Weird alternator bolt issue.

[mk2tmr2]

Last year about this time my alternator belt started squealing a bit. I chalked it up to cold weather but when I went to tighten the belt I found out that what had really happened is that the bottom mounting/pivot bolt had backed its way out. Fortunately, there's (what looks to be) an intake manifold support bracket behind the bolt that stopped it from backing all the way out. But the alternator was loose enough that you could wiggle it around manually. No big deal - it was a 2 minute fix to get the bolt back in and everything torqued down.

Yesterday the belt started squealing uncontrollably again. This time, older and wiser, I checked it and - yup - that dang bolt backed out again. But this time it wouldn't thread back in. I can push it all the way in until the bolt head seats. No threads.

I can see the threaded hole on the near side of the alternator and I don't see sheared-off bolt threads in the hole. So I'm at least hopeful that I don't have to re-tap anything. But it just seems very strange that such a low-stress bolt would A) back out all the way on its own and B) lose its threads.

Did this ever happen to anyone else?

[black roadster]

No.

[MadMerv]

Not on a Miata, but it sounds like you may have a bent shaft on the alternator which will cause it to vibrate and slowly back the bolt out.

[mk2tmr2]

Not on a Miata, but it sounds like you may have a bent shaft on the alternator which will cause it to vibrate and slowly back the bolt out.

I thought that too. But when I watch the pulley spin it's solid as a rock. No shimmy.

Could be overall motor vibration what with the stiffer motor mounts.

[mk2tmr2]

No.

Very nice. I didn't use LocTite on this bolt because it doesn't seem to be one that would need it. This bolt gets loosened every time you adjust the alternator. Typically adjustment bolts don't get loctite'd (at least when I'm doing the wrenching.)

[boisking]

But this time it wouldn't thread back in. I can push it all the way in until the bolt head seats. No threads.

Sounds to me like the threads are stripped in the mounting "ear" on the alternator housing. Being that the alternator housing is aluminum and the bolt is steel, it is not likely a stripped bolt. Cheap fix is to get the alternator onto a bench, were you can more clearly inspect the damage and install a re-thread kit (a.k.a "heli-coil"). That is unless, of course, you have a free-replacement warranty on an aftermarket alternator.

[mk2tmr2]

Sounds to me like the threads are stripped in the mounting "ear" on the alternator housing. Being that the alternator housing is aluminum and the bolt is steel, it is not likely a stripped bolt. Cheap fix is to get the alternator onto a bench, were you can more clearly inspect the damage and install a re-thread kit (a.k.a "heli-coil"). That is unless, of course, you have a free-replacement warranty on an aftermarket alternator.

makes sense. When I looked at I last night the threads in the alternator that were visible from the front looked fine. But as the bolt threads don't necessarily come all the way through, it may just be that these are the only remaining threads in the housing.

For the meantime I made a sweet retainer clip out of wire coat hanger that is doing a fantastic job of holding the pivot bolt in place (seriously). This alternator was on the car when I got it almost 20k miles ago. If it's old enuf for threads to fail I'll probably just put a new one on if it comes down to it.

[creek]

Gremlins! Seriously though, a little loctite of the removable variety wouldn't hurt. Lots of vibration there.