Didn't think this would happen again

So use a bolt like this where the unthreaded portion is sitting inside the hole in the bracket.

Thanks for all the answers.

 

I would take the bracket off and check to see how much play there is in the bracket with a bolt like you show in the post above. If it has or seems to have a lot play you might want to go to nut bolt place and get a bolt that has a little larger shoulder or have yours drilled and press a sleeve in to it to take any excessive slop out of the hole.

 

Makes sense, ill check in the morning. The hole in the bracket could be bigger than the bolt, when i hit a bump the bracket jumps up and down, till the bolt cant handle it and breaks. If the bolt was snug in the hole it wouldnt jump, and not cause any stress.

 

Again thanks to everybody who gave their input and help a kid out.

 

Just a guess, but it looks like both the upper and lower bracket mount on the same bolt. Maybe you could re-locate the upper bracket over to the intake manifold bolt, water neck bolt, or maybe over to the A/C compressor. Might take some stress off the lower bracket.

my 2¢

.bjb

 

The upper piece is what puts tension on the belt, and to me the top piece just "holds" the alt. out there. Seems like the strength would be in the lower bracket that kinda cradles the alt. and is what actually holds it.

 

That's unusual David. I ran a very similar bracket on the Chevelle when I had it, mine was for an early 70's Corvette as they still used the short water pump then. I never had any problem like that however and it was on the car for 12+ years. Lot's of good suggestions here, I hope one of them works out for you. I tend to agree with those saying that something is causing vibration, like the bolt bottoming out and never really being tight or needing a shouldered bolt.

 

With all the bracket options out there for that application. Why not just try a different bracket. Obviously something is not right about that cobination.

 

^^^This is interesting also David. My adjustment bracket bolted to the drivers side, top bolt of the water pump. Perhaps that is out of alignment and when you tighten it at the alternator it is putting tension on that bolt that is breaking. Just another thought (as if you don't have enough to sift through already!).

 

Ill check everything in the morning. Just start process of elimination.

Thanks again everybody

 

Could it be the top bolt never really stays tight enough to hold both brackets, causing lower bolt to shear under stress and vibration?

 

I see what your saying, but the upper bracket is between the bolt head and bracket, the bolt is still pushing towards the motor, its just thicker there. Which might help because it takes up threads and the bolt doesnt bottom out, like it might be on the right one. If it was behind the bracket it would put the two bolts in a bind against each other.

 

This pic may help to better explain what I was saying David. You can see how the upper bracket extends down and attaches to one of the water pump bolts. Essentially it turned it into a third mounting point for the alternator instead of all the weight being supported by the two bolts into the cylinder head. It may not make a big difference, but it's probably worth switching over anyway.

 

I see what your saying. How did that line up? The water pump bolt is farther off the block, seems like it would put that out in front of the alternator. Yours also looks straighter than mine. Mine is more of a gradual curve, where yours was straight through out till it gets to the alternator then bends around the alternator. That might have something to do with it.

 

Here's another pic David to show how it attached to the alternator.

 

Cheap common hardware washer or no washer at all, as in 'pic, doesn't provide bolt head proper support. Check for alignment of contact surfaces, as mentioned. Spotface the bracket under bolt head and use an "AN" sized washer and a lock washer. Stud idea is good too.

 

Lots of great suggestions so far. I like the idea of making sure you're using the right length bolt so you're not getting a false sense of torque. Might be one of those instances where you need, for example, a 2 3/8" instead of the more common 2 1/2" bolt.

How about an out-of-balance condition with the alternator. Bad rotor or damaged or cracked fan or pulley? Do you have another alternator that you could swap out?

And this is going to sound counter productive but how about dropping back from a grade 8 to a grade 5 bolt? GM had some issues with broken head bolts on some 2.5 engines several years ago and the fix was to make the repair with new head bolts that were one hardness grade lower. The original bolts, while strong, were a bit too brittle. The replacement bolts provided just a little bit more elasticity. And I can't imagine that alternator mounting requires grade 8 hardware.

 

This will sound strange, but I had a simillar problem with my coupe. I'm running a SBC with a home made alternator bracket to get it low on the left side of the engine. After breaking 2 grade 8 bolts, I installed a grade 5 stud and nut and my problem went away. Each bolt broke on a hard shift, thinking that mount bolt needs a little "give" that a grade 8 bolt doesn't have.

 

Since you already have the bracket and it looks like a good original GM piece it should work well.

The breaking of the bolts tells me it is experiencing stress in your application that could be from road bumps (shock coming through the suspension) or extra stress from your engine.

As you can see with my first picture that GM bracket mounts to 2 holes on the head that are flat and paralell to each other. The hole closest to the intake is even with the other hole.

If your head does not have the even mounting bosses like that (For example the outer one is not out as far as the inner one) then obviously you would need a spacer to even the mounting points out.

If so then the best spacer is a length of heavy wall tubing cut and machined to the proper length.

Once you have that sorted out you can proceed to the solution.

My approach to solving the problem would be to add extra support to the bracket.

I would come off the bottom of the bracket (right under where the alternator mounts on it) with a couple of tabs welded to it for a couple of bolts.

Then I would design a support plate that would bolt to the two tabs under the alternator and go from there to the front 2 holes on your exhaust manifold.

Of course it would be slightly different depending on whether you were running headers or cast iron manifolds.

That would give additional support to your alternator mount and you would not break any more bolts.

I'll post a couple of pictures to give you the general idea, if you need I can draw a diagram amd scan and post it.

The first is what you have, the second is the support idea.

 

Ok, got the old bolt out, and put in a slightly shorter stud and nut. We'll see how this works out.

Thanks for all the help guys.

 

So you don't think the extra support would do the trick ?