failed flywheel bolt

[Peterfrans]

Last saturday I was finaly ready to put the engine back in the car, only needed to bolt on the flywheel and clutch and join the engine and gearbox. Easy job until a brand new flywheel bold said "ping" when torque-ing it. See the result in the attached pictures. Not trusting the rest of the bolts I decided to re-use the old bolts (with a little red loctite).
I am curious what could have caused this. The bolts were porsche originalteile from my local pc.

[Jim Sims]

Bolt the wrong length? Lubricant on threads (can result in over stressing the screw during tightening)? Improper manufacture and/or storage? Exposure to hydrogen? Based on the images posted I would conjecture having the threads lubricated caused the problem. Jim

[sayah]

Why does the interior of this bolt look cast?

[mwildt]

I'd bring it back to wherever you bought it. If it's Original Porsche parts it's warrented for 2 years (probably not covering bolts, but who knows), and if it's in a bad batch they mostlikely would like to know.

Michael

[Peterfrans]

The bolt is the correct length, was not lubricated except for some loctite. I took it out of the plastic bag just before using it. My best guess is a manufacturing defect. I told the parts guy at the pc today, but he was not very interested ...

[ChrisBennet]

I'd check the torque wrench as well.
-Chris

[Wayne 962]

Originally posted by sayah
Why does the interior of this bolt look cast?

It's probably cast. I can't think of any other way to make a bolt?

Check the torque wrench - you may be overtightening this...

-Wayne

[Peterfrans]

Afik bolts are forged with rolled threads. Cast would make it brittle.
I am going to check the torquewrench tomorrow

[Jim Sims]

All high strength, steel fasteners start from cast ingots or continuously cast bar. These ingots or bars are then forged, upset, rolled and heat treated to make the fasteners.

[Porsche_monkey]

Take the bolt to a heat treat shop and ask them to hardness test it. That looks like a brittle failure to me. Possibly a 'quality control' issue?

[David]

Brittle fracture since it failed instantly. It better not be cast. These bolts should be made from bar stock, have rolled threads and forged heads. It appears that this bolt was drilled too deep for the socket head, this reduced the cross section to where the specified torque was too much. A good material lab could section the bolt and see if the head was in fact forged, I doubt it was. At the minimum, material should be checked. It should be chrom-moly steel with about 0.4% carbon and the hardness should be around 37-40 Rockwell C.

[Porsche_monkey]

What David said.

[ChrisBennet]

The dealer may not care but I bet someone at Porsche does. Where exactly did you get the bolt i.e. what dealer?
-Chris

[Porsche_monkey]

The more I think about this the more I agree with what (I believe) Chris is implying. Looks like a 'knock off bolt'. That failure would be what I would expect from a low priced 'off-shore' bolt. I've heard stories of cheap bolts being stamped as 10.9 etc when they are just plain crap.

[Peterfrans]

I checked my torque wrenches today and they are not out of spec. I think the drilled to deep theory makes the most sense.
I bought the bolts at my local porsche dealership here in the Hague, the Netherlands. I think I will email porsche and see if they care.

[AndreasLanghoff]

Yes, this happened to me recently. I bought my bolts from mittelmotor in Germany. One bolt broke as yours, so I did reuse the old ones.
Mittelmotor say that they sold me original porsche bolts and refunded me quickly.
My old original bolts were only about 3mm deep in the 12point groove with little more in the centre. The new "mittelmotor" bolts were 4mm deep and very conical in the centre.
Please don't nail me down on the exact measurements as I have deleted the relative files already.


Good luck and ask for a refund!

Andreas

[TimT]

The appearance of that bolt doesnt look like rolled threads....

It appears that the failure started at a stress riser...

The grain structure, and obvious poor machining lead me to believe that the bolt is a cheap counterfiet, sold as OEM..

Caveat Emptor..

You can see that the threads are cut..and where the cutter pulled away from the bolt head..

just MHO and im sure others will disagree

[TimT]

Also, bolts are kinda my business and livelyhood....been rehabing bridges here in NYC for over 20 years...

the bolt failure shown could also be a "head burst"

[garibaldi]

That doesn't look like any Porsche flywheel bolt I have seen.

That bolt looks like it was made in a wok.

[garibaldi]

here is an old used flywheel bolt from a 911 i just found in my toolbox.