Torque for driveshaft 12 pt. bolts (short)

[dziadzu42]

The Haynes manual  says 38 - 42 Nm  or 28 - 31 lbf ft for "all other models" the catagory  which I believe  my 79  r65 is in.  But then in the text  it  says " on all models the torque setting is specified to take into account the leverage which results from the use of the special crowfoot-type spanner BMW tool no. 00.2.560."  
I will be using a Northwoods Airheads torque wrench adaptor and they give a formula to calculate what your torque should be. But are the torque figs. above already adjusted for an added length?  Also they look kind of high. The Northwoods Airheads people say 25 to 27 ft.lbs?  which, with his torque wrench adapter in use, is lowered to 22.7 ft. lbs (with a torque wrench that is 14.5 in. long.)  Can someone just tell me the proper torque and I'll figure out what my wrench should be set at?

[Barry]

I think the Haynes figure is too high. I'd go with 25 -27 ftlbs on clean unlubed threads which several other sources besides Northwoods would support. Some use a drop of Loctite blue but others omit that and don't have the bolts come loose.  

[Tony Smith]

My thoughts

Clean dry threads.
New "torque to yield' bolts - no washers.


The thing with "torque to yield" bolts is that it is obvious when you have reached max torque and the bolt is "yielding".


I do mine with a 12 point ring spanner and nip all four bolts up to "firm" evenly and in three stages, I then apply more force on each bolt in turn until I feel  it yield and stop immediately.


My calibrated fingers tell me that this is around 20 ft/lbs.

In my opinion these sort of bolts should be specified as % of a turn past yield and not by a torque figure.

[georgesgiralt]

Hello !
My '85 factory manual says 40 Nm. As I'm a metric man, I let you do the conversion in Imperial units.
Beware that the adapter you put on the torque wrench can, if misplaced, cause a change in the setting of the torque wrench...

[mrclubike]

 If you put the torque adapter on at 90deg to the wrench it will read as if it was a normal socket installed.
No need to do any calculating  

That being said I use the method  Tony uses with a little blue Loctite

[Tony Smith]

if you must use a torque wrench and a 'crows foot' adaptor, here is some helpful info

https://www.motorcraftservice.com/renderers/torquewrench/wrench_formula_main_en.asp

[dziadzu42]

Tony,  so when you "feel it yield", does it begin to turn more easily?  And is that the point you stop and do no further turning?

[peteremc]

I'm also very interested to know what indication(s) there may be when you reach the point of "yield". I'll be doing this in a couple of weeks (I hope) as part of reassembly, and probably will not have the use of an adapter to measure torque settings for tightening these tricky little fellas.

[Tony Smith]

How do I know when the bolt is "torqued to yield'?


A deceptively simple question that is very hard to give an answer to. I'll start by saying it is one of those things that when you do it you will say "Ah Ha!, so that's what he meant".


As you tighten a bolt, the force requirement to continue turning increases, with a bolt that is designed to be torqued to yield you will reach a point where the force required to continue turning the bolt more or less ceases to increase, that is when I stop.

[Barry]

You might want to practice with some spare bolts to get the "feel"    Tony will have been acquiring this skill for many many years so a little practice could be in order.

I know what he means by you'll know it when you experience it and I've never stripped a thread in my life. For what it's worth I think that getting close to a torque setting by feel is quite difficult. If you can get within 10% then you are doing remarkably well.  Torquing to yield is a different thing as that is approx. 25% past the usual torque setting and is the point where the relationship between applied torque and stretch in the bolt changes.  That's why you can feel it.

Best illustrated on this diagram where the yield point is the start of the top curve or the start of the plastic range where the bolt will no longer return to it's original length. That's why they are reckoned to be single use. For stretch bolts the aim is to get somewhere between the yield point and the ultimate tensile strength i.e. not past the top of the curve. The downwards part of the curve nicely answers the earlier question of what's happening when the bolt gets easier to turn. That would mean you have gone much too far and the bolt is about to fail.

[dziadzu42]

Thank you gentlemen for this information. I had never heard of the expression "torque to yield" before, and will try this on some bolts.  As for the driveshaft 12 pointers, I believe I will stick to a torque wrench, (unless I really get it down with scrap bolts) !!!

[Tony Smith]

Thank you gentlemen for this information. I had never heard of the expression "torque to yield" before, and will try this on some bolts.  As for the driveshaft 12 pointers, I believe I will stick to a torque wrench, (unless I really get it down with scrap bolts) !!!

The ideal bolts to practice with are the old ones - if you break a couple you will be adept at knowing what the "feel" is. Hogging down on some conventional bolts is much harder as they will simply break.

[peteremc]

Thanks for the info gentlemen. A lot of very helpful knowledge and experience here.

[suecanada]

Sorry for the possibly stupid question but having stripped a few bolts in my time despite being weak of hand, I am wondering what is actually "yielding" here...the bolt thread or the thread in the hole? How would either be any good anymore? Is it the feel you get when you crush a "crushwasher" then stop turning??

[Barry]

It's not the thread in the hole only the bolt that may yield or permanently stretch in this case which is why they are designed to be one use bolts.  There has been some debate on various forums about whether or not these bolts can be re-used. The augment goes that if the bolt really did yield the thread pitch would now be slightly longer and it wouldn't turn freely. Conversely if your used bolt does turn freely then it can't have stretched and therefore can't have reached the yield point. Some people do re-use them on this evidence.

As far as reusing bolts is concerned my take on it is that this is a critical high load application because there is a safety implication if these bolts came loose.  Because of this the torque setting is set higher than the typical 75% of  yield. Given that torquing a bolt isn't a precise method of applying load to an accuracy of better than + or - 25 %  then even when the correct torque is applied some bolts may yield and some may not.  So the bolts may be designated one use only but if they haven't yielded and stretched then there is no good reason not to re-use them.