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BMW Motorcycles:

The Slash 2  (/2) Oil Cleaning Slingers

Copyright 2023, R. Fleischer

There is very little specific Slash 2 or /2 information on this website. There is a listing of the rear drive ratios in  The main reason there is so little specific /2 (and other even earlier BMW bikes) information is that my huge three ring binder of notes 'disappeared', and it was so many years since I worked on these bikes, that I did not want to furnish possibly faulty memory information.  However, there is a lot of information in the carburetor, electrical's, and other areas of the website that are applicable.

Slingers, background information:

The /5, slash 5, also known as the Airhead, began production in late 1969 with the 1970 models, and the new /5 era motorcycles were very different in many respects.  After WWII, in general, the street bike models had the camshaft located higher up, and lubrication was not the best, but adequate.  The location of the pushrod tubes is immediately recognizable as a pre-1970 twin cylinder motorcycle, as they are located above the cylinders.  The many changes included the front forks, the main frame, etc.

For the purpose of this article, I will use Slash 2, or /2, to include all those pre-1970 engines that had no removable and replaceable oil filter element, but had 'slingers' on the crankshaft, and this includes the boxer twins as well as the single cylinder bikes such as the R26 and R27, etc.  BMW made all of these various engines for a long time.  The information generally applies to other motorcycles besides those actually identified as /2; that is, generally, but not exclusively, all BMW motorcycles built before 1970.    BMW also built race engines, and these were often very different in layout, so they are not included here.

Some basics on the pre-Airhead engines:

There was no oil filter such as a paper pleated filter, as there is on the 1970+ Airheads.   On the /2, the oil from the oil pump was 'filtered' by sending it into closely separated plates or plate and backing, that were part of the crankshaft assembly.  One plate is called a slinger, where the harsher of the contaminants would be deposited into a hard layer along the edge of what was actually a shallow cup.  The capacity of the cavities was limited.   If the slingers were not cleaned at reasonable intervals (you will not think the mileage reasonable! ...30,000 miles!), the slingers could fill-up, clog the oil passageway, & some very expensive engine parts would be ruined. The material the slingers gathered/filtered, by centrifugal force, tight spacing, & deposited in themselves ...was rather hard.  These deposits were not removed by adding detergent additives to the oil or by the detergent additives modern engine oils come with.  Oils were not as good back then either, and, in truth, the deposit build-ups did, somewhat, depend modestly upon the type of oil used.  Modern SG oils work well with the old bikes, but the deposits will still build up as things wear, and the slingers/cup will fill up with enough miles.

The material deposited in the slingers was somewhat magnetic, and consisted mostly of worn engine metals and carbon.

The over-all condition of the engine, including such as blowby from wear in such as valve guides and piston rings; how the bike was driven; under what conditions & what type of oil; ...all had effects on how fast the slingers filled-up.

In order to clean the slingers, you must disassemble the engine!  There is NO OTHER WAY.   If you failed to clean the slingers (sometimes you had to replace them, they could be damaged upon removal, depending on where they were in the engine version), your engine WILL eventually fill up the slingers and that causes oil starvation and WILL RUIN OR GREATLY DAMAGE THE ENGINE, INCLUDING THE CRANKSHAFT.   This creates a very expensive repair.

Before getting into slinger cleaning, I will comment here on something that is not discussed ...or, not like I will here.  My comments in this section may be controversial.   I want to make it CLEAR that this section is completely my own opinion.

If you have a /5 or later Airhead, you have a pleated-papter oil filter that you remove and replace at every oil change; or, every other oil change.  The implication from discussions about the /2 era 'no filter, but has slingers and they fill up eventually', is that the /5 and later oil filters catch and remove the stuff that would otherwise be deposited in slingers.

That is misleading.  It is true that the purpose of the /5 and later filter is to catch small or larger wear, etc., particles.  But, the filtering medium must allow oil to flow.  Thus, the filtering medium, a special type of porous paper (and, rightly so) can not filter out particles that are smaller than a certain size.  Thus, many of the particles that are trapped in a /2 slinger, are not going to be filtered-out by the /5+ era filter.  Another way of saying all this is that the /5+ filter is not as efficient at filtering as we, or the engine design engineers, would like.  In fact, to make it much more efficient is totally impractical.   So, your conclusion perhaps might be that the /5+ era engines would wear faster than the /2 era engines.    The truth is much more complicated.

The /5+ era engines have much better breather systems.  The oil passageways are different.  The oil pressure system is different.  The oil temperatures in various areas are different.  There are MANY other differences, including engine design that has much reduced metallic deposits.   The engine oils that became widely available and in nearly 100% universal usage, have much more sophisticated and efficient additives.   The final change...and a very big change... was when Tetra Ethyl Lead (TEL) was removed from all road gasoline's and TEL and associated oil wearing components had caused a very large percentage of the deposits seen in the pre-/5 era.

Engine blowby (which greatly contaminates the oil) was reduced by more sophisticated and better designs and machining.  Blowby contains combustion components, including burned oil (carbon), etc.   The Airhead engine had an oiling route changed that was done for other reasons primarily, but it did help, IMO, with deposits; and perhaps the reed breather helped some ....but, the final and BIG change was the change to Nikasil/Galnikal coated cylinders, which greatly reduced blowby.   There are a number of interesting things done at the heads (valve gear), that affect deposits, but they are too nerdy to get into here.

The over-all effect is a very considerable reduction in various particles that are developed.........and......what is developed tends, strongly, to be of extremely small size, is not much trapped by the filter of the /5+ era, and remains in suspension (due to the detergents and other chemicals in the oil), dissolved, etc., in the engine oil, and is thus removed from the engine at oil changes.

One might well be thinking that modern oils will ..or might allow, ...extended mileage between slinger cleaning.  That appears to be so.   You also might be thinking that extending the mileage between oil changes, would be OK.  Possible, but not a good idea; after all, what you are after is the minimum wear and the maximizing of miles & time, safely, between slinger work.    Another thought might be that synthetic engine oils might be best.   I am not convinced.   I do think that semi-synthetic multigrade oils would be a very good idea. Further, I think the oil should be of a relatively narrow range.  That is, that 20W40 (1:2) would be better than 20W50 (1:2.5) or 15W50 (3.33) or 10W50 (1:5), all these in declining order.   Probably, depending on weather conditions, the 20W40 or 20W50 would be your best choice.  I won't get into oil compounding and viscosity range hardly here, just to mention that the additives, that allow wider and wider ranges, and the detergents, all tend to 'burn off', or deteriorate, faster.

Back around 2007 or so, I wrote an in-depth pretty-much step-by-step article on how to do slinger servicing & placed it in this website in a file which I did upload to the Internet, but the website had no reference to it in such as the Technical Articles List.   I would give the URL (website article address) to those needing it. This was done because I did not want to become widely known as any sort of pre-Airhead expert, & there are web groups and clubs devoted to them. I had mixed feelings about having the article, and, passing out its address. My misgivings turned out to be true, because one of the results was a considerable number of private E-messages dealing with details already spelled out in the article, in depth. Another misgiving also was that I would start being asked things about the /2 era bikes that I was not up-to-date about, ETC. LOTS of ETC. I had been pondering for some time about removing the article.  I had been also pondering about how to reduce my private E-mail traffic, which as taking up as much as 150 hours a month!!!...yes, really!

On October 11, 2010, I saw a vastly shorter article, quickly more pertinent & to the point, and covering the basic outline of the procedure, without being a step-by-step article. It was posted on the Airheads List by Mike Arman.   I decided to edit it liberally, & post it here, remove the original article I had written, and I figured out how to at least start minimizing my private E-mail hours.

The information, below, is reasonably complete enough on how to do the job.  I still have mixed feelings about owner's doing this work without a full understanding & following instructions carefully.


To clean the slingers requires some special tools.  You will probably want to remove the engine from the frame.  You will have to remove everything in front, that means the magneto & generator.  The timing chest has a lot of Allen screws & you must heat the cover. You will have to remove the cylinders & pistons.   Don't try to remove the rods at this point. Removing of the large flywheel nut is required, locking the flywheel (like one does with an Airhead) in order to undo that nut. You will be removing the oil pump drive nut which is a left-hand thread, and remove the gear (on a taper). You will rotate the crankshaft & remove all 4 of the front cam bearing housing screws, then heat the top of the crankcase quite hot & remove the camshaft with the rear cam bearing.

There is a small bearing forward of the main crank gear; it needs to be heated quite hot & then pulled off.  You will have to put the flywheel back temporarily & lock it again so you can do the gear work.  You can then remove the front main bearing retainer ...with a heated crankcase.  If the old style main bearing retainer is cast iron, it may be cracked.

As Mike pointed out, for the R50S, R69, and R69S you'll need a special tool which keeps the self-aligning rear main bearing aligned. Fasten tool into place, heat the rear of the crankcase, pull the crank forward & "roll" it & the rods out of the crankcase.  Remove the rear main bearing which had better be very tight.  If it comes off easily, the rear of the crank is necked, this can be fixed at high cost.

Now you can remove & clean the slingers. The deposits are HARD and difficult to remove completely.  It may be easier to buy new slingers.

At this point you want to inspect everything & get new bearings (rear main is $$$).  If you use standard number 6207 for the main bearings, be sure they have metal cages and 11 or 13 balls not use bearings that have a plastic cage and 9 ball bearings. The other bearings are not very expensive.  You will need new gaskets & seals, rear main seal, etc.

If the engine is relatively worn, maybe you need to hone the cylinders, get new piston rings, maybe even pistons.  How are the valves, valve seats, valve guides?  What about the generator brushes, ignition points, etc?  Think about honing the cylinders, touching up the valve seats, new generator brushes, points, condenser, plugs and maybe wires.

Re-assembly is rather involved.  Better get a good book.  Mike recommended the BMW Blue Book manual.

Maybe you should be in touch with Vetch! ...and/or farm the job out ...or?

NOTE:   The singles are different in some ways.  No need to get into them here, you would easily see the differences during disassembly.

This is a slightly edited copy of information from

Bench Mark Works, Craig "VETCH" Vechorik:  An excellent source for information, parts, and technical help.    Two ""divisions"", one in Mississippi, one in Canada.  Snowbum disagrees only with his website remarks on not using GL5 oil on /5 Airheads and later, which I discuss elsewhere's on my site.   Vetch stocks parts for the old BMW's ...and quite a few for the later bikes, including manuals & other literature.  Well worth your time to browse his site.  Almost a must if you have a pre-Airhead (before 1970 models).    check it out for /2 parts sketches...etc.

I don't typically discuss the /2 and /3 bikes; or, really, anything built before December 1969.  I also do not "follow" Vech's site, nor the sites for the older bikes. Two reasons.  One is that my huge notebook on the /2 and /3 era bikes was somehow lost.   The other reason is that these old bikes have a specialized following, some of who are much more knowledgeable than I am my memory has faded on them over the years, as I got further & further away from working on them; and, I do NOT keep up with information, changes, etc.   There are some sources for information, such as a /2 LIST on the internet (and other lists or groups), and in a very few really good books. One of the books that is just about a must to own is the Barrington Motor Works BMW /2 Restoration and Service Manual.  This is a well done book, reviewed by many 'experts' in the field before it was published.  The book is roughly $100, and worth it.  or call 607-664-2673.

rev. dates:
05/28/2012:  Clean up article a bit, narrow it too.
10/14/2012:  Add QR code; add language button; update Google Ad-Sense code.
09/22/2014:  Add a few references, clarify a few details.  Fix layout.
03/30/2016:  Update metacode, scripts, layout, fonts, colors.
11/18/2016:  Metas, scripts, layout, cleanup, minor improvements in clarity, links.
04/26/2018:  Reduce HTML, colors, fonts.  Improve layout.  Improved comments.

Copyright 2023, R. Fleischer

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Last check/edit: Saturday, August 19, 2023