Bought a '75 R90S in April (odometer read 20k miles) and decided to check and address the wearing parts. Retained stock pistons and cylinders (see info below), honed cylinder bores with spring-loaded 3 stone hone for cross hatch finish and installed new OEM rings. All engine seals replaced, new clutch, new transmission inlet seal, Heads were cleaned, valve seats lapped, valve guide clearances were not excessive. Engine was broken in using varying throttle and engine braking and has gone 800 miles since.
After rides, I notice oil had been leaking on the right side where the starter cover and intake cover meet.
The crankcase vent has been upgraded to the reed valve type, the reed opening oriented to the 11 o'clock position. The hose that leads to the right carb inlet is new. Opened the air inlet housing expecting to see a puddle under the hose clamp, I did not.More details:
Plugs show evidence of excess oiling...not dripping oil but black and some carbon buildup one one side of electrode.Piston OD (at skirt, 90 deg. from pin)
Left 3.5405" Right 3.5405"Cylinder ID
Left 3.545" Right 3.545"Ring gaps...rings purchased from dealer. Noticed that the top and mid gaps were out of spec but decided to take my chances. Mistake?
Left top .022" Right top .022" Spec .012" - .018"
Left mid .024" Right mid .024" Spec .012" - .018"
Left oil .013" Right oil .014" Spec .010" - .016"Looking for opinions on cause/source of oil leak.
Ring gaps...rings purchased from dealer. Noticed that the top and mid gaps were out of spec but decided to take my chances. Mistake?
Left top .022" Right top .022" Spec .012" - .018"
Left mid .024" Right mid .024" Spec .012" - .018"
Left oil .013" Right oil .014" Spec .010" - .016"Looking for opinions on cause/source of oil leak.
Every set of rings I've purchased on the last 15 years has had about the same ring gaps as you've measured. None of them gave me any trouble. I worry a LOT more about not enough ring gap, not a few thousandths too much.
800 miles isn't nearly enough to break in the rings, and my guess is that as the miles accumulate, the oil in the airbox will greatly reduce or disappear altogether. I didn't see it mentioned in your comment, but you should be accelerating hard for brief periods of time during break in to force the rings against the cylinder walls. A spirited ride on a twisty road is a good way to achieve that.
Thank you Scot. I did ride it as you suggested, paying attention to varying loads and RPM. I'm thinking the cylinder honing didn't provide adequate cross hatch texture, but I'll give it a little more time to see if it improves.
R90S Oil leaking from crankcase vent area
If it were leaking from the area of the vent, then oil would be dribbling out of the small hole in the side of the block, just below the "W" in the BMW engine logo. You can see the hole in your photo, and it's clean.
1. My advice is and would have been to leave the engine alone. An Airhead with only 20K miles is not fully broken in. How could it possibly have any "burned up" parts ?
2. You say your plugs are black from burning oil, but you don't give any details on the plugs, mixture, timing, fuel, plug reading process, etc. Lots of stuff contribute to black plugs and I'm just wondering how you reached this conclusion with apparently no supporting data.
3. Lots more info missing on type of oil being run and the hows and whys of your riding. Not trying to discourage you in any way, but there's a lot more to this than simply changing the rings and checking the plugs.
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
R90S Oil leaking from crankcase vent areaIf it were leaking from the area of the vent, then oil would be dribbling out of the small hole in the side of the block, just below the "W" in the BMW engine logo. You can see the hole in your photo, and it's clean.
1. My advice is and would have been to leave the engine alone. An Airhead with only 20K miles is not fully broken in. How could it possibly have any "burned up" parts ?
2. You say your plugs are black from burning oil, but you don't give any details on the plugs, mixture, timing, fuel, plug reading process, etc. Lots of stuff contribute to black plugs and I'm just wondering how you reached this conclusion with apparently no supporting data.
3. Lots more info missing on type of oil being run and the hows and whys of your riding. Not trying to discourage you in any way, but there's a lot more to this than simply changing the rings and checking the plugs.
Hope this helps.
Richard, thanks for the reply. The small hole you refer to has indeed been dry, but the location pointed by the wooden dowel has been wet with fresh oil after each ride.
Response to your points:
1. The bike had no documented history and the previous owner only had it for a couple years, didn't ride it much. He bought it from a man who owned the Fresno BMW dealership years ago, who claimed the mileage was original, but the bike originated in Chicago area. Was it really 20k or ??? I wasn't convinced. Pulled transmission to check the splines and grease them. While I was at it, removed the clutch and although it was within spec there was some discoloration so I sent it to Southland and had it redone. Decided to replace the rear main seal, so pulled the flywheel after blocking the crank with a strap across the front (mistake, not a firm blocking). At some point in changing the seal, the crank moved and the forward thrust washer came off its pins inside the crankcase.
Pulled both cylinders to access the thrust washer and was able to reposition it on the pins (very lucky indeed) without pulling the crank. That's why the cylinders were removed, but while I was in there I decided to install new rings and hone the cylinder. And yes, the crank was properly blocked until the flywheel was installed.
2. Plug reading process...just my best guess, don't claim to be an expert. Have been through the carbs to replace o-rings and gaskets, synched using vacuum gauge. New points and condenser, points gap set at .016", timing set with a strobe light.
3. Castrol GTX 20W50 Dino oil. How do I ride? Typical ride is between 30 and 100 miles, some hills and curves, shifting from between 3000 and 5000 depending on my mood. First rides after putting it back together was a lot of on throttle, off throttle, hills, etc.
Why did I post this? Hoped someone would share their experience with a similar leak, or say my ring gaps were wrong, or my honing job was wrong, or ???? Maybe advise me to put more miles on it...too soon to be worried...like Scot said. If no one responded, I was OK with that too. Really like the bike and enjoy riding it. This problem will be solved, I was just hoping to gain some knowledge from others on this site.
OK. Gotcha.
• The reference to the hole was in response to your title, not the body of the message. That oil could be from the air filter or flywheel seal. But, the breather could have deposited excess oil within the air box. New air filter, I hope.
• The GTX is not a good oil for the Airhead motor. It needs to be API rated SF or SG or SH. GTX is probably SM or SN. But it won't affect the break-in of the rings.
• When you strobe timed it, was it at high speed to achieve Full Advance using the "F" mark ?
• Still no plug numbers. Most of the new plug charts are listing colder plugs than optimal. Your problem could simply be 1) the wrong temp plug, or 2) not doing "plug chops". I'd like you to take your plug wrench with you on your next ride. After a long stretch (at least 3 miles) of constant mid-range throttle, hit the "kill" button and coast with the clutch disengaged to the side of the road. Then look at your plug.
• The way you ride and are breaking the rings in sounds fine.
All the best.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
OK, so I've been thinking about the breather and whether it is truly seated or ??? so I pulled the intake and starter covers to take another look. Initially no smoking gun...no standing oil, etc. Then I pulled the breather cover and noticed the gasket was wet, indicating it wasn't being compressed.
The cover is aluminum casting, and the inside diameter of the mating surface for the gasket isn't round...it was contacting the edge of the screw head for the reed valve. The gasket appeared to be fully seated, but it lacked just enough to leak.
Used the Dremel to remove some aluminum, problem solved. Will test ride tomorrow, but I expect this will work.
There are actually 2 "breather covers", both rather small and held in place by the same 4mm screws. Since these covers never really come off, I usually seal them to the engine without a gasket, using only LocTite brand "gasket maker" 510 or 515. But what you did was an excellent fix.
Later this winter, when the riding season slows down, you'll want to revisit the top engine cover area. This time to re-pack the starter motor bearings. The OEM grease inside the starter motor bearings has hardened with age and the starter motor is really working against itself. The bearings themselves are OK, it's the aging grease inside the bearings that may burn up your $300-$400 starter motor.
All the best.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
3. Castrol GTX 20W50 Dino oil
I strongly recommend you reconsider your choice of oil. Castrol GTX in 20/50 is a good weight choice, but it falls far short in the amount of ZDDP needed to adequately protect the cam and followers in the engine. I believe you'll find a very thorough discussion of oil in the tech pages, written by Bob (Snowbum) Fleischer. Spectro 20/50 is one highly regarded choice, I've had good luck with Mobile 1 15/50 synthetic, but recently have started using Lucas oil products as they are widely available at most automotive parts stores and provide more than the recommended levels of ZDDP.
3. Castrol GTX 20W50 Dino oil
I strongly recommend you reconsider your choice of oil. Castrol GTX in 20/50 is a good weight choice, but it falls far short in the amount of ZDDP needed to adequately protect the cam and followers in the engine. I believe you'll find a very thorough discussion of oil in the tech pages, written by Bob (Snowbum) Fleischer. Spectro 20/50 is one highly regarded choice, I've had good luck with Mobile 1 15/50 synthetic, but recently have started using Lucas oil products as they are widely available at most automotive parts stores and provide more than the recommended levels of ZDDP.
Appreciate the heads up Scot. Coincidentally I changed my oil yesterday to the Castrol GTX Classic 20W50, which has 1400 ppm zinc. I started using GTX in '73 when I bought my Norton 850, wasn't aware that over time the zinc content had decreased.
@ Yes, Castol has an entire line of oils for 1980's vehicles, but they are slow sellers and not available at every oil dealer. And to add insult to injury, Castol may be changing the name of that product. Over the last 25 years I've noted about 3 of their products designed for older motors, but have never seen them all in one place at the same time.
@ Another product that is very easy to find is Valvoline VR1 20/50 Racing Oil. By the quart it is fairly expensive, but their big jug is fairly reasonable and seems to be carried by 2 or 3 of the large auto parts store chains.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
Local Walmart has the Classic in quarts for $4.68. Amazon has 6-quart packs for $28.
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