Initial torque, final torque?

[Twisties]

What do BMW mean when they specify an initial and final torque, and why is this done?  What is the proper technique for handling these situations?

 

I usually encounter this with crush washers and spark plugs.  In that scenario I presume it is intended to provide a brief moment for the crush washer to deform and seat before final tightening.  In these cases the initial and final torques are usually pretty close to one another, or even the same. I usually figure by the time I've reset the torque wrench it's good to go to final torque.

 

Right now I am replacing shocks and find that for the rear shock the bottom screw is installed with Loctite 243 (blue) and an initial torque of something like 20 N m and the final torque is almost 60 N m.... a large difference.   This seems to be very different.  What is going on here?  How long between initial and final torques?

 

Note that I put this in Motorcycle Talk because the question isn't bike specific and we don't have a general maintenance forum.

[dirtrider]

1 hour ago, Twisties said:

What do BMW mean when they specify an initial and final torque, and why is this done?  What is the proper technique for handling these situations?

 

I usually encounter this with crush washers and spark plugs.  In that scenario I presume it is intended to provide a brief moment for the crush washer to deform and seat before final tightening.  In these cases the initial and final torques are usually pretty close to one another, or even the same. I usually figure by the time I've reset the torque wrench it's good to go to final torque.

 

Right now I am replacing shocks and find that for the rear shock the bottom screw is installed with Loctite 243 (blue) and an initial torque of something like 20 N m and the final torque is almost 60 N m.... a large difference.   This seems to be very different.  What is going on here?  How long between initial and final torques?

 

Note that I put this in Motorcycle Talk because the question isn't bike specific and we don't have a general maintenance forum.

 

Evening Twisties

 

Good question & one for thought.

 

As a rule an initial torque then a final torque is used to get two parts to pull together evenly without warping.  

 

But on the rear strut that wouldn't seem to be a requirement.

 

While no one knows for  sure what goes through BMW engineering design, release, & development engineer's heads my guess would be that (2) step torque requirement on the rear strut bolt is due to the specification for using somewhat thick  grease or Lock-Tite on the bolt threads.  

 

The initial lower torque allows the grease or Lock-tite to ooze out of the thread mating area so the final torque would be true & consistent.    

This would be doubly true if a greased bolt is torqued into a blind hole as it allows the grease compression hydraulic force to decrease at a lower initial torque (grease ooze)  before the final high torque could hydraulically  crack the casting. 

 

 

[Twisties]

Thanks dirtrider!