Clutch bolts keep snapping - Proper Torque

[Justin B.]

The Haynes book lists clutch cover (pressure plate) torque on r45/r65 as "Not Available"  The 23 Nm value is for pre-'81 "full-size" boxers.  I would suggest using a value from the following chart I found on some engineering related web site.  You can see from the chart that the value for a 6mm grade 12.9 fastener is 12 lb/ft which is about 16.3 Nm, count your lucky stars (always look for the silver lining...) you snapped the bolt instead of pulling some threads!

                  U.S. Bolt Threads

Coarse thread:

Thread Size      SAE Grade 2      SAE Grade 5      SAE Grade 8

¼ - 20                49-65 lb-in            75-100 lb-in          107-143 lb-in
5/16 -18      101-134 lb-in        157-210 lb-in               220-305 lb-in
3/8 - 16          15-20 lb-ft            23-31 lb-ft             33-44 lb-ft
7/16 – 14         24-30 lb-ft            37-50 lb-ft            53-70 lb-ft
½ -13                37-49 lb-ft             57-75 lb-ft            80-107 lb-ft
9/16 – 12         53-70 lb-ft            82-109 lb-ft          115-154 lb-ft
5/8 – 11          73-97 lb-ft           113-151 lb-ft          159-211 lb-ft
¾ - 10             129-173 lb-ft          200-266 lb-ft         282-376 lb-ft

Fine thread:

¼ - 28                  55-75 lb-in      85-115 lb-in          120-163 lb-in
5/16 – 24       112-150 lb-in          173-230 lb-in      245-325 lb-in
3/8 – 24           17-23 lb-ft        26-35 lb-ft            37-50 lb-ft
7/16 – 20           27-36 lb-ft       42-55 lb-ft              59-78 lb-ft
½ - 20                  41-55 lb-ft        64-85 lb-ft            90-120 lb-ft
9/16 – 18           59-79 lb-ft      92-122 lb-ft          129-172 lb-ft
5/8 – 18          83-110 lb-ft      128-170 lb ft         180-240 lb-ft
¾ - 16               144-193 lb-ft      223-298 lb-ft         315-420 lb-ft

                  Metric Bolt Threads

Thread Size      Low Grade      Grade 8.8            Grade 10.9      Grade 12.9

5 mm            5 lb-ft                       7 lb-ft
6 mm            3-5 lb-ft                7 lb-ft                 10 lb-ft            12 lb-ft
8 mm            8-12 lb-ft            17 lb-ft               24 lb-ft            29 lb-ft
10 mm            15-22 lb-ft             33 lb-ft               47 lb-ft           57 lb-ft
12 mm            39 lb-ft             59 lb-ft                83 lb-ft           100 lb-ft
14 mm            60 lb-ft             101 lb-ft              131 lb-ft         158 lb-ft
16 mm            60-94 lb-ft           146 lb-ft                    202 lb-ft        247 lb-ft
18 mm            60-130 lb-ft           201 lb-ft                   283 lb-ft       340 lb-ft
20 mm            166-188 lb-ft          285 lb-ft                  401 lb-ft      482 lb-ft

*** Multiply lb-ft by 1.3558 to get N-m

[not-so-fast-ed]

The book for my Mono lists "20 +2 Nm".
Most of the book shows "+-", but not this specific clutch "Housing cover to flywheel" torque.
Does this mean 20 to 22 Nm ?
Not sure what size bolt they are talking about...

Ed

 :-/

[vestandpants]

i wondered that yesterday ed, as i held aloft yet another sheared bolt.

after doing some of my own research (thanks dad for this link http://www.mechanical-fitter.co.uk/torque.htm ) and what you guys have posted it does seem that haynes and BMW's own repair manual (i have an original) have got this particular torque setting wrong. the tensile grade of 6mm 8.8 bolt is WAY lower than 23nm, in fact i was lucky to get it to 15nm and have it hold fast.

so for next time when im armed with new bolts i'll torque them to 10-12nm and if it isnt right, there is no harm taking the 'box off again and nipping them all up another turn or two  

[nhmaf]

Hmm,
This is curious.  My Haynes manual doesn't have any specs at all for the torque value for these bolts.   My Clymer manual has the suspicious 23 N-m or 17 lb-ft value for the pre-81 models, and from 1981+later it shows 20-22 N-m or 15-16 lb-ft.   I know from my notes that I tightened mine up to 15 lb-ft when  I replaced the clutch on my 1982 LS.    some of the bolts made a slight 'squeak' but none showed any signs of failure or thread pulling...    I wonder if perhaps you might have gotten some non-hardened bolts ?   In any case, according to the chart from Justin, I should think 12-13 N-m for your bolts would probably be just fine as well.   I put a little bit of anti-seize on my bolts when I installed them - a thread lubricant does cause a change to torque values for equivalent "tightness".

 :-/

[nothing]

Snowbum recommends 7.5 foot-pounds (approximately 10 Nm) with Loctite Blue threadlocker for the 79-80 R65 clutch bolts. He discusses the confusion/misinformation about this torque spec here: http://bmwmotorcycletech.info/clutch.htm 2/3 down the page, red paragraph.

[steve hawkins]

This is an excellent snippet of info and a candidate for the 'R65 Facts or whatever we are calling it, don't you think?

Cheers

Steve H