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Discussion Starter #1
Hi folks , I'm new on the "block"....got an 86- 300ZX. The timing belt was removed & reinstalled without properley torquing the crank pulley nut. Approx one week later the engine seized due to the keyway failure on the crank pulley....any comments, observations, conclusions would be helpful.
Herb
 

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Discussion Starter #4 (Edited)
If the crank pulley nut was not tightened to specs , then when the crank nut came loose , the keyway would be the first to fail....& the engine stops.
Would like to know how many 300zx's have had keyway failures for any reason? Where could I get this statistic?
 

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ZPOOLNZ
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Umm the key is a weak point on the crank. But you mean the sproket not the pulley. What typically happens is not a torque spec issue, but a crank issue. That is just a weak point.
 

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bagwingca said:
If the crank pulley nut was not tightened to specs , then when the crank nut came loose , the keyway would be the first to fail....& the engine stops.
Would like to know how many 300zx's have had keyway failures for any reason? Where could I get this statistic?
Right, it might make the engine stop, but it is not "seized". Not unless something else went wrong. You might have some valve damage from the pistons hitting the valves, but I doubt your engine is seized.
 

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Discussion Starter #7
♣Zen31ZR♣ said:
Right, it might make the engine stop, but it is not "seized". Not unless something else went wrong. You might have some valve damage from the pistons hitting the valves, but I doubt your engine is seized.
You're right , it stopped. It did not over heat . Yes possible valve damage. There were alot of iron filings left around the crank bearing ....presumably from the gradual wearing away of the keyway.
Would imagine that a poorly torqued nut would set up the keyway for imminent failure?
 

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Discussion Starter #8
JAMESZ said:
Umm the key is a weak point on the crank. But you mean the sproket not the pulley. What typically happens is not a torque spec issue, but a crank issue. That is just a weak point.
I am not aware of keyways failing if nuts are properly torqued
What do you mean by a "crank issue"
 

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ZPOOLNZ
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bagwingca said:
What do you mean by a "crank issue"
I mean it is a weak point on the crank; sometimes the crank fractures/beaks/chips in at the key causing the problem you described. And yes it is usually caused by improper torque, but it is also just a weakpoint/bad design. Do you mean sproket? Because there is no key for the pulley.
 

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JAMESZ said:
I mean it is a weak point on the crank; sometimes the crank fractures/beaks/chips in at the key causing the problem you described. And yes it is usually caused by improper torque, but it is also just a weakpoint/bad design. Do you mean sproket? Because there is no key for the pulley.
They kinda share the same keyway. At least on my car. The crank pulley, being used for timing light orientation, must always be in a specific clock position, so it has a keyway slot as well.
This sort of thing is a bit unusual, I don't think I've ever seen a keyway failure that didn't involve some other problem. Usually either the key is the wrong size, or it is not completely in the slot, or the size of the key slot in the pulley or sprocket does not match the size of the key slot in the crank. I would suspect at least one of those as being the problem in this case.
 

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Discussion Starter #11
♣Zen31ZR♣ said:
They kinda share the same keyway. At least on my car. The crank pulley, being used for timing light orientation, must always be in a specific clock position, so it has a keyway slot as well.
This sort of thing is a bit unusual, I don't think I've ever seen a keyway failure that didn't involve some other problem. Usually either the key is the wrong size, or it is not completely in the slot, or the size of the key slot in the pulley or sprocket does not match the size of the key slot in the crank. I would suspect at least one of those as being the problem in this case.
In my case, the timing belt was removed ....reportedly pried off with a little screw driver ie. ( only the upper housing was removed) in order to fix a leak on the right manifold & reportedly put back the same way. This, as I later discover, by an unlicensed mechanic. And approx one week later, the key way goes & the engine stops.
 

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bagwingca said:
In my case, the timing belt was removed ....reportedly pried off with a little screw driver ie. ( only the upper housing was removed) in order to fix a leak on the right manifold & reportedly put back the same way. This, as I later discover, by an unlicensed mechanic. And approx one week later, the key way goes & the engine stops.
I'd say it was an unrelated incident. One will not cause the other to happen, in this case. Something else happened.
 

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Discussion Starter #13
♣Zen31ZR♣ said:
I'd say it was an unrelated incident. One will not cause the other to happen, in this case. Something else happened.
The engine had starting difficulty after the job. The timing could not have been checked on the lower pulley since the claim was that the housing was not removed. These engines need to have precise timing.Perhaps the valves were damaged due to the improper timing. Other opinions seem to suggest that the housing was removed & the lower nut not tightened properly
 

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bagwingca said:
The engine had starting difficulty after the job. The timing could not have been checked on the lower pulley since the claim was that the housing was not removed. These engines need to have precise timing.Perhaps the valves were damaged due to the improper timing. Other opinions seem to suggest that the housing was removed & the lower nut not tightened properly
Valve damage won't cause keyway failure. Neither will the crank bolt being either loose or missing entirely. Truthfully, the crank bolts only job is to push the crank pulley onto the crank snout. In essence, at that point you conceivably could remove the bolt and run the engine without it and have no damage. The crank pulley is a very tight press fit, it will not move again once installed till acted on by an outside source such as a pulley puller.... I don't suggest doing this, this is just for an example. Ive already stated above what will cause keyway failure.
 

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Discussion Starter #16
♣Zen31ZR♣ said:
Valve damage won't cause keyway failure. Neither will the crank bolt being either loose or missing entirely. Truthfully, the crank bolts only job is to push the crank pulley onto the crank snout. In essence, at that point you conceivably could remove the bolt and run the engine without it and have no damage. The crank pulley is a very tight press fit, it will not move again once installed till acted on by an outside source such as a pulley puller.... I don't suggest doing this, this is just for an example. Ive already stated above what will cause keyway failure.
Thanx for sharing your opinions.
 

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bagwingca said:
Thanx for sharing your opinions.
I've seen some pretty screwy stuff in 15 years under the hood. I saw a situation where someone had apparently lost the key for the crank slot, and superglued a piece of wood in place to serve as a key...... I also saw where someone had used a piece of a hex key tool as a substitute. It was also the improper size, and caused the exact same situation as what you are describing above. Eventually it fell out and allowed the crank sprocket to spin......
 

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or you do what i did and not pay attention when putting the pulley back on causing it to hit the key way and break part of it off. So I lined it back up properly and tightened down the pulley. It still runs like a champ and I drift it every day on my way to work, so well see what happens when it breaks lol.
 
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