My drive sprocket on the crank has wear in the keyway, key is worn on one side, and timing trigger (stamped steel) is very sloppy in the keyway. Does anyone know what causes this? Its relatively low mileage.

 

Can well imagine that this is pretty tough on controlling the timing.

Most times Key Way elongation is from a constant jarring, or repeated impact or lack of smooth operation. I'm gonna guess that the side that beat up is the on the normal rotation direction side.

someone may have placed a improper size key into the crank shaft, giving it slightly more wiggle room and over time it beat itself up, making for even more room. Like from a prior repair? (example 5/16 vs 8mm, dimensionally very close, .3125=5/16 & .315=8mm) but that is remote.

Another possibility is that steel on the drive sprocket was not tempered or hardened correctly to stand the abuse of the impulses of the firing order. Might be a superseded part number for better quality or after market part?

might want to double check the flywheel if it is on tight enough or whether your compression varies widely from cylinder to cylinder.

are you heavy on the throttle and brake? or... have you had a lot of back fires from the engine? was the harmonic balancer damaged or loose?

 

The key way on the crank in my mustang failed that was the thing that made me rebuild the engine. It can be fixed but the crank has to come out of the engine to do it.

 

Only complaint I had about the harmonic balancer was there was a stripped bolt hole for the boost ring, not sure if that over time could cause the issue but I will definitely replace it. I tended to be heavy footed. The key way in the crank is tight still, all wear was on easy to replace parts (key, sprocket, trigger) , there is literally a smooth cup mark indented into the key just above where it sits flush to crank. The trigger wheel deposited material to crank.

The crank was brand new when this crank went in, hopefully it was correct back then. Backfiring, well there was a few times but not common. I have ordered an accufab trigger wheel, hopefully its up to snuff but obviously I want to nip the true issue this time.