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(Transmission output flange area)
includes a bit on push-starting too

For: BMW Airhead Motorcycles

Copyright, 2014, R. Fleischer


REFERENCE:  June 1983 BMW Factory Service Information bulletin (S.I.)  June 1983  26 005 83 (2078)

You WILL have numerous occasions to remove or check the tightness of the 4 bolts that are a 12 point type that fasten the forward end of the Airhead Motorcycle driveshaft to the transmission output flange, if you own your airhead for enough miles or time. This can be for removal of the transmission for a thorough spline clean and lubrication, or transmission overhaul, or a new clutch or crankshaft output seal, oil pump seal....or some other reason, including replacing the rubber driveshaft-to-transmission boot, and just plain checking those bolts!  You do NOT want them to loosen!

Generally, removing the U-joint bolts is NOT needed for a simple transmission input shaft cleaning and re-greasing (aka 'clutch splines').  It is a good idea to schedule a checking of the tightness of the four U-joint bolts at regular intervals. For that purpose, never loosen first, just check in the tightening direction.  If even slightly loose, remove, inspect the bolt, and if OK, clean the bolt AND flange threads so they are clean and dry, and use a drop or two of Loctite BLUE (#242 or equivalent) and retighten to 29 MAXIMUM footpounds of torque (20-22 is probably acceptable, but I use 25-29).   If you have lock washers, remove them, replace the bolts with the later shorter type (which appear almost the same, but are very slightly shorter). Apply the drop or so of Loctite BLUE onto clean, dry threads!!
If you want to be anal about the torque, considering that Loctite is a MILD lubricant, use 23-26 foot-pounds.

If you fail to use the shorter bolts after removing the lock-washers, you can damage the area behind the flange.  Details further on in this article.

If the flange was totally loosened and U-joint separated (as in for a new boot, etc.), tighten in a cross-pattern, evenly, in stages.  If the flanges were separated, be sure the surfaces are clean before reassembly.

The threads on the bolts and in the flange should be oil-free [clean and dry]. This is per BMW, and I agree. BMW says to replace these bolts (non-washered type) every time they are removed.  I do not. What I DO is to screw them in with fingers, and if they are smooth, I assume they have not been stretched (especially just under the head, visual look-see), and I reuse them, with 1 or 2 small drops of BLUE #242 Loctite AFTER cleaning the male and female threads with a quick evaporating solvent. Please be advised that the bolt COULD have had some stretching just beneath the head, and not in the engaging threads, which would not necessarily be revealed by my screw-in test, so this is YOUR decision here as to using new bolts, or not.  UNscrewing first to check and then torqueing for tightness is NOT a good breaks any existing Loctite bond, and just adds to wear.  I look at the bolts, threads, and at the sharp point of shank to head, beneath the bolt head, using a 10X eye loupe....I look for cracks or other distress.
   I have NEVER had a reused set of these bolts break....nor loosen....that I personally installed.

Background:  The earliest bolts with split type lock-washers were not torqued as tightly, per specification, as the later non-washered ones (20-22 footpounds was generally used early-on, but there were even lower specifications quite early-on) (now is 40 Nm....29 ftlbs max.), so these early lesser-torqued ones would not have stretched, unless GREATLY over-tightened. If you greatly over-torqued the bolts...considerably beyond 29 ftlbs I think... you might even break them or damage the flange, besides stretching them.   I will NOT use the early style 'long' U-joint bolts with any type of washer, and especially not split WILL have serious problems if even just one washer cracks or breaks or otherwise allows bolt loosening; this has been known to happen well after assembly.   I use ONLY the later, slightly shorter bolts, withOUT lockwashers.  I use blue Loctite on already cleaned and dried threads.  

It is acceptable to tighten those bolts by hand without a torque wrench.   You do not have to use as much as 29 footpounds, which is a maximum....and somewhat less is OK.    If you do not use a torque wrench with an adaptor, you can probably get away with using the tool kit wrench, which is a 12 point box wrench style, and use "a good grunt" on the bolts using that wrench.


If you wish to measure the bolts:  Measure the bolts threaded area length. 

The official book value on the old bolts were 14.5 mm (actually measured 14.4 mm or 0.568"),  and were part 26-11-1-239-414, and were used with the troublesome lock-washer 07-11-9-930-840.   NEVER EVER reuse these, unless a dire emergency.

The new bolt, that began being used in 1983, are 13 mm, and part number 26-11-1-242-297.   That means a 1.5 mm (0.059") difference, officially. 
  BUT....In practice, the bolts will ACTUALLY MEASURE almost exactly 1/2 inch; or, for the metric folks, 12.7 mm and a difference of  1.7 mm. ... this very slightly shorter bolt MUST be used if the lock washer is NOT used, to avoid seal damage.

Could you shorten the original bolt and use it withOUT the split lockwasher:  YES, but you must NOT heat the bolt much during the grinding/shortening.  The bolts are very tough hardened metal, NOT change their heat treatment by getting them quite hot. So, if shortening the longer ones, do a wee bit at a time, allowing cooling between grinding attempts.  If the temperature of the bolt during grinding is hotter than you can comfortably hold in your bare hands, it is likely too hot.  Do not leave the ground-shorter bolt with messy sharp threads, etc., at the end.

BMW says that at 29 ft lbs, the bolt stretches. I do NOT believe that.   Some manuals and literature have the torque at 27.5-31 for the 1977-80 models, and 29.0 for the 1981 models. Note that these were before the 29 specified by BMW in 1983. 

I use 29 ftlbs as the maximum, ALL models and years....and have NOT seen any bolts stretch from that.   I actually torque, as noted earlier, to 25-29 foot-pounds.

NOTE that BMW's specification is for 40 Nm, without the "max" I have added.  I have never had a problem using 29 foot-pounds, clean and dry, this is even with BLUE Loctite on the bolt threads.  

To make this very clear:  I personally use 25-29 footpounds, on cleaned threads with a bit of blue-Loctite and I DO reuse the bolts if they look and feel OK.  I have done them when a torque wrench and adapter was not present, by using "A good grunt" with a 6 inch wrench.  

When using a torque wrench, using an extension adapter straight-out, these extend the length of the torque wrench, increasing the actual torque from that indicated by the wrench setting, so the torque wrench setting needs to be reduced to compensate.  Information is on this site on how to adjust for that factor, link at end of this article. 

 When using a torque wrench with an added wrench (or adapter) at 90 to the torque wrench barrel, no correction is needed, and you can just use the proper 25-29 foot-pounds setting.  

Do NOT use RED Loctite, it requires a LOT of heat to loosen and remove a bolt installed with RED Loctite.

HISTORY of the bolts and washers........and push starting.........and some GOSSIP/etc:

The forces at this flange (which is a TAPER fit to the transmission output shaft (and is why bump starting is a poor idea, although acceptable, but NOT IN FIRST GEAR, and probably not in second) and bolts are very high;....and,  in 1983, BMW eliminated the lockwasher, as some had broken (many back even to the /2 days!), and then a bolt would really loosen up....and soon all 4 were loose.  When the flange and U-joint parted company, the rear of the transmission could be torn off!  This had happened to one of my own R60 bikes; which is how I purchased it so cheaply!
The early old original longer bolts, with the split lockwashers, at one time had a tightening specification of 17-19 ftlbs, and that was likely adequate then, as Loctite was not then available for use, still, I will NOT use those longer bolts and lock-washers.  I consider the lock-washers a dangerous item.

Push starting is best done in at least 3rd.  The reason is that 1st or 2nd might cause the rear wheel to slip, and it is also easier on the flange taper, etc., to not use sudden high shock forces.  You do NOT want the taper fit to usually damages it.

I have been informed a few times by various owners that BMW dealership parts departments have stated that BMW has either gone back to the washer design, or at least some dealers are selling the original bolts and lock-washers.  This is NOT TRUE, and is due to a mix-up with BMW.  Only the new shorter bolts are to be used, withOUT washers.  NOTE that the new shorter bolts need to be inspected VERY closely to see that they are shorter!...see my figures well above.

Gossip, unconfirmed (best way to put this, as I have no intention of mentioning my sources for this information!):
Some time after the bulletin came out (see very beginning of this article for bulletin number), BMW discovered that they had a large amount of the old longer bolts and washers in stock, and some dipshit decided that it would be a good idea to distribute a "Parts Bulletin" recommending that they be used for the pre-1980 models. was a Parts Department guy who wrote it, and he vanished from BMW shortly thereafter,, because the warranty claims started coming in again.

My own 10 mm 12 point BMW tool 88-88-6-002-560 was manufactured by Hans Schubert in Germany, for BMW, and there may have been other makers at various times. Mine was sold with marking as "00-2-560" for a part number. I have a feeling that this tool is still available, but the price in my 1995 BMW book was $54.58 retail and $32.75 dealer cost. That is pretty pricey, even back then.  It is true that one can use a double ended BOX wrench, and then an Allen adapter to your square drive on the torque wrench.   I have seen combo box wrenches with 10 mm on one end and 8, 9, 11, 12 mm on the other.    The original BMW 12 point box end wrench which some use for this purpose is OK.  You must either use that wrench with a goodly grunt; or, use it with a torque wrench, and calculate the reduced torque wrench setting (unless the usage is at 90).  

The actual real BMW tool mentioned above (that I owned), was simply a square female drive with a slot.  The actual tool is simple, and PICTURES and descriptive information is here: in the  It works very nicely with a torque wrench.

If you have a reasonably good feel you can probably tighten these 4 bolts without any torque wrench, using Loctite BLUE.     That's why I say 'a good grunt'.   The proper bolts are very tough metal.

For those that do not fully understand torque wrenches and adaptors at various angles, please DO NOT use your 'adaptor' at anything except almost perfectly straight out (that means parallel...IN LINE WITH THE TUBE OF THE TORQUE WRENCH) to the clicker torque wrench.  That way, no angle is involved to mess up the calculations.  That makes calculations easy, and no leverage problems or interferences.  Do the calculations and reduce the torque wrench setting appropriately, to compensate for the extension. STORE that clicker wrench with its setting close to zero (standard advice). 

NOTE, however, that if the adapter is at 90 degrees, no calculations are needed.

Here is how to do the calculations for YOUR torque wrench:

04/04/2003:  revise strictly for clarity
05/10/2003:  add some items previously in hints.htm section of website, and revise entire article.
01/14/2004:  Clarifications about the -560 wrench, use of the torque wrench, and hyperlink to the tools.htm
02/15/2005:  a few more clarifications
01/30/2008:  Clarify that 29 footpounds is the maximum, and that 20-22 is likely OK with Loctite blue;
                    minor other changes (grammar)
03/08/2008:  Add exact measured bolts information, in bold and color; do some minor clarifying
06/17/2011:  Clean up article, add hyperlink, etc.
09/26/2012:  Add QR code, language button, update Google code
09/27/2012:  Clean up for clarity regarding the new bolts, torques, push-starting.
03/29/2014:  Clean up possible confusing information on torque wrench use/settings.  Two more edits,
                    one later in the morning, and one in the evening.

Copyright, 2014, R. Fleischer

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