The four transmission output flange 10 mm 12 point bolts (& lockwashers?)
For: BMW Motorcycles
© Copyright, 2014, R. Fleischer
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'), if the transmission is left in the motorcycle. 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.
88-88-6-002-560 or equivalent BMW tool allowed a torque wrench to properly fit the driveshaft bolts at the transmission output flange. USUALLY a "home" tool. The BMW-sold tool consists of a 3/8" square drive female part with a slit in the side. Pressed & brazed into that slit is a 12 point 10 mm off-set box end wrench. That wrench was simply a double ended wrench of some sort, cut to be shorter, one end discarded. Crude! See photos below. When using this tool, typical is to use it straight-out with the torque wrench, not at any angle to the torque wrench. The center-to-center distance of the working ends of this tool, compared mathematically to the distance between the working center of the torque wrench square drive & the center of pressure of the torque wrench handle, will determine the setting THAT WILL BE LOWER than the torque wrench setting. The official maximum torque amount is 29 foot-pounds, on clean and dry threads. In ALL instances except use at 90°, the torque wrench will be set to LESS than 29 ftlbs if the wrench is used straight out.If the tool is at 90° to the tubular torque wrench, there is NO correction needed. You can use the dual box wrench (10/12mm), with your glove to protect your hand, & fairly easily get near the 29 ftlbs, which is my maximum for you. I am fine with 25-29; and, using Loctite BLUE, using a real torque wrench and adapter.
NOTE: I am adding this note because there have been rare instances in which someone has questioned just what BMW means by its torque figures. This question arises now and then for other areas of the bike, not just the output flange. This is because using a torque wrench at a limited clearance area of such as the driveshaft U-joint flange bolts requires an adaptor, see above 4 photos, which "can" add to the working-length of the torque wrench, INcreasing the actual torque on the bolt from the value set on the torque wrench......unless the adaptor is used at 90°. My torque wrench article describes using extensions more precisely; but, here I will add brief commentary. When this question comes up, it almost always is in regards to the tightening method for the 4 driveshaft bolts, that hold the U-joint flange to the transmission output flange.
>>>BMW's torque figures are the value on the bolt itself, whether or not you use an adaptor from the torque wrench to the bolt. This is standard for industry, & this applies unless specifically noted to be different by the manufacturer. BMW never made this clear & BMW's own wording can be wrongly interpreted.
The factory specified torque is what IS to be applied to the BOLT, & you MUST calculate the adjustment factor for the torque wrench if the adaptor is not used at 90°. In every instance, where the adaptor extends the working distance of the torque wrench, the torque wrench will need to be set to a value LESS than the value of the actual torque you want applied to the bolt head. I suggest that if you use the adaptor, you use it at 90°, which is a bit awkward; or, straight-out with the correction value. Any angle besides those TWO requires trigonometry calculations. To make this 100% clear, if you use the adapter straight out from the torque wrench, you MUST do calculations, but if at 90° NO special calculations are needed.
If you are just checking to be sure the 4 bolts are tight; try tightening them, using the setup of your choice, see prior paragraphs. 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 drop of Loctite BLUE (#242 or equivalent). 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 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. I have already done that for you by suggesting 25 foot-pounds instead of the maximum, which is 29 foot-pounds.
If you fail to use the shorter bolts after removing the lock-washers, you can damage the area behind the flange.
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, except that I suggest you use a drop of the Loctite mentioned, and use 25 foot-pounds of torque at the bolt itself. 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.
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, hence the no-spraying.
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.
An explanation is that BMW could be
correct, but I believe that at 29 footpounds the bolt stretch is NOT PERMANENT.
Do NOT use RED Loctite,as it requires a LOT of heat to loosen & remove a bolt.
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, unless careful.
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 source for this information:
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:
PUSH-STARTING: IF YOU HAVE TO PUSH-START, IT IS BEST DONE IN 3rd GEAR, NOT FIRST
IF YOU HAVE TO PUSH-START, IT IS BEST DONE IN 3rd GEAR, NOT FIRST GEAR!
some additional references:
REFERENCE-1: BMW Factory Service Information bulletin (S.I.) June 1983 26 005 83 (2078)
REFERENCE-2: http://www.gunsmoke.com/motorcycling/r100rt/transmission/ There is a fair amount of information in this article by Scot Marburger, about the output flange bolts, threads, theory about threads.
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 article.
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.
03/14/2016: Update metacodes slightly; update section on the tools, adding edited material from the tools article.
05/29/2016: Final updating. No changes in text information. Rechecked, minor clarity changes, 06/14/2017.
©Copyright, 2014, R. Fleischer
Last check/edit: Wednesday, June 14, 2017