What About Those Oil Slingers?

Sun, 03/25/2012 - 13:43 -- darryl


The changeover to the /5 series of motorcycles in 1970 was a watershed for BMW. Not since the introduction of the R5 in 1936 had they so completely redesigned nearly every aspect of the bikes in their lineup. The entirely new motor for 1970 had plain bearings oiled by a high pressure system that also pushed that oil through a modern pleated paper filter. All previous motors had used roller bearings for the crank- and camshafts, low pressure oiling systems, and some form of filtering that is generally refered to as a slinger.

In a slinger oiling system, the oil is pumped into the main bearings at either end of the crankshaft, and is then captured on the inner side of the bearing by the slingers, which centrifuge the oil and then direct it into the hollow crankpins. From there, the oil leaks out through laterally drilled holes, oiling the connecting rod big end roller bearings and being flung around the inside of the crankcase, which serves to oil the connecting rod small ends, the piston skirts, the camshaft and the valvetrain.

On the bikes through the R12/R17, these slingers are simply grooves machined into the outside of the crankwebs. From the R5 on, they are separate pieces made from sheet metal and screwed onto the crankweb.

In those long gone days, it was much more difficult to rack up huge mileages on a vehicle. BMW seems to have considered the slingers to be sufficient for the expected lifetime of the vehicle, or at least until there was reason to remove and service the crankshaft.

Why Must the Crank Be Removed to Inspect the Slingers?

Everyone asks this question — all the time. Surely there must be some magic way to clean them without tearing the whole enigne apart, or at least to look at them so that you know whether you need to tear the whole engine apart. What kind of design is this, anyway? BMWs are supposed to be easy to maintain!

What is a slinger?

This is a slinger. On the twins, there are two of them, one behind each of the main bearings.

Oil is pumped through the main bearings and exits onto the face of the slinger. Because it's mounted on the crankshaft, it's spinning at high speed, and the oil is flung into the cupped outer lip. Any particulates in the oil are compressed into the lip and held there. The constant spinning of the crank presses this mass together and squeezes out all of the oil that was carrying it.

In the photo above, you can see two holes in the face of the slinger, one is for the screw that fixes the slinger to the crank web (this is a 500cc R51/3, there are two of these on 600cc engines). The other is bigger, and open, and lines up over where the crank pin is located. Below, you can see how these holes match to the crankweb.

As the cleaned oil fills up the lip area, it falls into the hole. The crank pin is hollow, and also is drilled laterally. The oil therefore leaks out of the pin and lubricates the conrod big end bearing. Without this lubrication, the bearing would score the pin and sieze. Worse things — much worse — would then follow directly. Actually, if you're lucky, as the oil flow slows to nothing, it will wear the conrod bearing and crank pin and you'll start to hear all the loosening parts banging together on each power stroke. You still need a crank repair in this case.

Can I see the slingers without getting this deep?

No. To get to the point you see above, you have to remove the timing chest cover, the timing gears, and the front main bearing holder.


Getting to the rear slinger is more work; you have to remove the crank and pull the rear main bearing off of it.


My thumb is resting on the rear slinger, and against the rear main bearing.

Although you can see the outside of the lip of the slingers through the cylinder holes, that's not of much use. And because the main bearings fit closely into slingers, and the slingers face the inside of the case, you can't use a borescope or similar tool to snake around and get a glimpse.  You can't look in from the top, there aren't any holes in the crankcase casting there. And you can't see in from the bottom, because there's usually a perforated windage plate down there. But even if it's missing, you'd still get this same kind of view.

Do I really have to check these things?

Well, you don't have to do anything. But if the holes in the crank pins fill up, your bike will suddenly stop running, after making some expensive noises. The problem is, there's no outward indication that this is happening. One day things are fine, and the next, you need a new crank. There aren't any new cranks anymore. So you need a specialist who can take one apart and put it back together, successfully.

The picture on the left is a clean slinger. The the others show a slinger out of an R60/2, that was right on the edge of problems. (The lip is crushed because this is a rear slinger and to get the rear main bearing off, it's easiest to use tire irons against the slinger to push the bearing.) Bearings and slingers are still available, and not really very expensive. Since you're in this far, consider replacing the bearings.


Look at the third picture, the one taken from the back. See how the crud is migrating into the hole? Its next destinations are the small feed holes in the crank pin. This slinger came out of a motor with 63,000 miles on it.

So, if you don't know what condition the slingers are in, you're taking a very big chance. Changing the oil frequently may put off the day somewhat, but the usual guidance is that they need cleaning every 40,000-60,000 miles. Which was a heckuva long time and distance for a motorcycle to travel when these bikes were new. Eisenhower proposed the Interstate system in the 1950s; nearly every road then was what we'd call a backroad today.

Are you sure that there's no magic solvent that will clean them out without disassembly?

The short answer is YES — I'm quite sure. When you have one of these in your hands and you go at it with a srewdriver or a pick, you'll find that the crud is packed in that lip solidly. It's like dried clay, and comes out in chunks.

Consider now where the slingers are. If any of this crud came loose while the motor was assembled, there are only two places it can go. Into the main bearing that covers the open face, or into the hollow pin. You don't want either of those things to happen. Even if there were a solvent that could loosen up the crud, do you really want that stuff to run through your bearings?

Don't believe the odometer!

At this late date, odds are that the odometer on your baby does not function, or at least has spent some time when it did not function. You are the nth owner, and the whole chain is no longer known. If the odometer did work the whole time, how many times has it flipped over to 00000 again?

Most of these pictures are of my '53 R51/3. When I got it, allegedly only the third owner of the bike, the odometer read 36,000+ kms. But when I opened up the engine, it was on its 2nd rebore and needed a third (as well as a valve job). Probably, this bike has 236,000 kms (143k miles) on it, if the odometer was running the whole time.

(And now that I've had the odometer restored, it's back to 00000. I wonder what the 4th owner will think of that? ;-)

So, bite the bullet and do the job. Then you will know what condition things are in and you won't have any sleepless nights worrying about it.