2004 V6 3.9 Mustang Convertible

[Richmond]

Balancer needs to be removed, inspect both it and the crank / crank key for damage. If your lucky, bolt just fell out, odd time they break off in the crank itself. There are options if it's broken off inside. If the balancer is indeed damaged, gotta be 110% sure you get the 3.9L version.

I inspected the crank pulley and it looks good. no signs of wear. The crank also looks fine. You mention a crank key , does that fit in the slot on the crank pulley. If so what is it made of? Would you happen to have a picture that I could reference.

[ZR]

Key fits into the slot cut in the crankshaft, prevents the balancer from spinning.
Had the balancer slipped part way off the crankshaft?

[Richmond]

yes It had moved perhaps between a 1/4 inch to 1/2 an inch.

but I believe the key is still in place.

the slot on the balancer looks to be in perfect condition.

[ZR]

That would explain loosing crank trigger signal.
Next is balancer off to see how far below the crank face the bolts broken.

[Richmond]

The bolt is recessed at least 1/2 inch. No threads are exposed.

[ZR]

Do you mean recessed 1/2 below face of the balancer or face of the crank itself?

[Richmond]

the face of the crank.

[ZR]

I'd think your resigned to a large easy out and at least some heat (may have loctite). Drilling perfectly centered / straight is a must, as is using a quality easy out and not overheating. Be sure to pack around the seal with a rag to prevent any chips from dropping into the engine.

[Richmond]

I followed you instructions and I have removed the broken Crank pulley bolt.

everything looks great. crank pulley balancer, crank treads, etc. and I have a new bolt and washer .

But I still don't know what caused the problem. Is there anything else I can do to determine the cause?

[ZR]

My opinion, poor quality torque to yield bolts + engine harmonics. If it was mine, would much rather see a conventional torque ARP or similar bolt in it's place.