Didn't think this would happen again

This is the second time this bolt snapped off clean in the head. Whats going on? Why did the same bolt in the same hole break the same way twice? Now ill listen to what squirrel posted in the 1st thread. Adding a extra support for the alternator.

Heres the link to the 1st time it happened.
http://jalopyjournal.com/forum/showthread.php?t=707408

After Bowling Green we replaced the temporary bolt with a Grade 8. Well today, the grade 8 bolt snapped. Got up this morning and loaded the truck up to go to a side job. Truck ran great, I drove it easy, never got on it. Turned into the subdivision of side job and heard a noise, almost like what a dry bearing would make, i thought it was something on the front end. But the noise got louder with more RPM. Got to the side job house, pop the hood, and same thing, belts loose, alternator is against fender. So we ended up having to tow it home.

So why did this happen the same way twice?

 

show some photos of the offending bracket.

 

all i see are pics of you under the hood post a pic of the problem area.

 

Here you go

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thats strange for sure. here are some theorys.

1. there is something in the bottom of the bolt hole not letting the bolt tighten correctly. evetually the vibration breaks the bolt. or the bolt is barley too long.

2. the bracket is warped or not flush across the the back putting outward force on the broken bolt when you tighten the top bolt. again vibration eventually breaks the bolt.

i would get a different braket that bolts to the exhaust manifold or move it the other side, unless you have air.

 

take the braket off and put a straight edge across it to see if its flush. it's almost got to be something to do with that lower braket.

 

It might be, ill have to check. It can really only go in this spot, no threads on other side for mount, and not enough room for it go down low.

 

If the bracket is not flat, best solution would just have to buy a new bracket right? Grind the high spot out?

 

Does the bracket rest against the front of the Head when the bolt is tight ? It looks as if there should be a thick washer/shim between the bracket and head ?
If there is nothing between the two it is flexing getting stretched as the bracket moves up and down and is braking/shearing off the bolt from being stretched as the bracket is flexing up and down.

 

I would try another bracket just in case this one has a slight twist which is causing the bolt to fail.

 

I have seen those alt. brackets before what is it off of originally ?

 

i wouldn't grind it. no way to get it true across both pads. take it to the machine shop and have them put it on the belt sander.

 

Tried this out before the 1st time, put a spacer between bracket and head. This is what we thought caused the 1st one. Now that i think about it. We drove from August 2011 to June 2012, without problems or a spacer, noticed the alignment might be a hair off, added spacer mid June 2012. Went to BG and broke. took spacer out added grade 8 and snapped again.

 

This is what was on it when we bought the motor. 1972 350 out of gmc truck.

 

What are you using for a spacer between the bracket and the head? I can see witness marks in the first picture that appear to me made by some kind of spacer, but it almost looks polished like it was moving a bit, and only appears to have been making contact with the head on one side of the bolt.

EDIT: I just saw your post where you said you removed the spacer. You definitely need one to fill that void, otherwise the bracket will definitely vibrate and break bolts. You need a spacer that the ends are parallel on, and the thicker the wall the better. Have someone spin one out on a lathe for you, tighten it up properly, and you should be good to go.

 

Is the hole in the head heli coiled or have loose fitting threads ? In the pictures it looks as if it has been opened up a little bit.

Bee interesting to see how the bracket was originally used, if it had a space.

 

also, the way you've got that skinny arm on the top, when the engine is running, it's constantly trying to pull the alternator forward and down. if you were to put the big braket on top that bolts to the intake and water pump that would make the top of the alternator more stable and put much less stress on the lower braket.

 

Never heli colied, on way to Bowling Green, we drilled and tapped this hole.

Do you have a picture of this type of bracket or a part number?

 

I had the same thing happen in my Nomad years ago.
Bolt broke off in the head, replaced it with a grade 8.
Next day same thing!!! Went down to bolt store bought
a stud & grade 8 nut. Installed the stud, never broke again.....

 

Think about the possibility of vibration induced metal fatigue caused by the alternator. If the frequency at the location of the bolt is causing the correct resonance it could cause the failure.There could be something in the alternator that is out of balance causing the vibration. You could try to add an additional bracket at the rear of the alternator to help stabilize the unit or change the length of the bracket to change its frequency. Adding some weight to the alternator housing might also help. Take a look at the attached file.

 

I seem to recall there being a bracket that bolted to that stud on the lower water pump bolt to the bolt that holds the alternator.

 

Best I can do right now. This is on the p***enger side. They made them for both sides






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Were there any threads on the portion that broke off? Grade 8 bolts (which are mainly Chinese **** if you bought them from a hardware store) normally have only a portion of the bolt threaded. Therefore, you can tighten the bolt until it is tight and the alternator bracket is still not tight against the engine head. This creates a "stress riser" at the outer most thread and eventually the bolt will break at that thread. The same thing happens if the bolt "bottoms out" in the hole. So with a new bolt measure from the last thread to the underside of the bolt head and make sure that this distance is less than the thickness of the bracket. Now screw the bolt all the way into the hole and make sure that the distance from the underside of the bolt to the engine head is still less than the alternator bracket is thick. If it is, you are "good to go". Squirrel is one smart guy but in this case I have a hard time believing that you are shearing bolts of that size because you have an alternator hanging on the end... the alternator forces, rotational and bump "g" forces just won't generate that type of shearing action if the bracket is actually down tight against the heads. Good luck.

 

Maybe try bolting bracket to head, but bolting upper bracket with a spacer to third lower hole in head. I think the earlier mentioned upper bracket was a p***enger side,long water pump only piece?

 

Never thought about vibration putting stress on the bolt, till it cant take it anymore.

Now the bracket isnt very thick, so when i tighten the bolt down there are to many threads or body of the bolt "holding" the alt. The stud would give more body holding the bracket.

 

very good point.

 

A lot of good thoughts on this. One other thing is how does the bolt fit through the hole is it snug or ? You probably need a bolt or stud that the shoulder is in the bracket and not the threads.

 

Dont know if there are any threads on broken portion, ill have to compare to the other one.

So Marcosmadness Maybe try a shorter bolt to make sure the bolt tightens up before bottoming out inside the head.

 

How bout a stout spring washer? That looks a lot of weight/ force to bear on one puny bolt.

 

Sounds good.