Quote:
Originally Posted by Tom F2
Factory manual, page E101, says to oil the threads of the bolts lightly and tighten them to 150 Nm. I think that all of the Porsche torque specs are for lightly oiled threads, unless otherwise specified. This is for an early, six-bolt flywheel.
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The 6 bolt flywheel does, however, use an M12 Bolt, hence the higher torque.
Discussions about Torque to Yield fasteners (TTY) and Porsche engines continues to surface and in general this category of bolts doesn't seem to be very well understood.
In recent Engine Builder magazine review of TTY fasteners by Mahle-Clevite there are several inaccurate statements which don't help.
Basically TTY fasteners are tightened beyond their yield point and therefore deform permanently.
There are two reasons for using fasteners in this manner.
The first is when the clamping force needed for safe operation is greater than can be produced by an 'elastic' bolt. This used to be the case for the con rod bolts in early 2.0 litre engines but this changed when the rod was re-designed on the 2.2 litre engine.
Mahle-Clevite suggest that when a bolt is stretched beyond its yield point it loses some of its elasticity - this just isn't true. Even the most basic examination of a material's tensile stress/strain curve will show that this statement is incorrect.
The second reason is when nut runners are used and several fasteners are tightened at the same time.
By reducing the initial torque and then tightening to an angle the preload in likely to be more uniform with less scatter which is a good think.
I don't believe that Porsche flywheel bolts are TTY bolts and can be safely re-used unless they show signs of damage.
If you always use new bolts then the scatter of preload will increase due to variation in thread surface finish.
With regard to balance - I am not use small variations in the weight of lock wire will influence balance at the centre of the flywheel.
The bolts, for example are not match weighed and will vary.
I used to manufacture very high speed diaphragm couplings (100 000rpm +) and we used to critically match weigh bolts.