SBC Lower Left Alternator Mount

[oldracer22]

I have used two different lower alternator mounts. They have both broken the lower bolt to the block. I assume this is because it is in single shear. I just discovered this, and it is a pain to repair. The bolt is broken off flush with the block. I have to remove the radiator to make room for a drill and easy out. Has anybody else had this problem? Know of any fixes so it doesn't happen again?

 

[donsrods]

No, that is pretty unusual. I used one on the 350 sbc in my 23 and had no issues with it in the 3 years and lotsa miles I drove it.

This might sound odd, but I notice you are using grade 8 bolts, which in most circumstances are preferred. But I THINK I remember hearing something about grade 8's being more brittle than grade 5's, and that might be contributing to the problem. Someone with more knowledge of bolt characteristics will have to tell you more about that than me, though. I just don't know.

Another possibility is that you have some imbalance, maybe by an out of round crank pulley or something like that, which is setting up some vibrations that eventually crack the bolt.

Here is the setup I had on my 23.

Don

Here, I found a better picture of mine.

 

[RustyNCA]

I have used two different lower alternator mounts. They have both broken the lower bolt to the block. I assume this is because it is in single shear. I just discovered this, and it is a pain to repair. The bolt is broken off flush with the block. I have to remove the radiator to make room for a drill and easy out. Has anybody else had this problem? Know of any fixes so it doesn't happen again?

Looks like you are running the same thing I am, I have never had a single problem with mine.

 

[kenny c]

I tend to agree with Don some thing is out of round or one of the pulleys may be crushed or spread and causing the belt tension to fluctuate. Ken

 

[sneakysnake]

Ive had this happen on power steering pumps that mount there(late 60's).
I would say bolt grade is playing a factor and may look at a brace to go from the back of alt back to block.

 

[Old Iron]

If it's going through the bracket, through the alternator, through the spacer and into the block, that is double shear and not single.
Do you have the spacer between the back of the alternator and the block ?
Is that a 3/8" bolt ? If so it's supposed to be a 7/16" bolt, for the pivot of an alternator.
Just another couple of thoughts.

 

[oldracer22]

I have the alt. and bracket off and have better pictures. When I bought the bracket I thought that it would be mounted in doublem shear also, but,after more thought, being as the outside bracket is not fastened to anything solid , it is really only single shear. The pivot bolt is 3/8". A 7/16" wil fit not fit through the alt. or the bracket.However, the 3/8 bolt is a pretty sloppy fit, allowing the alt to cock a little bit. This could be part of the problem.. I think I will drill the alt and bracket for 7/16 to get a tighter fit. This may help. I will try grade 5 bolts also. And I am going to fab a bracket to go from the alt to the block. This should help stabilize the alt. It,s hard to tell if the lower engine pulley is out of round, but I have an aluminum pulley, so I will replace the lower pulley also. Thanks, guys. Jim

 

[sneakysnake]

Just curious, Are the bolts threads bottoming out before the bracket is snugged up?Just from the pic looks as if the bolts are to long.

 

[oldracer22]

The bolts look too long because alignment issues require the alternator to be shimmed out around 1/4". I shortened 4" long Grade 8 bolts for this installation because that's what I had. This time I went and bought 3-1/2" Grade 5 bolts. They still, of course, require the shims for alignment. Long story short, no, they weren't bottomed out.

 

[sneakysnake]

The bolts look too long because alignment issues require the alternator to be shimmed out around 1/4". I shortened 4" long Grade 8 bolts for this installation because that's what I had. This time I went and bought 3-1/2" Grade 5 bolts. They still, of course, require the shims for alignment. Long story short, no, they weren't bottomed out.

What are you using for shims? Is it something that could give the bracket some flexing?A 1/4"bar with two holes will give better stability than just washers.

 

[oldracer22]

Sneaky snake, That is true. I hadn't thought about that. I will make a plate. Thanks

 

[sgtpontiac]

Might be a dumb question...

When you say it's shimmed out 1/4"....are you taking about the bracket that should be tight to the motor..the rear bracket? Or are you talking about between the front and rear bracket...??

 

[oldracer22]

sgtpontiac, If I put the bracket against the block, the pulleys won't line up.. The alternator has to be shimmed away from the block to line up with the water pump pulley. This is caused because I am using a short pump and an aluminum timing chain cover. The back of the pump hit the cover before the pump hit the block, so I made spacers 1/4" thick between the pump and the block. This obviously moved the pulley out 1/4". I had to space the lower pulley out also for proper alignment. I hope this makes sense.

 

[Sumfuncomet]

Bracket issues

Dont build it yourself....although you could....GM hired some great engineers. Look in junkyards for an engineered bracket or buy a setup from Alan Grove. He makes great stuff, relatively inexpensive. Just Google his name.

 

[sgtpontiac]

Sorry it took so long to respond....lost the post somehow..

sgtpontiac, If I put the bracket against the block, the pulleys won't line up.. The alternator has to be shimmed away from the block to line up with the water pump pulley. This is caused because I am using a short pump and an aluminum timing chain cover. The back of the pump hit the cover before the pump hit the block, so I made spacers 1/4" thick between the pump and the block. This obviously moved the pulley out 1/4". I had to space the lower pulley out also for proper alignment. I hope this makes sense.

makes sense that it needed spacers between the pump and the block....I guess what I was leading up to was if the brackets are not mounted solidly somewhere....like to the block... that any bracket that is shimmed out will have enough vibration/harmonics to create problems....then add different vibration/harmonics from one accessory to another and you amplify it 10 fold....jmho.....if the brackets are not mounted someplace that is flat and cannot flex, you always risk the possibility of vibration / flex stress cracking issues...again, jmho

 

[klink]

Do the holes have good chamfers? If not, this is also a potential problem. If the holes were done with a punch, they could have a sharp edge at one end or both, which will cut into the fastener. If the holes are laser cut they may have an edge or burr where the laser started and stopped, this could also cut into the fastener. Make sure the holes are clean (free of burrs) and round.

 

[donsrods]

I think the guys are on to something regarding those shims. I bet they are letting the whole bracket assembly move around slightly, fracturing the bolts. Can you otherwise move the alternator out that 1/4 inch by modifying the alternator itself so it is thinner by 1/4 inch in the front ?

Don

 

[oldracer22]

I have chamfered the holes in the bracket. I have fabbed a plate instead of the shim washers, and I have fabbed a support bracket from the back of the alt to the block. I have changed from grade 8 bolts to grade 5. The only thing left to do is road test it. BUT, it is 25 degrees and there is 6" of new snow outside and the car is a roadster. The road test may have to wait until spring now.Thanks for all the suggestions guys.