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Thinking about buying a1989 R100RS, have some oil leak questions..


New member
Hello everyone, I’ve been around here a while but this is my first post, hope that’s OK to dive right in…

I’ve had a few oilheads, but have never owned an airhead and would like to change that. I’ve been looking for a monolever R100RS and am about to make an offer on a white 1989 that seems to be in pretty good shape, ODO at just over 44K miles. It’s only been ridden a few hundred miles per year over the last several years, but the owner shared a log of what has been done (starter, fork seals, steering bearings, etc). The bike is not nearby so I’ve not seen it in person, but have gotten images and a few videos.

The main concern I have is the amount of oil coating the oil pan, wondering if it’s indicative of something really bad or if it’s just been neglected, sitting too long and just needs some attention. The owner is saying that it just needs an oil pan gasket, no oil coming from behind the front engine cover.

From the blackened color of the front of the pan, I would guess that it does at least need a new gasket. But I also see that the oil cooler lines and adapter are oily and completely black as well. I’ve attached two images that I pulled from the video he shared.

So my question is: does this look like a straightforward pan gasket leak that has discolored the pan and the oil cooler parts? Or something more?

I know it’s difficult/impossible to diagnose with just a few photos, but thought I’d try to see if this is no big deal.



I have a garden spray bottle (holds about a liter) and I put about 85% kerosene and 15% marvel mystery oil in this bottle. The MMO is mostly to keep the pump lubricated and I found it lengthens the life of the sprayer (and the MMO smells better). I do this outside, usually with an old piece of card board beneath the bike. I give the lower pan, area beneath the cylinders, and other external engine / transmission / final drive area a nice spray with the kero / MMO mixture. I use an old potato brush to scrub the external surface, then a water spray rinse. I might repeat with another spray bottle containing a 10:1 mixture of Simple Green and water (one part SG and 10ish parts water, sweeten to taste :) ). Another rinse.

I like to work on clean bikes. For my airheads I have found it quite useful to replace the pan gasket and valve cover gaskets with reusable silicone gaskets. I have found that these silicone gaskets greatly reduce weepage and other small oil leaks. It is possible that you might find one or more of the oil pan screws to be stripped. I gather that this is fairly common, although I have been fortunate in not having this on my prior airheads. Follow the cleaning and torque instructions that accompany the silicone gaskets.

I use to get the silicone gaskets from Rocky Point Cycle, but the owner retired and the new owner may have closed up shop.

A place called RealGaskets has these silicone gaskets for BMW airhead bikes. Link follows -

One other thing that I'll add, on bikes over 25 or so years I have found it necessary to replace all rubber bits. This includes seals and gaskets. If such hasn't been done on your airhead, perhaps it would also benefit from a "rubber refresh".
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Welcome to the forum! Certainly, the first place to consider are those pan bolts. They need to be checked every so often. There's a torque value but if you put a 10mm socket on a nut-driver handle, you'll know if the nuts turn a bit...plus you won't over torque the bolts that way. A second source of oily fluid could be the rear main seal or could be the input seal on the transmission. If the transmission, then the oil will have a sulphur smell as it will be gear oil. If you look at the space that is directly underneath the transmission, you'll see a bit of a shelf. Leaks out of the rear main seal will collect on this shelf. Thirdly, oil can leak from the oil pressure sending until that is below the dipstick on the left side of the engine. However, if the right side of the oil pan is oily, hard to see that it could come from the leak on the left side, so the oily mess would be coming from somewhere else.

And you've mentioned the oil cooler so that is another source.
Welcome to the forum!
I’m guessing the MMO is Marvel Mystery Oil?
It is a twenty five year old bike so some seepage is likely.
For that, I like a preliminary wipe down with a gasoline soaked rag. The next step would be using some Gunk Foaming Engine Cleaner-
Cardboard under the bike is a good idea.
Rinse with hot water. Probably take a couple of tries.
Good luck.
I really can't say for sure if you have a large problem or a small one from the pictures. Regardless, there could be a seeping oil pan. The oil cooler line seals could be leaking. The list of possibles is not terribly long.

I will go out on a limb and say IF you have good mechanical skills nothing that is seeping would be a major terrible chore to fix.

The hardest item to replace would be the rear main seal behind the flywheel and I really don't see any sign it is leaking. That would require a lot of tear down to get to as well as the tool to seat the new seal in it's proper place.

I don't know anything about the current owner, maybe he will be good enough to check the oil pan bolts and then give a good cleaning.

LOL, personally it would not stall a sale if it were me buying the bike. Good luck, St.
Thanks for the comments, I appreciate the input. What seemed odd to me is that the oil cooler pipes almost seem painted black. I was wondering if they were originally black, but pretty hard to determine from only a few images.

He did share an image that shows the area under the left cylinder and it appears fairly clean to me.

If I buy it, I plan to fly and ride it home, so I will ask him to clean it up and check to see if any bolts are loose.

Be aware that the 10 mm oil pan bolts are *very* easy to overtorque, stripping out the threads in the bottom of the crankcase. The usual fix is a Helicoil or Timesert, which if not done correctly can result in a nagging leak. I'd ask whether any of the oil pan bolt threads have had to be repaired in the past. That said, the oil grunge pattern is not consistent with only one or two thread repairs, so likely not a problem.

In addition to the potential sources identified above, leaks can original from the four pushrod tube seals as the seals harden over time. Oil can also come from around the cylinder base gaskets (the latter is fairly rare).

Another possibility is leakage from the gasket at the base of the oil cooler thermostat cover over the oil filter (front lower right corner of the crankcase).

At the rear of the engine, in addition to the crank rear main and tranny input seals noted above, the oil pump cover (located vertically below the rear main seal) can also leak with age.

Oil pan gasket is the easiest first-step. But from what can be seen of the two sump pictures, it appears that most of the leakage is at the rear, which means removing the tranny and clutch/flywheel to get access to inspect the three primary seals in this region. Don't be put off by this, unless you do not have basic wrenching skills. There are a couple special tools that help with the process, but it isn't rocket science and the process is well documented across the Internet.

One last note: be aware that oil can wick its way *forward* along a sump, potentially misleading one to conclude there is a front-region leak. In the first picture, there does not appear to be any leakage from the front cover above the sump, so no indication of crank seal or camshaft seal leakage. But I'd want to see pictures of the right side of the engine to see whether there is potentially a leak from the thermostat housing, right-side push rod tube seals, or the right side of the oil pan gasket that explains the apparently greater amount of grunge on the forward right underside of the oil pan.

FWIW, if the rest of the bike is sound and you're not dependent on a dealer having to do all the work ($$$), what I see there is nothing that would scare me away from the bike.
Mark “mcampy”, Welcome to the Airhead World !

In Mark’s post above, he pretty much nailed the areas to look at; happy that he mentioned the Push Rod Shroud Tubes Seals :)

Regarding your question about the Oil Cooler Lines: in the mid-70 builds, the steel fittings were cadmium plated; there are health issues not only with the cadmium plating, waste from the process but, handling of cadmium plated parts. BMWAG and the Euro communities banned cadmium plating so… BMW went to black paint on the steel fittings

The fourteen(14) bolts securing the Oil Sump are: M6-1.0 thread; which require a Metric 10 socket/wrench to service. I use the minimum book torque value, Blue LOCTITE and a sequential cross-pattern while torquing the pan to the crankcase.

As you’ve been around “Beemers” before, the importance of using a Torque Wrench on fasteners… cannot be understated ! The air-cooled Airheads are even more so ! Torque by “feel or grunt” will come back to bite you and… be expensive !
… thinking further; fifty some years working on these Airheads, the items I find most often are: fasteners too tight, lack of anti-seize on critical areas, irregular greasing.
Regarding the oil situation, I totally get your concern. It's like playing detective with those pics and descriptions. The blackened pan and oily bits do make you wonder what's going on under the hood.

I'd say it's wise to be cautious. Maybe consider getting a second opinion from a mechanic if possible. Sometimes a pro's eye can spot things we might miss, you know?