Re: [ST] copper grease and torque settings

["Young, Mike" <myoung@xxxxxxxxxxx>]

>Problem is for something like piston rod head bolts, the type of grease
>to apply is specified and more and more it's a torque/angle procedure.
>It's the case for the 955i engine.
>But most of the cases, nothing is added to the torque spec.
>That would probably mean it's for a brand new perfectly clean thread
and 
>bolt .
>Hence for some critical nuts, getting a new one each time is specified.
>The surface shape of the thread would probably change after the first 
>fastening.


Current engineering practice on critical threaded fasteners is tending
toward "Torque to Yield" fasteners. These fasteners (bolts usually) are
torqued to a specified clamping load and then given an additional
fraction of a turn which literally stretches the bolt into the "elastic"
part of its strength. The steel alloys used in high strength fasteners
often perform better from a fatigue standpoint if the static load on the
fastener is in the elastic range. 
I've seen torquing instructions on connecting rod bolts that call for
the fastener to be tightened to a specific (and relatively low) torque,
measured for length, and then tightened further until the fastener is
some small amount longer. Those same instructions call for discarding
the fastener any time they are removed, and replacing them with new. 
With all that said, I've personally cheated with "single use" head bolts
and used them twice. I don't think I'd feel comfortable using them three
times, but needs must on occasion. 
I don't think the wheel fastening in single sided swing arm type
applications fall into the single use "torque to yield" category.
(Having to replace the spindle every time you took the wheel of the bike
would make for very unhappy owners, and eventually very low sales
numbers.)

Mike 
'06 ST
Seattle


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