knock sound cause found! Now, why did it happen?!?!

[SupraPieces]

I had this happen on my motor before, so for it to happen again is understandable. I was driving at speed when my motor developed a knock-like sound. I thought is was rod knock. Upon inspecting I found the crank pulley bolt was loose. As it turned out, the key had slipped and rounded out the crankshaft and pulley walls. I thought the crankshaft was supposed to be made of the strongest metals. For a pulley to slip a woodruff key and destroy the keyway on the crankshaft tells me the metal it is made of is not if the highest quality.

I fixed it by widening the keyway and using a larger key. But just today, I found the pulley loose again. I tightened it and drove home from work.

I also recently purchased a 1991 7mgte motor and during the teardown I found that it too had a slipped crank pulley... WTF!!!

Anyone have any explanation for this issue?

 

[IJ.]

195ft/lbs...

Once it's slipped the snout of the crank loses it's press fit and there's not much to be done to save it, the ATI SuperDampers while a bit expensive are a much tighter fit and could be an option to save the crank/engine.

 

[CyFi6]

People that dont properly torque the pulley bolt is the cause. Lots of people just impact that puppy on and call it good, but it needs to be torqued. Once it comes loose a couple things happen. First the pulley itself gets messed up- the inside surface gets widened and scarred. Then the keyway gets torn up so now the keyway wont properly locate the pulley. Then the snout of the crank gets beat up so bad that the outside diameter is decreased to the point that even a new pulley wont have a tight slip fit any more like it should. Finally, the threaded hole for the bolt gets widened from the pulley smacking all over the place so now it is unlikely that the bolt will hold torque without backing off. Once the crank, bolt, and pulley have been damaged, they all need to be replaced. There are ghetto repairs that may work temporarily, or expensive repairs that can work if they crank isn't totally chewed up, but in most cases a new crank, pulley, key, and bolt are in order.

 

[SupraPieces]

Ok... that makes sense as to why my problem came back... even though I torque to 195ft/lbs if it slipped before and widened the threads on the snout and therefore slipped again... but mybquestion is why does this happen to start with? Should the key and pulley get destroyed before destroying the crank?

 

[IJ.]

Ok... that makes sense as to why my problem came back... even though I torque to 195ft/lbs if it slipped before and widened the threads on the snout and therefore slipped again... but mybquestion is why does this happen to start with? Should the key and pulley get destroyed before destroying the crank?

People that dont properly torque the pulley bolt is the cause. Lots of people just impact that puppy on and call it good, but it needs to be torqued. Once it comes loose a couple things happen. First the pulley itself gets messed up- the inside surface gets widened and scarred. Then the keyway gets torn up so now the keyway wont properly locate the pulley. Then the snout of the crank gets beat up so bad that the outside diameter is decreased to the point that even a new pulley wont have a tight slip fit any more like it should. Finally, the threaded hole for the bolt gets widened from the pulley smacking all over the place so now it is unlikely that the bolt will hold torque without backing off. Once the crank, bolt, and pulley have been damaged, they all need to be replaced. There are ghetto repairs that may work temporarily, or expensive repairs that can work if they crank isn't totally chewed up, but in most cases a new crank, pulley, key, and bolt are in order.

 

[SupraPieces]

Ok good... I bought another crank for my build, a 6m one, with a good snout and keys in place... just wanted to.make sure it wasn't some shitty defect in the crankshafts

 

[IJ.]

Cranks are fine it's an owner/mechanic defect