings from or for BMW, the gaps will tend to be a bit big. If you have standard cylinders and standard pistons that you are going to reuse, usually buy a set of the next over-size rings and file them to fit (about .006" gap).

If you have a standard size piston and standard size cylinder within the wear tolerances and you put in standard rings, the ring gap is usually .010" or better. As the rings wear in and take on the shape of the cylinder, the end gap gets larger. It's not unusual after 30K miles to find a ring gap of .025" plus and still not be using oil.

Don't worry, be happy!
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Old 07-13-2014, 04:11 PM   #4
jackd
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+1 I re-ringed my GS many years ago (Nikasil jugs) to cure a smoking problem. There was no wear on the jugs and so I ordered standard BMW rings from MAX BMW. I did measure them before I bolted it together and the gap was .025" - from what I remember this was over the allowable limit. I called them up and asked them what was happening here. Rusty assured me not to worry and throw it back together. I did as advised and she seated the rings in just fine - and no smoking was to be had. I'm still scratching my head on this one. I personally wouldn't rip it apart to check - I'd go riding...
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Old 07-13-2014, 04:43 PM   #5
disston
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As Jackd has stated there have been many similar reports.

When this happened on my iron cylinder R90 I was very upset and made the dealer take the parts back. I was going to use Hastings rings or some other aftermarket but a NOS pair of pistons in my size showed up. The new rings on the NOS pistons were checked and they were the correct size.

I may be the last Airhead with properly fitted rings. But all the others report no problems.

I did get a super good deal on the pair of pistons. Paid about what you might pay today for one piston.
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Old 07-13-2014, 05:01 PM   #6
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Some say don't worry, some do. Guess it's kinda like life.

Funny, I don't worry much in general, but I want my ring gaps within spec so I bought first over rings and filed (actually ground) them to fit as my pistons were in spec and I had heard too much about new standard rings being "loose".

Too much old racer in me to let it go I guess.
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Old 07-13-2014, 05:13 PM   #7
disston
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The objection to using the next oversize rings has been that the curvature won't be correct. I have always thought that maybe it would work better than this conservative view.

Chollo9, any idea, how much material do you have to grind off each ring to get them in spec?
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Old 07-13-2014, 06:20 PM   #8
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Quote:
Originally Posted by disston View Post
The objection to using the next oversize rings has been that the curvature won't be correct. I have always thought that maybe it would work better than this conservative view.

Chollo9, any idea, how much material do you have to grind off each ring to get them in spec?
There's the curvature but don't forget the rings' tension. I suspect it's too high but I don't have any over the long hall comparative experience to tell me that. I have installed over sized rings. The difference in tension was very noticeable. Rings are a huge source of power robbing friction. I don't think over sized rings are the answer although I do wish BMW would gap their rings to spec. I have checked a lot of brand new BMW ring gaps and I have never measured one that wasn't out of spec too wide.
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Old 07-13-2014, 09:29 PM   #9
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My experience has been that the gaps need to be checked.
I went to install some R90 rings a few years back, all Std. size and the gap was way over spec.
Bought the next size up and had no ring gap. I ended up using the "zero gap" rings and filed each side simultaneously, with a double sided points file to achieve the exact ring gap. I was amazed how soft the ring material was on the ends....only took a few minutes to "size" each one. I fitted each ring in the cylinder bore to check the gap as I "sized" them. Stopped filing when the gap was centered in the specification range. I pinched the points file in a vice and then held the ring in both hands to file both sides of the ring at the same time...it worked. I performed the exact same exercise on my 1978 R100S engine a couple years ago as I had the same ring gap issues.
mine weren't the Nicisil variety though.
Just my experience, but I only know enough to be dangerous......
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bmwhacker screwed with this post 07-13-2014 at 09:38 PM
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Old 07-14-2014, 04:37 AM   #10
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If I recall, there was a fair amount of overlap when I placed the oversize rings in the bore, so, no gap + some more (technical, eh?). I tried the same method bmwhacker used and decided there had to be a better way. I rigged up the thing below (dremel handpiece and assorted blocks and clamps) and ground them to fit, very slowly with light pressure, switching sides and checking regularly for gap and square. It was fast and accurate.

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Old 07-14-2014, 08:02 AM   #11
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I have only replaced the rings in 3 Airheads, all with Nikisil cylinders. All 3 had new rings that were either very close to being out of spec or slightly slightly wider gap than specified. All ran great, excellent compression and no oil burning.

Chuck
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Old 07-14-2014, 09:04 AM   #12
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Quote:
Originally Posted by chollo9 View Post
If I recall, there was a fair amount of overlap when I placed the oversize rings in the bore, so, no gap + some more (technical, eh?). I tried the same method bmwhacker used and decided there had to be a better way. I rigged up the thing below (dremel handpiece and assorted blocks and clamps) and ground them to fit, very slowly with light pressure, switching sides and checking regularly for gap and square. It was fast and accurate.

Nice jig, that's a great idea brought to realization.
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Old 07-16-2014, 03:04 PM   #13
SickASeeinHarleys OP
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Chollo, that's quite a setup you devised!! Is that on a milling machine table?

After mulling over the responses, I'm going to leave the rings as they are. My main fear was that the gaps would be too small, but from the responses, it sounds like the opposite is true.


Thanks to all for your time & input. Hope to be on the road soon.


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Old 07-16-2014, 03:23 PM   #14
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Thanks, if you never throw anything away, you often find what you need! The table is a cheap X-Y unit that's mounted to my drill press.
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Old 07-16-2014, 06:30 PM   #15
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For years I insisted upon file-fit rings (usually .010' oversize) from my aftermarket supplier, but on a recent R75 build no rings were available in a file-fit. I put the motor together with ring gaps similar to what you'd get in an OEM set. To my mild surprise, this was one of the nicest running R75s I've ever built or ridden.

Ring manufacturers have been telling for just as many years that large ring gaps don't hurt anything and now I concede their point. (I suppose they might know more than me..) Hot rodders and race builders have been for some time deliberately been opening up the second ring gap in the theory that the gap allows the top ring to settle in its groove more quickly.
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