The four transmission output flange 10 mm 12 point bolts.
For: BMW Motorcycles
© Copyright, 2014, R. Fleischer
REFERENCE-1: BMW Factory Service Information bulletin (S.I.)
June 1983 26 005 83 (2078)
There is a fair amount of information in this article by Scot
Marburger, about the output flange bolts, threads, theory
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 universal joint to the transmission output flange. This can
be for removal of the transmission for a transmission input spline cleaning &
lubrication (but, see next paragraph); or, perhaps for a 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, & just checking
those bolts. You do NOT want them to loosen!
Removing the U-joint bolts is NOT needed for a simple transmission input shaft
cleaning & 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, & if OK, & screws in smoothly with
fingers, then clean the bolt AND flange threads so they are clean & dry & use one
or two drops of Loctite BLUE (#242 or equivalent); retighten to 35 +- 3 Nm, which
is same as 26 +- 2 foot-pounds. You can replace the bolt(s) if you think they could
have been stretched, or otherwise been damaged, or if you just want to be
absolutely sure the bolts are OK. 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 two of Loctite BLUE onto
clean, dry threads!! DO NOT INSTALL WASHERS OF ANY TYPE! Don't
bother to take into account that Loctite is a MILD lubricant.
Snowbum uses ONE drop of Loctite BLUE.
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 & U-joint separated (for a new boot, etc.), then
EVENLY tighten in a cross-pattern in stages. Be sure the surfaces are clean &
totally undamaged by nicks, particularly at the edges, before reassembly. It is
important that the flanges have NO metal, not in the slightest, sticking up anyplace
that would prevent the flanges from coming totally, flatly, together. Failure to follow
this advice can result in massive damage, if the flange 'disconnects' from
the transmission; which happens if forces cause the bolts to loosen.
The threads on the bolts & in the flange should be oil-free [clean & dry]. This is per
BMW; I agree. BMW says to replace these bolts (non-washered type) every time
they are removed. I personally do not always do that; what YOU DO, is up to
YOU. See Scot's article, link at the top of this article.
What I DO is to screw them in with fingers; if they are smooth I assume they have
not been stretched ..especially just under the head, visual 10X eye-loupe look-see,
then I usually reuse them, with 1 drop of BLUE #242 Loctite AFTER cleaning the
male & female threads with a quick evaporating solvent. DO NOT spray any solvents
into the transmission flange threads; instead, use a Q-tip or equivalent, moistened
with solvent. Don't use Q-tips that have cores of plastic. I do not want solvents to
get on the seal behind the flange.
The bolt COULD have had some stretching just beneath the head & not in the
engaging threads, which would not necessarily be revealed by my screw-in test
(less likely, but possible with the 10X loupe look), so this is YOUR decision here
as to using new bolts, or not. Certainly new bolts should be the safest. If you use
new OR USED bolts, DO MEASURE THE LENGTH & DO INSPECT THEM.
When just checking at regular intervals for tightness, UNscrewing first to check &
then re-torqueing for tightness is not a good idea....it breaks any existing Loctite
bond, adds to wear; you are then re-torquing on used hardened Loctite, which
changes things somewhat. I look at the bolts, threads, and at the sharp point of
shank to head, beneath the bolt head, using my 10X eye loupe....I look for almost
microscopic cracks or other distress. I have NEVER had a re-used set of these
bolts break....nor loosen....that I personally installed. YMMV!!
The earliest bolts for the /2 & the Airheads were used with split type lock-washers,
& were not torqued as tightly, per specification, as the later non-lock-washered ones
(20-22 footpounds was generally used early-on, but there were even lower
specifications quite early-on), so all these early lesser-torqued ones would not have
stretched, unless considerably over-tightened. If you greatly over-torqued the bolts...
considerably beyond 30 ftlbs I think... you might even break them or damage the
flange threads area, besides stretching threads.
In an emergency 'in the field', with none of the shorter BMW bolts being available,
I would re-use the old bolts AND with lockwashers, until I could replace the bolts.
You WILL have serious problems if even just one washer cracks or breaks or has
spread slightly during tightening...these things allow bolt loosening & has been known
to happen well after assembly. It is acceptable to tighten those bolts by using a hand
wrench & not a torque wrench, this is for Field Situations, that is, you are on a
ride/tour, & have a reason to tighten the bolts. So, how do you tighten these bolts
if no torque wrench?: Use a 12 point box wrench wrench that FITS the cramped
area AND is about 6" long; use "a good grunt" on the bolts using that wrench.
You probably already have that particular wrench, it is somewhat thin-looking but not
fragile. Here is the description in my tools article,
71-11-1-237-847 10 x 12 box end wrench, which replaces 71-11-1-230-579.
To measure the bolts length, to be sure you have the correct ones: measure the
distance from the underside of the head to the end of the bolt.
The official book value on the old bolts, 26-11-1-230-414 was 14.5 mm
(actually measured by me at 14.4 mm or 0.568"). These were the longer
ones that were used with the troublesome lock-washer 07-11-9-930-840.
NEVER EVER reuse these lock-washers, unless a dire emergency. If you
absolutely have to reuse these old longer bolts, install new lock-washers,
even if from the hardware store.
The new bolts, that began being used in 1983, are 13 mm, part number
26-11-1-242-297. That means a 1.5 mm (0.059") difference, officially.
The bolts will ACTUALLY MEASURE almost exactly 1/2 inch; or, for the
metric folks, 12.7 mm and a real difference of 1.7 mm. ... these very slightly
shorter bolts MUST be used if the lock washer is NOT used (and it is NOT TO BE
USED!!!, to avoid seal damage).
Could you shorten the original bolts & use them withOUT the split
lockwasher: YES, but you must NOT heat the bolt much during the grinding-
shortening. The bolts are very tough hardened metal, BUT....you do NOT want to
change their heat treatment by getting them quite hot. So, if shortening the longer
ones, do a SMALL bit of grinding at a time, allowing plenty of 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 messy
sharp threads, etc., at the end.
BMW says that at 29 ft lbs, the bolt stretches. I do NOT believe that; I think it
takes a fair amount more. A bit more nerdy explanation is that BMW could be
correct, but I believe that at 29 footpounds the bolt stretch is NOT PERMANENT.
Some manuals & literature have the torque at 18 foot-pounds on the /5 models;
27.5-31 foot-pounds for the 1977-80 models; 29 for the 1981 models. Note
that these were before the 29 specified by BMW in 1983. The last of the
Airheads, owners manual, says 35 +- 3 Nm, which is 26 +- 2 foot-pounds,
I agree with that; and believe that 26 is OK with good reused bolts, as
well as brand-new ones. I personally set my accurate torque wrench at
When using a torque wrench, using an extension adapter straight-out, the
adaptor extends the working length of the torque wrench, increasing the actual
torque from that indicated by the wrench setting. 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. Snowbum uses it STRAIGHT OUT.
When using a torque wrench with an added wrench (or adapter) at 90° to the
torque wrench barrel, no correction is needed, & you can use the actual
desired torque setting just as it says on the torque wrench.
Do NOT use RED Loctite, as it requires a LOT of heat to loosen & remove a bolt.
Using any Loctite (Snowbum uses ONE drop of BLUE, a medium holding Loctite)
gives some added insurance, while not acting hardly as a lubricant, which increases
effective torque slightly.
HISTORY of the bolts and washers.
The transmission output flange is a TAPER FIT to the transmission
output shaft. This is why bump starting is a poor idea, although
acceptable, but NOT IN FIRST GEAR, and probably not second gear,
Forces on the flange & bolts are substantial, especially during shifting
gears or sudden braking. The ACTUAL transmission of power is, via
the flange contacting area....yes...by the FLAT SURFACES.... but those
are pressured together BY THE BOLTS under discussion...which do
carry a shear loading as well. BMW has had some problems with these
bolts backing-out (unscrewing, becoming loose, use whatever words
you like) over the years....all the way back into the /2 era, etc.
In 1983, BMW eliminated the lockwasher, as many had broken (back
even in the /2 days!). The result of an even one slightly loose bolt was
the bolt would continue loosen....and soon all 4 were loose, or, even
before, the U-joint & transmission flange parted company. When the
flange and U-joint parted company, the rear of the transmission could
be torn off!! I had personal experience with a few of these as the Chief
Technician for a dealership. This had happened to an almost-new
R60/2 bike; which is how I purchased it so cheaply!
The original longer bolts, with the split lock-washers, at one time had
a tightening specification of only 17-19 ftlbs. The major problem with
split-lock-washers is the washers are capable of spreading during the
bolt tightening; they are also brittle, & can crack.
Ever since BMW changed the bolts and eliminated the washers, there
has been MISinformation. I have been informed a few times by various
owners that their BMW dealership parts departments personnel stated
that BMW has either gone back to the washer design, or at least some
dealers are/were selling the original bolts & lock-washers. DUE TO A
MIXUP BY BMW, ANY INFORMATION ABOUT A REVERSAL TO THE OLD
STYLE BOLTS & LOCK-WASHERS IS NOT TRUE. Only the new shorter
bolts are to be used, withOUT washers.
|I have no intention of mentioning my sources for this:
Some time after the bulletin came out: Factory Service Information bulletin (S.I.) June 1983 26 005 83 (2078); BMW discovered that they had a large amount of the old longer bolts & washers in stock, a BMW factory parts guy decided that it would be a good idea to write & distribute a "Parts Bulletin" recommending that they be used for the pre-1980 models; he vanished from BMW shortly thereafter because the warranty claims started coming in again.
Adaptor tool for your torque wrench, for your shop,
not necessarily for the bike tool tray:
For years I used a 10 mm 12 point BMW tool/adapter... an old number for it
was 88-88-6-002-560, manufactured by Hans Schubert in Germany for BMW;
there may have been other makers at various times. Mine was sold with
marking as "00-2-560". MAYBE 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, & then an Allen adapter to your square drive on the torque wrench.
There is a BMW combo box wrenches with 10 mm on one end & 12 mm on
the other end. You must either use that wrench with a goodly grunt; or,
use it with a torque wrench; calculate the reduced torque wrench setting
(unless the usage is at 90°).
The actual real BMW -560 tool mentioned above (that I owned), was simply
a square female drive with a slot, with a box wrench piece in the slot. The
actual tool is simple; PICTURES & descriptive information is here:
It works very nicely with a torque wrench. There are now at least two
sources for a very small version, one-piece, laser-cut, that is quite neat-O.
See that tools article, it has photos of the BMW tool AND the aftermarket
tool. I am FINE with you making your own tool, or, purchasing an aftermarket
version. I think BMW's price for the tool, now, if even available, would be
Here is how to do the calculations for YOUR torque wrench:
PUSH-STARTING: IF YOU HAVE TO PUSH-START, IT IS BEST DONE IN 3rd GEAR, NOT FIRST GEAR!
If you are on dry pavement,
IF YOU HAVE TO PUSH-START, IT IS BEST DONE IN 3rd GEAR, NOT FIRST GEAR!While one reason is that 1st or 2nd might cause the rear wheel to slip and not transmit enough force to rotate the engine properly...it is also easier on the flange taper to not use sudden very high shock forces when suddenly engaging the clutch. You do NOT want the taper fit to slip....that usually damages it.
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.
08/16/2015: Clean up a bit, and fix excessive verboseness in two places, and set down latest specs for torque.
08/22/2015: Fix error in bolt part number, plus some minor clarifications and cleanup.
02/09/2016: Update meta-codes. Narrow article. Increase font size and use more left justification. Change
horizontal lines. Clarify some details. Remove excessive redundancies.
© Copyright, 2014, R. Fleischer