Bent and threaded lower rear shock bolt

[glennis]

Mornin' ppl,

Was in the process of replacing my fecked clutch slave cylinder and whilst removing the rear shock I noticed the lower mounting bolt was quite bent. I'm in Kenya at the moment and managed to source a replacement bolt but when putting it in it would not tighten and feels like the thread in the swing arm is stuffed. I use a decent torque wrench to tighten bolts so I think the thread was damaged removing the bent bolt.

Not sure what to do about this, so would like some advice on what approach to take before I let some Kenyan mechanic loose on my bike.

Cheers

 

[Bendy toy]

Check the swing arm has not cracked around the bolt hole. If its just a failed thread, you will need an insert. Helicoils are much easier to get hold of but can unscrew themselves. Saying that they were (maybe still are) used extensively by Rolls Royce on new aero engines to provide good strength into soft alloys.

This is for an M12 repair, buts uses a larger standard tap (M16) to prepare the hole. Whatever type you use, they should be retained with high strength thread lock and allowed to cure before fitting the new bolt normal strength threadlock.
http://www.ebay.co.uk/itm/Threaded-..._Metalworking_Supplies_ET&hash=item257db569bb

This is an M12 helicoil set. Everything in one set so might be the most practical option.
http://www.ebay.co.uk/itm/15pc-THRE...t=UK_Hand_Tools_Equipment&hash=item2ec54d2b66

 

[Ian J Hartley]

M12 repair???

Are you sure? The bolt is an M10 x 80.

I would have faith and go and talk to any motorcycle / small engineering workshop in Kenya and tell them the type of repair you need. Africa is the home of bright creative thinking, when it comes to repairing machinery to keep it going.

I think you will be pleasantly surprised.

After all the worse case senario is you need a replacement trailing arm.

 

[Ian J Hartley]

The other thing I would investigate is why the bolt has bent.

There are no bending forces on the bolt, only shear forces. The bolt passes through the outer trailing arm lug, which has a bush. This together with the thread on the trailing arm should prevent the bolt from flexing.

If you've lost the bush, or the bush is badly worn, then the bolt will bend.

This leads me to think. You must have had a really hard time getting the bent bolt out past the outer bush. If the bolt came out easily then probably no bush and hence the torqueing problem you encountered.

All conjecture, I'm just thinking out loud.

 

[Bendy toy]

Sorry, I should have said M12 was a guess on size and to treat the eBay items as examples. The same suppliers offer different sizes of insert. Der check the swing-arm alloy is OK. It takes some force to bend a high tensile M10.

I'm not a big fan of torque wrenches for everything - there's no feel of what is going on so all to easy to strip a thread. Special bolts in the engine and thinks like wheel bolts - accurate torque every time. This one is important enough to need a torque wrench but well worth a dry fit by hand to be sure its running ok before leaning on the click oh dear its too late device.

 

[Ian J Hartley]

The way I read this is you have several options

1. Helicoil the damaged thread

2. Drill deeper into the swinging arm caseing and tap to M10 and fit a longer bolt. This depends on how much depth of casing is available before you break into the tunnel for the drive shaft.

3. Drill right through the swinging arm caseing into drive shaft tunnel and fit a nut on the end with an even longer bolt. This would mean removing the final drive and prop shaft to tighten the nut. Make sure you use either good quality loctite, or castellated nut and split pin. You do not want the nut coming loose next to the drive shaft. It would also mean that your swinging arm and rear shock would effectively become a single unit. Not good if you have to replace the shock.

4. You might be able to Tig weld the damage thread and machine and cut a new thread, but it's a tight spot to Tig and the welding and machining would have to be good.

5. Get a replacement swinging arm caseing.

I'm sure there are other options.

Hope this helps

 

[glennis]

There is a workshop here and the guy has heli-coils. Has anyone tried this before on the swing arm on lower mount for the rear shock and can you get the tool to install it through the opposite mounting hole??? The bolt torques up to about 30Nm. Any more and it feels like it will strip completely. Other suggestions still welcome.

 

[glennis]

Any comments on using a thread locking compound to compensate. I can torque the bolt to around 20-25nm. Is it possible to add a compound with a breakaway force of lets say 30mn, to bring the total breaking force to 50-55nm. The bolt is supposed to be tightened to 58nm. Of course this would be a temporary fix until I return to Europe (1-2months). What do you reakon?? Or is this complete bollocks

 

[Pukmeister]

On my old '04 bike I fitted Wilbers shocks, then refitted the BMW originals prior to selling it at which point I picked up the top shock mount bolt by mistake and screwed it into the swing arm. It went solid after about 3 threads at which point I realised my mistake. I managed to remove it again thankfully with minor damage to the threads.

I had to draw out the plain shouldered steel bush from the swingarm casting into an oversize socket using a nut bolt and washers which then allowed me to screw in a tap and clean up the threads. The bush was drifted back in afterwards using a small deep socket as a drift.

Perhaps your mechanic could do the same eg remove the shouldered steel bush from the swingarm casting and retap the original threads. It's worth a try with nothing to lose. If that fails, helicoil is the next best thing which requires removal of the shouldered steel bush anyway.

The bolt is secured with loctite as standard.

Still not sure how you bent the bolt, unless the bolt shank is the wrong diameter or the shock lower eye hole is too big for the bolt, the bolt material is not properly heat treated, or the shouldered guide bush is absent from the casting allowing the bolt to flex.

 

[glennis]

Thanks for everyones advice so far.

Pukmeister - Was it necessary to apply heat to the swing arm in order to remove the bush. And once out could you used the same bush once the thread had been repaired?

 

[glennis]

Oh, I now recall why the bolt may be bent. In India last year I snapped the rear shock in half - might have had something to do with that

 

[Bendy toy]

Glueing the bolt with thread lock "might" work but if it fails the consequences are serious. At least drill the bolt head and lock wire it. But that has to be the very last resort. If a new bush is not available a helicoil has to be a good option.

The cause isn't clear, but quite possible his rear shock collapsed. A locked rear wheel will cause that sort of snake.
http://www.ukgser.com/forums/showthread.php?t=325325&highlight=desert+crash

 

[Pukmeister]

I didn't use heat to remove the bush but it is an interference (press) fit so heat will help.

The same bush was reused without any issues.

Must have been some 'abuse' on bad roads to snap a shock in half??

 

[glennis]

Cheers for advice guys

I had a new bolt made and the threads are a little thicker than the OEM bolt. The bolt is a class 12.9 bolt and has been torqued up to around 45nm and has a generous amount of thread lock applied. The German mechanic, an ex BMW mechanic, who made it seems to think this will be okay. The recommended torque setting for this bolt is 58nm. So I think it should get me back to Europe where I can then have it repaired properly.

Does anyone know what class the OEM bolt for the lower rear shock should be? A 10.9 was installed on my bike!

With respect to the shock snapping in half, it gave way in India where they like to place unmarked speed humps on motorways but to be honest I haven't treated it too kindly over the last few years on this trip! The shock was an Ohlins with 30km's on it. I did contact them and explained that the piston rod had snapped where it enters the bottom mount. They offered to replace the shock at half the cost as it had not been serviced in the recommended 20k service interval...what I should have done was pop down to the local Ohlins service centre in Mongolia . They shipped another one out to me which I though was pretty decent.

A quick off topic question: Why does BMW use V10 fluid in the clutch system? For a bike that's built to travel the world, this stuff is impossible to get outside of the western world...pain in the butt!

Thanks again guys

 

[Pukmeister]

Vitamol V10 is a fairly basic mineral oil. I bet you could temporarily use something else as a hydraulic medium. I believe it is the same as Citroen hydraulic suspension fluid but any low viscosity mineral based hydraulic oil would probably do.

At a push I would chuck motor oil in it to get me home. Anything but DOT fluid which will wreck the seals.

I doubt the OEM shock bolt needs to be any more than 10.9 as it isn't a high tensile application, all the forces are in shear.

You could have asked the German fella to make two bolts whilst he was tooled up for it so you could carry a spare just in case you have further issues on the ride home.

Be sure to check the existing bolt every few days. Good luck with the ride home.

 

[DrFarkoff]

The bolt is a class 12.9 bolt and has been torqued up to around 45nm and has a generous amount of thread lock applied

I'd be careful here! grade 10.9 is a high tensile grade,

grade 8.8 is normal tensile strength for fasteners (unless its stuff from china where I have seen 4.6 )

Grade 12.9 is higher again

If I recall correctly from my time in QC on gas and petrochemical sites here and overseas the new bolt may have great tensile strength but the higher the value the less resistant to sheer forces

Please check it out before you go really hard on it!

 

[Pukmeister]

You ideally want low ductility and high toughness. 10.9 grade should get you home.