R90/6 Piston Ring Thickness Specs

I am in the middle a top end and cylinder base overhaul of my R90/6. While looking up specs to check for piston ring wear limits (as in gap between the rings and the ring groove in the piston) I have come up with a bit of a surprise. I have several sources which I have crossed checked for the specs of ring thickness and piston groove width and they all agree. However my pistons and the corresponding rings are all narrower. I can find no reference to this online. My pistons do appear to be stock and OEM with the standard stamping on the crown showing the diameter (89.975), the marking VORN (Front) and the arrow showing facing forward.
Were the pistons made with narrower grooves at one time? My machine type number is 2476. Can anyone help me with this situation as I'm not presently sure how to order the correct ring kits for my machine.
I do see three different ring sets available on a BMW microfiche but since there are no details, I assume these for different bore over sizes. Maybe one of them is for the shallow ring set but there are no details and I thought I would check with the community first.
Thank you.

Dimensions in question are shown below. Hope it retains its formatting. Not too sure how to do a table in a post!:

....................Ring Thickness widely published.........Actual Ring Thickness........Groove in piston actual
Top................1.75 mm (0.068")........................1.47 (0.058)........................1.63 (0.064)
Middle............2.00 (0.078)...............................1.72 ((0.068).......................1.76 (0.069)
Oil Control.......4.00 (0.156)...............................3.38 (0.133)........................3.42 (0.135)

Can't help much. But I looked at my Haynes, and all of the ring thicknesses listed are larger than you show for a /6. It certainly doesn't help your situation.

20774 said:

Can't help much. But I looked at my Haynes, and all of the ring thicknesses listed are larger than you show for a /6. It certainly doesn't help your situation.

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Thanks. I had looked at Haynes, Clymer, a copy of an original BMW service manual and some other specs that someone had published they all read the same but different than mine.

Never seen much published on piston grooves. Just for grins, I took one of my OEM /7 pistons that were replaced after a top end a while back. Groove thickness measures:

top -- 1.88mm
middle -- 2.05
bottom -- 3.98

Not sure if the /7 grooves would be different than a /6.

20774 said:

Never seen much published on piston grooves. Just for grins, I took one of my OEM /7 pistons that were replaced after a top end a while back. Groove thickness measures:

top -- 1.88mm
middle -- 2.05
bottom -- 3.98

Not sure if the /7 grooves would be different than a /6.

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Seem to be very close, within the limits of possible measurement error, to the published figures that I mentioned above, and therefore larger than the dimensions that I have recorded on my own piston and rings.

Rebore?

Are you going to rebore the cylinders? If so at that point, I think you would end up with new pistons one size larger and the rings to match. I ask this because if the bike has a lot of miles on it or is sucking oil, the cylinder may not always be cylindrical. It's dimensions may be out of spec as well, despite not having any visible damage. So putting new rings in may not be the best idea.

Of course I very well could be reading your post incorrectly and if so I apologize. I hate to see you put things back together only to have to take them apart again later. St.

Steven, thank you for raising that question. Nothing like a good question to put things in perspective. I will admit it was never my intention to do a rebore and this all started through wanting to put a fresh set of pushrod tube seals in. However, now that I am at this stage and you have raised the question, of course whatever rings I have now become a moot point if I do a rebore. The problem is I do not have an internal micrometer and there’s a little bit of not wanting to know what I will find. Everything looks really good, no serious signs of blow by. Prior to pulling the machine apart, there was no visible oil burning in terms of smoke and it’s difficult to say if any significant amount was being burned because of the pushrod tube oil leaks. Yes, there is a significant amount of wear on the top ring in terms of thickness. I guess I could make up a rod that is almost the same diameter as the narrowest part of the bore and measure for ovality with feeler gauges. I could also take the cylinders to a machine shop and get them to do the measurements for me. I will of course also measure the ring end gap and make sure that it is not insanely wider than the spec. I do agree that putting new rings in on a cylinder that has not been deglazed could actually result in a drop in ring performance.
Thank you.

On my /7 top end, I needed to measure the cylinder specs and I had a bore gage to work with. I wasn't confident in my readings so took it to a machine shop. They were able give the numbers which indicated it was out of spec. Rebores on 1000cc jugs is problematic so I opted for an aftermarket kit as replacement w/ BMW parts was too expensive.

Yes

I laud the effort to take things to a machine shop for measure. A good shop will have the proper mics to measure and decipher your specs. LOL, I had a neighbor years ago who had a machine shop in his basement and he had the slickest set of three way micrometers I had ever seen. My last rebuild job, he measured, told me "rebore" and I then found a shop who knew about BMW airhead cylinders. Sadly he passed away form cancer. Now, my local airhead shop measured things for the other two rebuilds, they don't have the slick mics Matt did but they do okay anyway.

I don't recall, how many miles on the bike? Good luck, St.

STEVENRANKIN said:

I laud the effort to take things to a machine shop for measure. A good shop will have the proper mics to measure…
I don't recall, how many miles on the bike? Good luck, St.

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The machine has about 96,000 miles on it. I have had it since 1979! Only been to Coast to Coast once otherwise pretty minor trips. I do have a very high-quality digital caliper, Mitutoyo and a dial gauge which is not good enough quality. Problem is that having moved from the big city five years ago, there are very few machine shops within a reasonable distance but I am looking. From all my preliminary measurements there seems to be very little bore wear or ovality, but I can only measure to 0.1 mm (0.004”).
I will say that the rings I have really worn, though I cannot get any firm specs on them since I still don’t know where these pistons with shallow ring grooves came from, whether it was just a temporary production change or whatever. From all the casting marks and top stampings they appear to be genuine OEM.
I’ve measured the ring gap at 40 thou for the top two rings and 70 for the oil control ring.
I will get the bore properly measured if I can find someone to do it but it’s frustrating because I do believe that a set of rings would be a really good idea, just don’t know where I can get them. I do think that a rebore unless proven absolutely necessary is an expensive way to resolve something that was not the primary reason for me tearing the machine apart. (Pushrod seal leaks). Thanks for all your comments though, much appreciated even though I am trying to put this motor back together with a minimum fuss and minimum expense.

Maybe

Well, maybe you can cut cost by buying just a new set of pistons and rings if the bores are good? One of the better BMW dealers like Max or Rubber Chicken Racing, Ted porter, Boxer two valve, should be able to work with you to get the rings and matching pistons. Just a thought. St.

Thanks Steven for your comment, which I have only just read. In the meantime, I have acquired an adequate quality bore gauge and determined that the cylinders are not badly worn and certainly OK in terms of ovality, so I'm thinking new rings. However, having heard nothing from anyone on this forum or at BMW Canada that can shed light on why I have totally different rings than mentioned in any of the specs that I have seen, I certainly cannot easily buy replacements, unless I deal with a ring manufacturer directly and somehow find a match. Therefore, your suggestion of new pistons and rings seems to be a good corrective measure. It's just the cost that makes me hesitate. According to prices that I have seen, the total would approach $1000 CDN. Since the machine will unlikely be used for high further mileage by me, I'm tempted to put it back together, as is, and see what happens. If I have significant oil burning, I'll get new pistons and rings per your suggestion.

Thanks

Thanks for the follow up, I wish I could be more help. Sadly in my first rebuild I was faced with the dilemma of replacing Nikesail coated cylinders. BMW at the time was the ONLY place to get replacement cylinders and they wanted nearly $1K apiece for just the cylinders. Add to that the cost of new pistons and rings and well, things were not looking good. I was able to find a machine shop that was willing to bore out the niasail, and press in cast iron sleeves. My local airhead shop found after market pistons and rings of a decent quality and price. So, that is what I did. That combination has been good for over 100K miles.

Now of course, there are the new conversion kits to bump an R80 To 1000cc, and they are nikasail lined bores to boot and a reasonable cost. Next time I have to rebuild, I will purchase one of these kits, and install. So far, I have heard good things about the quality. I have not gotten a straight answer if the bump from 800cc to 1000cc made a big difference but, the overall cost is still cheaper than BMW official parts.

Please understand, I have nothing against BMW parts but if I can get same quality for less money, bump the perforce up a bit I will go for it. I wish that these kits were available back when I needed to my rebuild.

I don't know how much of a purist you are, I know some people who insist on only BMW parts, that is fine for them. Cast iron bores can be bored out and there are companies making pistons and ring combos to fit of equal quality to BMW sourced parts.

I will try to get you the make of the pistons and rings I installed, give me a few days as I have to touch base with my local shop. St.

brewer said:

While looking up specs to check for piston ring wear limits (as in gap between the rings and the ring groove in the piston) I have come up with a bit of a surprise. I have several sources which I have crossed checked for the specs of ring thickness and piston groove width and they all agree. However my pistons and the corresponding rings are all narrower.

Click to expand...

You're of course making it difficult for yourself looking for non BMW dealer/BMW specialist parts.

You have learned also that the aftermarket isn't so trustworthy.

Working with BMW specialists, even BMW dealers seems most feasible.

I suppose an aftermarket piston manufacturer could have done whatever it wanted. I'd think there is next to zero probability any American manufacturer ever cared.

No problems

I got the job done right at half the cost of BMW I now have over 140k on the rebuild. The bike is running strong, uses 0 oil. So no, I don't believe I made it difficult for myself and yes, I would do the same thing again if I have to.

I will agree there are bad aftermarket parts out there, always have been however, a good reputable shop will not sell junk.

Those of us who still have airheads and want to keep them running should be thankful for people like Herr Siebenrock and others who have brought some NLA parts back onto the market, or made lower cost and in my experience just a good quality parts to the market. These are not fly by night Ebay guys who can sell junk and disappear, they are in the business for the long haul. No I have no sympathy for BMW's market and price strategy. They quit selling bikes I am interested in back in 84. Not to mention they put a good dealership out of business with their nonsense. If I can get good replacement parts from a small company for a good price I chose the little guy over BMW.

So yeah, if it takes in my case to find out at 145K miles on this rebuild with non BMW parts I need to have the jugs pistons and rings replaced all I can say is this set of NON BMW parts lasted far longer than the so called wonder cylinders BMW put on the bike at the factory they lasted only 90K miles and yes, I do regular oil and filter changes. I would have been happy back then to use official BMW parts but as I have said time and again when this topic comes up, the cost was nearly three times higher. Okay, I could have made a mistake and had the engine blow up at 100K miles with the non BMW parts but it didn't.

The next time I need to rebuild my R80RT, it will have the 1000cc jugs installed with a set of 40mm intake heads and 40mm carbs, to almost match the performance of my RS. I will at that time buy a set of the nikasal kits available from a OM company, I sure won't installing BMW parts. I think by the time this happens, and something goes wrong I will most likely be done riding and will be sittin on a river bank fishing I hope and pray. St.

A Cautionary Tale!

As the author of this thread, I am so embarrassed by my most recent finding that I could have just remained silent. However, I think the following account could have value as a cautionary tale.
I am a licenced mechanical engineer and a lifetime motorcyclist. I think of myself as a reasonably good troubleshooter, but I really blew it here! This is an extreme example of tunnel vision. My problem is that I have on several occasions in my life uncovered unusual situations, like the time that I found out that an importer of car headlights had been selling UK spec bulbs for over five years in Canada without knowing about it. Low beam was into oncoming drivers.
For those of you who might not have read the thread in detail, I was surprised to find out that I had piston rings and their accompanying grooves that were much narrower than spec. Narrower by a lot, so new standard rings could surely not fit. I am familiar with "running changes" being made on production lines for various reasons, so thought this might be my situation, but found it strange that there was no record anywhere (I searched everywhere). I posted to this forum, I contacted BMW Motorrad Canada (brushed me off), the Service Manager of my local BMW Motorrad dealer (very helpful but had not heard of any such changes in 1974), two other expert local BMW mechanics, etc. Everything turned up blank. After spotting some damage to my right side piston and bore, I resigned myself to a rebore which would, as Steven Franklin had pointed out above, solve my issue as I would be fitting new pistons.
Off I go to a machine shop that specializes in BMW Airheads two hours away. While there the owner, who is aware of my original concerns, shows me a couple of new pistons that he has in stock. To demonstrate the fact that my grooves were narrower, we offered them up to one of my pistons and they fit, perfectly. You can imagine how I felt.
Within hours back at home I had completed tests that resulted in the chart below.
My Mitutoyo digital caliper, one of the early ones, see below, was reading low by 15%, pretty consistently in the 0-5 mm range. The error was large enough that I believed it to be a true measurement. I never checked it against my vernier caliper because in my mind, why would I? That was the main mistake. I had a top of the line digital that was consistent but apparently inaccurate. As soon as I suspected the inevitable, a quick check with the $20 vernier verified the inaccuracy. I didn't have a micrometer in my tool box (I do now). I do think that the Mitutoyo has been somehow damaged in my ownership, rather than just failed, but I'm not looking for the reason and obviously not using it any more.
Therefore, if anyone is still thinking about my original predicament, please stop now.


HOWEVER, there remains a new mystery. Everywhere I look, the diameter of the rocker shaft is quoted as 14.5 mm. How come mine (verified by both mechanical caliper and mechanical micrometer) are 15.5? I discovered this while picking up other objects randomly to produce the chart attached. My machine is a 1974 R90/6.
I don't blame anyone for being cynical about my measurement technique and condition of tools, but I did verify this.

Surprise

I am frequently surprised at times by things I think I know about, am doing well, or what not.

I am very happy you situation turned out so well for you without having to spend a lot of money and effort.

LOL, thanks for sharing your story despite embarrassment. St.

It takes a Manly Man to admit a goof in a public forum ... Take comfort in knowing that you're not the only one who can make a mistake. Thanks for the heads-up and resolution.

I have a digital caliper, an analog ("manual") caliper, and a 1" micrometer sitting next to each other in one drawer of the main toolbox... If I get a weird number, it's good to be able to verify it on the spot (tho none of them have been "officially calibrated" for a long long time).

Now... is it possible the rocker shaft either picked up material from the rocker arm, or maybe is out of round...?

Good thinking there, got to check all possibilities, but if it had picked up a millimetre I’d be looking for material approaching the thickness of needle rollers attached to the rocker arm shaft and they look clean as a whistle and in good shape. I checked all four and they are all 15.5 mm. I cannot believe there would be a typo in the specifications which has propagated through to so many pieces of reference material. Another mystery but I sure am going to be careful how far I take it without double checking all my observations!!
The diameter of the rocker shafts is not really something that would be a determinant of their replacement so it probably hasn’t been noticed. Generally speaking I think one would look at them and if there is any scoring or pitting, that would be what would dictate their replacement.