Alternator "pivot" bolt failure

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zakimodo

zakimodo

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I have had two bots fail in as many years. The first failure I blamed on re using a 40+ year old bolt that was not in the greatest condition to begin with. This said, its held in there for nearly 10 years. This second bolt (new) has only been in for a year. Both bolts broke right at the cylinder head. I am at a loss as to what is causing this. Everything is in alignment, there is no binding of the bolt when I install it. Anyone else experienced this?

20200502_160447.jpg
 
Post some photos of the install. perhaps it is a vibration issue. I have never seen one fail on any car ( any manufacturer) I have ever owned
 
How tight are you running the belt?

Too tight can lead to alternator bearing wear and a broken bolt also.
 
Looks like a POS grade 5 bolt. Use a grade 8.
 
What year?
What Engine?

Small Block... Some of the brkts may be different year to year but the basic mounting should look like this.
20180919_182118.jpg
 
Dana67Dart said:
What year?
What Engine?

Small Block... Some of the brkts may be different year to year but the basic mounting should look like this.
Click to expand...

Car is a 74 duster, Engine is a 318 (78 block). I think the brackets are original to the engine. They are not original to the car (used to have a \6). Any way, setup looks like yours Dana67Dart.
Here is what I have in its current somewhat disassembled state.
20200502_193914.jpg
20200502_193902.jpg
 
I’ve never had one fail. It shouldn’t be under much stress and a low grade bolt should last forever... unless you aren’t running the outer bracket like in the photo.
 
In your photos you are not showing the spacer tubes. You are using them yes?
 
zakimodo said:
Car is a 74 duster, Engine is a 318 (78 block). I think the brackets are original to the engine. They are not original to the car (used to have a \6). Any way, setup looks like yours Dana67Dart.
Here is what I have in its current somewhat disassembled state.View attachment 1715521021 View attachment 1715521022
Click to expand...

Where`s the spacer ??-----grade 8 only when replacing any bolts .

posted before reading all posts !
 
A bent or out of true crank pulley can cause this. The belt gets tightened when the crank pulley is in a "loose' spot, then on every revolution when the belt gets tight, the bolt gets a little tug.

Start it without the alt belt and look at the crank pulley. Or just watch it closely while cranking.
 
Stock bolt is a POS grade 5 bolt, I've always replaced them with grade 8 bolts because I had some many problems with them breaking.
 
Have had 6 small blocks driven hard over 40 years and never broke a stock bolt. Chase threads lightly, lube threads and use a torque wrench
 
I had the same bolt break about 10 years ago too. It was the original bolt using the original hardware (spacers and brackets) the car rolled off the assembly line with (‘75 318). Thankfully there was enough of the sheared off threads left sticking out of the head I was able to remove without too much trouble.
 
Demonic said:
A bent or out of true crank pulley can cause this. The belt gets tightened when the crank pulley is in a "loose' spot, then on every revolution when the belt gets tight, the bolt gets a little tug.

Start it without the alt belt and look at the crank pulley. Or just watch it closely while cranking.
Click to expand...

This is interesting. I if I remember correctly, I put a new damper on a few months before the first bolt broke. I should be able to get a dial indicator on and check how true the crank puly is..
 
zakimodo said:
I should be able to get a dial indicator on and check how true the crank puly is
Click to expand...
I would check the insides of v grooves. Same thing could happen if the width of the v groove varies.
 
post 8 pic 2 - the bolt looks smaller in diameter than the bushing it goes thru in the alternator
 
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