BOLTS, THREADS, LOCK-WASHERS TO REMOVE, TOOLS, ETC.
BMW Airhead Motorcycle
© Copyright, 2013, R. Fleischer
You will almost certainly 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 an overhaul or a new clutch or crankshaft output seal, or some other reason, including replacing the rubber boot there, 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 those U-joint bolts at regular intervals. For that purpose, never loosen first, just check in the tightening direction. If even slightly loose, remove, clean the threads so they are clean and dry, and use a drop or two of Loctite BLUE #242 and retighten to 29 MAXIMUM footpounds of torque (20-22 is probably acceptable, but 29 is proper on all later Airheads). 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). Loctite BLUE on clean, dry threads!!
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 for a new boot), tighten 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, they may or may not have been stretched especially just under the head, but I assume not, 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 call here as to using new bolts, or not. AGAIN: Please decide for yourself whether or not to replace these bolts each time they are removed or unscrewed. UNscrewing direction first to then check and then torque for tightness is NOT a good idea....it breaks any existing Loctite bond, and just adds to wear.
I have NEVER had a reused set of these bolts break!!!
The earliest bolts with split type
lockwashers were not torqued as tightly as the later non-washered
ones (20-22 footpounds was generally used) (now is 29 ftlbs max.), so these lesser-torqued ones could not have stretched, unless
GREATLY over-tightened. At that point, you might even break a
bolt or damage the flange. I will NOT
use the early style 'long' U-joint bolts with the original added
split washers....you WILL have serious problems if the washers
crack or break; 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, even though I know that the setting of 29
ftlbs (max. at bolt) is increased SLIGHTLY by the Loctite
lubrication (NOT much actually).
It is acceptable to tighten those bolts by hand without a torque wrench, if you know what that should feel like. You do not have to use 29 footpounds, somewhat less is probably OK. If you do not use a torque wrench with an adaptor, and a known applied torque value, 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.
Measure the bolts threaded area length, do not add the head length.
Bolts are ALWAYS specified by full-threaded or shank length area.
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.
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, that is 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, do NOT change their heat treatment!
updated bolt is the one that is torqued to 40 Nm max (29 ftlbs
NOTE that SOME early literature calls for some of the early bolts with washers to be at this torque level....and 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. Yes, it is confusing.
I use 29 ftlbs as the maximum, ALL models and years....and have NOT seen any bolts stretch from that.
NOTE that BMW's specification is for 40 Nm, without the "max" I have added. I have never had a problem using 29 footpounds, clean and dry then adding BLUE Loctited bolt threads.
this very clear: I use 25-29 footpounds, on
threads, and I DO reuse the bolts if they look and feel OK.
A good grunt with a 6 inch wrench is usually OK if no torque wrench and special adapter to fit those bolts is available. Using an extension adapter, straight out, means the torque setting needs to be reduced, information is on this site on how to adjust for that factor.
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......:
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 bolts are very
high;....and, in 1983, BMW eliminated the lockwasher, as
some had broken (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 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, especially if Loctite blue is used on clean and dry threads; still, I will NOT use those longer bolts and lock-washers. I consider the lock-washers a dangerous item here.
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 easier on the flange taper, etc., to not use sudden high shock forces.
I have been informed a few times that BMW has either gone back to the washer design, or at least some dealers are selling the original bolts and lock-washers, and ERRONEOUSLY telling customers that BMW went back to the earlier longer bolts with 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.
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.
The actual BMW tool mentioned above (that I own), is simply a square female drive with a slot. The actual tool is simple, and PICTURES and descriptive information is in the TOOLS article on this site. 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.
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: IncTorqWr.htm
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.
© Copyright, 2013, R. Fleischer
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