ARP flywheel bolts, broken, anyone seen this?

[Poolman]

Anyone have any experience with ARP flywheel bolts breaking? Rebuilt 99 Cobra engine, NA, 375 RWH, running a few hundred miles. Had a strange noise from the clutch when the engine was running, trans in neutral, car stopped. Pulled the trans expecting clutch disc problem, but found five of the flywheel bolt heads snapped off, three were intact, one of those snapped on removal, two came out ok. The broken bolts screwed out of the crank by hand once flywheel was off. Crank is ok, flywheel seems ok, the noise was one of the broken bolt heads sliding on the clutch disk when flywheel was turning and disk was not. I was disgusted, didn't take any pictures last night. Any comments?

Bolts look like they broke in tension, head pulled off. Bolt shank is intact where it passes through the flywheel, no evidence of shear. Stock flywheel was installed and torqued by known and trusted engine builder. The arp bolts were new out of the package.

Things that make you go hmmm... and lighten your wallet. Should have just reused the oem bolts.

 

[NC85]

Bolts look like they broke in tension, head pulled off. Bolt shank is intact where it passes through the flywheel, no evidence of shear. Stock flywheel was installed and torqued by known and trusted engine builder. The arp bolts were new out of the package.

Things that make you go hmmm... and lighten your wallet. Should have just reused the oem bolts.

I've always had good luck with ARP hardware. Your case could be that 1 out 10,000 bad batches. It's also very difficult to rule out some sort of install error, however unlikely, unless you either performed the repair yourself or were literally beside the man each step.

 

[oldmodman]

The kind of shearing that you saw is generally caused by insufficient torque during assembly. Plus no Locktite on the threads.

Once the bolts are able to loosen just slightly the flywheel can shift back and forth. Eventually the side loading will snap off the loosest bolt head, then the next and so on.

As long as the bolt hole threads in the crank are OK and the holes in the flywheel are not elongated you can reuse both parts. just get another set of ARP bolts and tighten them with a torque wrench, in stages, using Locktite.

And it would be worth it for you to send the damaged and broken bolts back to ARP. If they detect anything wrong with them they will replace them for free.

I will say right up front that i have been using ARP bolts for over 20 years and have never had a failed bolt or stud from them. But anything is possible.

 

[bdcardinal]

Were they real ARP? There have been counterfeit ARP bolts on the market lately. They should have the ARP markings on the bolt head.

 

[OhIIICobra]

I don't see any grade 8 bolt that is properly torqued failing like that so quick at that power level. Maybe they were knock offs?

http://thecounterfeitreport.com/product/486/Bolts.html

 

[Poolman]

The kind of shearing that you saw is generally caused by insufficient torque during assembly. Plus no Locktite on the threads.

Once the bolts are able to loosen just slightly the flywheel can shift back and forth. Eventually the side loading will snap off the loosest bolt head, then the next and so on.

As long as the bolt hole threads in the crank are OK and the holes in the flywheel are not elongated you can reuse both parts. just get another set of ARP bolts and tighten them with a torque wrench, in stages, using Locktite.

And it would be worth it for you to send the damaged and broken bolts back to ARP. If they detect anything wrong with them they will replace them for free.

I will say right up front that i have been using ARP bolts for over 20 years and have never had a failed bolt or stud from them. But anything is possible.

I have had the same experience, will send back to ARP.

 

[Poolman]

I don't see any grade 8 bolt that is properly torqued failing like that so quick at that power level. Maybe they were knock offs?

http://thecounterfeitreport.com/product/486/Bolts.html

Bolt heads say ARP PRO. No alloy designation, maybe they are fake. Looked at the counterfeit link, also no dimple on the end of the bolt. Plus, there was no Locktite used. Maybe they backed out, or maybe not properly torqued, or maybe fake.

I can't remember where this set of bolts were purchased, but do remember they were in what looked like an authentic ARP package. There is a local shop where I bought some of my parts, ARP main and head studs. I am going to take them over there, see what they say. Then send to ARP.

Crank looks ok and flywheel looks ok as well. Engine ran very little after heard the odd noise.

Thanks for the info guys. Live and learn.