Flywheel torque question

[OldTee]

Yesterday while torquing the flywheel bolts on my 79SC to 88 ft lbs my socket failed. I had used 271 loctite. It failed at 60 ft lbs. Should I remove the bolts and re loctite em, or continue to 88 ft lbs with my new socket?

[rsscotty]

I think the proper torque for a nine bolt flywheel is 68 ft. lbs........not 88.

[OldTee]

Looks like we have a slight mistake in Dempseys book. text on page 139 is ... and for the 1978-89 911s is 120NM (88 ft-lbs) Figure 3-42 shows the process ...

AND Appendix A page 200 says Flywheel bolts (1978-89)..M10X1.25 90NM (66.3 ft lbs.

Porsche Technical Specifications (little white book) says 90NM

The Bible has a mistake........

[304065]

of course the later bolts, being smaller in diameter, take a lower torque setting.

You should remove the bolt, throw it in the garbage, and use a new bolt.

The flywheel bolts are "torque to yield" type which means they are stretched beyond the yield point. Once stretched to 90NM they cannot be used again. Once stretched to 120NM they REALLY should not be used again.

Did you feel the sickening feeling when they stretched, almost like they were going to give way and break? That's them elongating, it's amazing that you can feel it in your hands.

For the best explanation ever of TTY I quote the board's own Jim Sims:

When the stretch bolts (also known as torque to yield fasteners) yield they do not lose their clamping force but rather due to the shape of the metal's stress/strain curve the clamping force stays rather constant after yielding even as the bolt is turned a further half turn or so. This means that the required clamping force will always be achieved despite the variability of thread friction. The variability of thread friction causes the amount of bolt tension for a given torque to be a variable; the greater the friction the more torque is used to overcome it and therefore there is less to stretch (elastically and plastically) and tension the bolt. If the bolt is taken to yield then the clamping force needed to preload the bolt and rod end cap joint to prevent fatigue failure is guaranteed without regard to friction. Torque to yield fasteners should not be reused as the torque to yield procedure causes the bolt or screw to "neck" or reduce it's cross-section (at the cross-section of a thread root usually). Therefore the second time the bolt will not achieve the same clamping load as it necks even further during the second tightening. Torque to yield fasteners are typically made from less expensive metal (compare price with Raceware and ARP), are not as thread friction sensitive and may be correctly tightened with a simple torque wrench. This may be compared to a more elaborate bolt stretch measuring rig or the repetive tightening (five cycles of tightening and loosening to reach a constant and stable thread friction) sometimes required to achieve the correct tension in the more exotic superalloy rod bolts. Cheers, Jim.

[304065]

You aren't re-using your old $2 bolts are you. . . .

Fly wheel bolts conventional thinking...why

[OldTee]

No, all new bolts. I have plenty of time, will order another set. Thanks,

By the way, how does 271 loctite work? Is there a cure time?

[304065]

http://tds.loctite.com/tds5/docs/271-EN.pdf

[Eagledriver]

Is there a source that says these bolts are torque to yield. I can see why rod bolts are and they look delicate with the neckdown feature but flywheel bolts don't look like that. I've reused them and am not worried about it. I would however replace the ones you over torqued.

-Andy

[304065]

Andy,

Try it yourself. Buy a new screw from our host here for $2 and measure the length with a micrometer. Now install it in the flywheel and tighten to the correct torque. You will feel a sickening loose feeling while doing this, almost like you're going to shear the screw in half. This is the screw stretching beyond the yield point. Now take it out and measure it again, you will find that it has permanently stretched.

Perhaps we all make too much of these things (I for one plead guilty) but my own view is that if your flywheel comes loose, wouldn't you gladly pay $18 to avoid an engine drop, separation of the gearbox and engine and possible buggery of the crank snout? I certainly would.

More info here: Thread locker for G50 flywheel bolts?

[Inter911]

So what is the torque for the 6 bolt flywheel? I just did mine this weekend and used 110 ft lbs. Hopefully that was correct.

[304065]

Yes, six-bolt is 150NM (110 ft-lb)

[Eagledriver]

I used new on my 2.7 and didn't notice the effect you mention. That being said there have been instances of 6 bolt flywheels coming loose so an abundance of caution is a good idea. For 9 bolt flywheels it would be overkill IMO.

-Andy