1975 R90/6 Cylinders & Pistons

I have a 1975 R90/6 with about 35,000 original miles. Recently, I lost compression (about 1/2) in the right cylinder and it started burning oil. The left cylinder is within compression spec and no problems. When I took compression readings on the right cylinder after adding some oil, the compression momentarily increased to within spec. I concluded that meant worn or broken piston ring(s). I tore both cylinders down, expecting to find a broken ring or worse and everything looks okay, albeit the pistons are pretty carboned up and the valves are pretty dirty but nothing atrocious. Cylinder walls appear smooth with no scoring. I mic'd the offending cylinder and it averages 89.97mm (6 readings; 3 90 deg apart). The piston skirt averages 89.75mm, yielding a 0.05mm piston to bore clearance. If I'm reading the specs correctly, that's about 0.005mm over spec. Other than possibly worn valve guides if I misinterpreted my compression test, is the data I captured enough to cause oil burning that seemed to develop over night? My inclination is to buy matched cylinders, pistons and rings and not try to hone the old cylinders and get rings to fit. Is that the way to go and, if so, can anyone give me a source for new jugs, pistons and matching rings?

Rev1: After Kurt's reply, I rechecked my cylinder bore measurements. I had used a Harbor Freight digital caliper to measure my bore gauge and had trouble with that caliper giving variable readings a while back while rebuilding a VW Rabbit diesel engine. This time I measured more carefully and used a mechanical dial caliper for my readings. Interestingly, the right (problem) cylinder checked out to Kurt's and Clymer's specs. The average of 6 measurements top, bottom & middle of the cylinder and again 90 degrees from the other 3 measured an average of 90.009, within 0.001mm of the Grade B cylinder bore spec. The right piston crown was not marked A, B or C per Clymer's but was stamped 89.986, 0.006mm above the Grade C spec listed in Clymer's. I'm not sure what the number stamped on the piston crown means because the piston skirt measures an average of 89.878. I then measured the left cylinder - the cylinder that had compression in spec and was not giving me any problem - and I was surprised that the average bore diameter using the same measuring tools was 89.986, 0.014mm below the Grade A spec and the left piston was also stamped 89.986 but the skirt averaged 89.776. Anyway, these measurements are more accurate and I still don't know whether to buy standard rings or not and have yet to perform the valve leak test suggested by "copbike."

Welcome to the forum! I might have tried to do a leak down test after the compression test just to be sure. You said you added oil and the compression came up. Did you measure the ring gaps on the bad cylinder? I'm a bit confused about the cylinder dimension. 89.97 is actually smaller than spec....my Haynes said it's 90, 90.01, or 90.02 mm.

I'm wondering what a new set of rings might do. Kind of low mileage on the engine. Perhaps a change to newer valves and seats is warranted as well.

I did a top end on my /7 which ended being out of spec after 100K miles. I checked what new OEM cylinder, pistons, and rings would cost and I couldn't stomach that. I ended up going with a Siebenrock kit which includes all aluminum Nikasil-type cylinders, pistons, rings, and wrist pins. All parts were lighter than original and being aluminum, heat dissipation is better than the iron sleeved cylinders.

Note that your engine case opening should be 97mm...in 1976, BMW opened it to 99mm. So, whatever you end up getting, be sure that it is compatible.

Kurt, Thanks. You're quite right - I should have done a leak-down test. I used a digital caliper that could be off a hair, so I will recheck my measurements with a micrometer. I too was surprised that it was burning oil at 35K because it had been running fine, albeit it sits a lot, compounded because we don't have year around riding weather like San Antonio. I will take new readings and update my post. Assuming my readings turn out to be in spec in the vicinity of 90mm+, does that mean honing the cylinders, cleaning up the pistons and valves and standard rings might do the trick?

Compression loss

I have 2,800 air cooled engines under my belt (VW and Porsche) and I would suspect the cylinder heads are the problem. You can do a quick test of how the valves are seated with a little gasoline... Pour a thimblefull into the port and hold it up and see if any gas seeps by the seated valve head. If there is any at all then the valve is not seated correctly. You can do a quick 'valve job' by removing the keepers and spring and then lapping the valve to the seat with a hand suction cup lapper and lapping compound, and it only costs a few bucks. Before doing this though, you should try to wobble the valve back and forth in the guide... if you can detect any side to side movement of the valve stem in the guide, then the guide is bad. Bad guides or even bad valve stem seals can cause oil loss and contribute to the symptoms that you describe (the oil leaks from inside the valve cover, down the valve stem to the cylinder). The rings and pistons sound like they are fine, and it would be a shame to replace all of that and still have the same heads/valves. I would suggest that you do the gas test, check the guides, lap the valves and install new rings only. I would bet that will make it right. If the guides are bad, or even suspect, they can be replaced easily by a professional. On all the engines I rebuilt, I automatically drilled out the guts of each exhaust guide with a special drill bit that left about 1mm of the outside of the guide, then easily pounded out the guide (pneumatically). Then I would use an air hammer with a drift on the end to insert a new guide. A quick ream and the new valve would be perfect. I had a sailboat that was laid up in London for 6 months, and when we got back to the boat I suspected that the valves had some blow-by... especially when some of the valves are open for long periods of time in such an environment (on a 4 cylinder Perkins diesel). I pulled the head and did the gas test... and the valves leaked terribly. So, a quick valve lapping and everything was right again, and we were on our way.

Very good suggestions on the gas test of the valves. Your experience matches my suspicion based on prior experience and visually inspecting the valves and heads. I will do the gas test but the rings and cylinder bores look too good to be the problem. No wasted effort anyway because the pistons definitely could use a good cleaning and the valves sure need it. I have both a hand lapping tool as well as a reciprocating valve lapping tool so lapping the valves will be no problem. If the valve guides are suspect, I'll follow your advice. I bought a VW Rabbit new in 1977 and within 20K miles, I had new oil lying in the otherwise dry filter reservoir. I found out that VW used inferior valve guides so I pulled off the head and had bronze valve guides and new seals installed at a local machine shop and that solved the problem. I didn't see what I would consider excessive oil in the valve cover of the errant BMW cylinder but when I ran the bike after the problem developed, oil would drip out of the exhaust on that side. Now that I see what appears to be the relatively good condition of the rings with no broken rings, I'm more convinced it is valves and/or valve guides. Thanks for your help.

Darn valve guides

On air cooled VW and the early Porsche engines (up through the 356 and 912), the primary cause of engine failure is exhaust valve guides. Because of the high operating temperatures, the exhaust guides wear excessively, the valve starts to wobble, and then breaks catastrophically. This is particularly bad on the #3 cylinder because of the proximity of the oil cooler reducing the flow of cooling air. I have seen thousands of engines torn down and that #3 valve was invariably the reason for the first engine failure (if proper oil level was maintained). I don't know how much the guide problem affects BMWs (or Moto Guzzis), but I actually have never seen it on a motorcycle, personally. Probably because of the much better cooling on a bike, I suspect it is rare. But the guides are bound to wear some, and this manifests itself in symptoms of higher oil consumption. As for the rings, they should be replaced with new if the cylinder is removed (and, it is cheap enough). Allow me another story? Went to Alaska (in 1974, on my honeymoon) in my 40HP 1960 camper bus and at milepost 285 I spotted a derelict VW bus along the road, with the forlorn engine lying in the mud. On the way back we swung in and I grabbed the engine. Near Sacramento, in 100 degree weather the engine dropped a valve. At a rest area I spent 2 days removing the engine, taking the head and pistons/cylinders from one side of the derelict engine and putting them on the bad side of my engine. Both had failed at the #3 cylinder. Installed the engine and once again we were on our way (at no cost!). The only thing I was lacking was fresh oil, and someone at the rest area gave me enough new oil, not to mention sodas and water to drink. Now, VW transmission trouble while traveling is a whole other story... don't get me started

Iron Bores

wallymortar said:

I have a 1975 R90/6 with about 35,000 original miles. Recently, I lost compression (about 1/2) in the right cylinder and it started burning oil. The left cylinder is within compression spec and no problems. When I took compression readings on the right cylinder after adding some oil, the compression momentarily increased to within spec. I concluded that meant worn or broken piston ring(s). I tore both cylinders down, expecting to find a broken ring or worse and everything looks okay, albeit the pistons are pretty carboned up and the valves are pretty dirty but nothing atrocious. Cylinder walls appear smooth with no scoring. I mic'd the offending cylinder and it averages 89.97mm (6 readings; 3 90 deg apart). The piston skirt averages 89.75mm, yielding a 0.05mm piston to bore clearance. If I'm reading the specs correctly, that's about 0.005mm over spec. Other than possibly worn valve guides if I misinterpreted my compression test, is the data I captured enough to cause oil burning that seemed to develop over night? My inclination is to buy matched cylinders, pistons and rings and not try to hone the old cylinders and get rings to fit. Is that the way to go and, if so, can anyone give me a source for new jugs, pistons and matching rings?[/QUOTE

Those iron bores R90 and R100 had a hard time with the oil control rings. Luftmeiter used to sell a two part or three part oil ring that cut down on oil consumption. I think it was a Ford part.

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1975 R90/6 Cylinders & Pistons - Rev1

Thanks again for the additional info and great story. I had a 1975 VW "tintop" Westfalia camper buss with the 1.8L boxer engine. It was reliable and the engine never gave me any problems except the shutter door pivot pin wore out in the air control box, letting the door to fall loosely into the air control chamber so it wouldn't start or run. I rechecked piston and cylinder bore measurements and posted revisions in my original thread. I think my pistons and cylinders are okay and are within standard spec but I'd like a confirmation so I can follow up your recommendation and replace the rings with the proper size. Tomorrow I'll turn my attention to testing the valve seats and guides per your suggestion but I'm beginning to think valves are the problem based on the good info I've gotten from you and Kurt and my visual inspection of the cylinder bores, rings and pistons.

When I was working on my R100/7 top end, I tried to measure the cylinders and their ovality. I used something called a bore gauge. But I felt I didn't have the experience to use it and was getting variable readings. I took the cylinders to a local machine/engine shop and asked them to do the measurements. I trusted their readings and as it turned out, the bores were out of spec. Since going 1st over on 1000cc bores is problematic, I opted for the aftermarket solution.

Rings... a funny part

Rings have often been a problem in engines. But in your case, I suspect that they were fine. In the history of engine manufacturing there have been countless times where the rings were at fault for problems that would arise. I have a 1926 Buick that says in the owners manual the amount of oil to add with each gas fillup... and it was in the gallons per every 1000 miles or so. The problem was the poor rings... but manufacturing gradually got better and quality continues to improve, even to this day. That is why I would never use a set of original oem rings in any engine. Go with a modern set of premium rings and you cannot go wrong. Ring manufacturers have got it right by now. Valves are an altogether different animal. Take a look at this valve (OEM Pratt & Whitney 1820, out of a DC-3 Dakota) that I got at a fly-in for $1.00. I couldn't pass it up. What a beast!... and it is an intake valve. Lee Davis

Rings!

I would forget the bore readings that you have gotten. Just replace the rings with a standard set of a premium modern brand. The chance that a 30-40,000 mile pair of cylinders have worn beyond specs is virtually nonexistent. You are looking in the wrong place. As I mentioned before, it is almost certain to be a valve problem.

Parting out BMWs

A few years ago I got on a kick for buying old Moto Guzzis and BMWs and parting them out. The first was a 1982 BMW that I got for $900. It was a POS, but I made out on the parts. I bought 5 more 1980-84 BMWs on Craigslist and parted them all out. Know what the problem was that made the engines fail? On 5 of the 6 it was a blown head gasket. The engine was essentially a premium engine with just the addition of a new head gasket. On the 6th, I could not identify a problem. As for the Moto Guzzis, I could not bring myself to tear them down, so currently I have 7 Guzzis to restore (along with a BMW /5). I sure love European airheads.