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Old 09-09-2020, 09:50 AM
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andrewwynn andrewwynn is offline
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Part number for the Reinforcement Plate Bolts

Quote:
Originally Posted by Maruzo View Post
Well said and expertly backed up, Andrew. Thanks for informing us with your knowledge and your own experience!

CN90 said in another post that “head bolts, when you compare old vs new, are different. The old ones are longer from stretch. The stiffening plate bolts, they are big and fat, very much like a wheel lug. They don’t elongate with tightening. They will never break if you torque them properly.”

Any truth to his post?

Stiffening plate bolts use a nearly identical method to tension the bolt as head bolts and by definition will be longer afterwards. They are torque to yield installed meaning literally torque until they start to stretch past the point they will return to normal.

So the phrase about not elongating is patently incorrect. If they didn't stretch they weren't torqued to spec. (Also why they will absolutely snap in half after about five operations).

SP bolts are 10mm exactly the same as the head bolt on M54. Not sure if the same on M62, but on the ones I installed they didn't seem any different diameter than M10.

BMW wanted a way to get a consistent clamping force and since even using torque doesn't do a good job they choose stretch the bolt which achieves a very reliable amount of clamping force because when the bolt stretches it's fairly simple math of the yield strength of the metal and the cross section of the bolt.

When torquing to spec the clamping force will vary 10-20%, and if adding lubrication, maybe 40-50%.

Using TTY it won't matter if dry or oily because it's not turned by force but by angle, just has to be enough pre tension to get into the yield zone during the 90° part. It's a kind of fascinating method to get the job done really. They wanted 60,000# of force on the plate, not 35,000 ± 20,000 which is what would be achieved with torque wrench method.

When doing the destructive tests it becomes very obvious how it works by the 3-4th test as the torque to turn steps rising as you turn farther. Torque would rise to about 100 N·m until 40-50° and then just stay at 100 N·m as I turned the last half I could definitely feel that the bolt lost its ability to fight back and on the last time the bolt snapped before turning 45° into the 90° part of the tension routine.
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Last edited by andrewwynn; 09-09-2020 at 10:07 AM.
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