Cylinder head studs
 

Whilst torquing the cylinder head studs on our two R80GSs at Mr. Yut's in Bangkok, both bikes suffered a stripped thread. Andreas also had the same problem in Iran on his R80GS.

Seems to be quite common as there is an oil feed within the actual thread where the upper exhaust side stud locates

More of the legendary BMW reliability or are we missing something?

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Sean

Bit of both...

This isn't a rare problem unfortunately, and it always seems to come down to slightly overzealous torquing - or worse, bad calculations in translating from NM measurements to ft-lbs, or using Nm numbers in place of ft-lbs.

The manual specifies:

35 + 4 Nm OR
25 + 3 ft-lbs.

Note some manuals say 24-27 ft-lbs.

Obviously confusing the numbers and doing bolts up to 35-39 ft-lbs is going to do the threads in.

Not as obviously, going to the max torque listed of 39Nm or 28 ft-lbs is too much.

Experience on airheads says that safe torque is 24 ft-lbs or 35 Nm MAX.

A common problem is assuming that slightly tight will be good, so we always torque down to the high end of the range - I did so for years myself. Bad idea. And torquing down in three stages as the manuals suggest is also a VERY good idea as it prevents one bolt from being overtorqued due to the leverage effects of uneven torquing.

Who knows when the bolts were overtorqued, but it's certain that they were. However, knowing why doesn't help much when it's too late.

The fix is fairly simple, but difficult to carry out well. Just use helicoils or inserts, whatever seems appropriate and is available. It is absolutely critical to get them in straight, and also not block the oil passages.

A hand drill aligned by eye with the greatest care and assistance will result in a destroyed engine block - you have been warned.

It must be done with a jig or drill press locked at exactly 90 degrees to the surface. A very long drill bit drilling down through a cylinder bolt hole is possibly a way to do it accurately enough, if it's bolted down with the other three bolts, and a sleeve to hold the drill bit exactly in the centre of the bolt hole at the top.

Use Loctite on the insert or helicoils to ensure they don't back out or loosen. Make sure the inserts are long enough with lots of thread depth - two may be required.

Good luck, and take your time and be careful!!


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Grant Johnson

Seek, and ye shall find.
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One world, Two wheels.
www.HorizonsUnlimited.com

Hi Sean,how's it going??Good to see you out there doing it!!
I had lots of problems on an r100RT that had done in excess of 200K miles.
It doesn't help if the engine gets overheated especially if the bolts are at the top end of the tightening scale.Like Grant says:if you are worreid about not being able to get the hole square,then don't even attempt it!Use a greased rag behind the hole to catch the swarf from the drill,but also remove the sump to ensure there is none in the crankcase too.
Basically you will only have one chance!!
Good luck.
Bailey.

Hi chaps

Thanks for the info.

After double checking with my mate, we reckon that Mr. Yut was torquing our nuts to 42Nm! His information was from a book, which was obviously out of date. No wonder the threads stripped and 'he has seen the problem quite a few times'.

Anyway, we let him sort the problem out and he had new studs made with a larger thread and fitted them. Not sure how he drilled the hole, but so far all now seems AOK for a total cost of about $100. Not the end of the world by any means.

I also found info from the BMW owner's club of america which categorically states 35Nm maximum and ALWAYS slacken the nuts before torquing them or the studs WILL pull out.

Thanks again from Phnom Penh.



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Sean

Sean,

Just to reassure you, I had the same problem before I left the UK and on the road (also in Bangkok, strangely enough). I got Mr Yut to put a couple of inserts in and it's still going strong!!

Harvey

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no pipe, no slippers
www.chasingthesun.org

Hi Harvey.

Strangely I'm back in BKK. Mr. Yut has replaced the second stud after it failed on the run from Phnom Penh to Chiang Mai.

I hope that this is the last one....

Happy travels!

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Sean

Hi chaps

Well, the first stud replaced with an oversize in BKK has pulled itself out, without any outside interference. Dodgy repair?

The bike is rattling badly, but i rode it from Darwin to Sydney like this, so I guess it's not terminal.

A local BMW shop reckon they can put in an oversize insert to marry with the oversize stud. We shall see, but I have so little confidence now. Will my BMW always rattle?

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Sean

[This message has been edited by Sean Kelly (edited 15 March 2003).]