Another NA car breaks diff bolts...

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ponie1992

ponie1992

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Car is back on the ground and took er for a test drive. NVH has increased, but not as bad as I expected. Doesn't bother me either way. Now to go on one more drive, inspect and begin tear down for OPG's and Whipple.

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That’s for the subframe, need the rear diff bolt bushings, 16mm OD, 14mm ID
Are these just to take up the slack between the 14mm bolt shanks and OEM bushings?
If so, how long should be the sleeves?
 

shogun32

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Are these just to take up the slack between the 14mm bolt shanks and OEM bushings?
If so, how long should be the sleeves?
smidge shy of 2"
 

aleccolin

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Going back through this, I'm wondering if anyone has tried an old-school solution, why not just use a pinion snubber? I'm betting that would take most of the shock load off those diff bolts and still allow for low NVH. There's a good spot on the subframe to mount one up and you can even buy off the shelf and just fab a bracket for it and make it adjustable, run it down until it just touches the snout on the diff, and check it occasionally for wear.

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shogun32

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I'm wondering if anyone has tried an old-school solution, why not just use a pinion snubber?
because the freedom of movement is both up and down? And the morons at Ford put a 14mm bolt in a 16mm hole?
 

aleccolin

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because the freedom of movement is both up and down? And the morons at Ford put a 14mm bolt in a 16mm hole?
The primary force being applied is up, which is what snubbers are for. The only force pushing the pinion down is from decel which engine braking can only apply a small amount of force. Braking forces are translated from the hub into the chassis directly.
 

shogun32

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wheel hop would suggest there is very serious force downward as well. I'm sure it would help somewhat but the snout is not where the damaging articulation is happening, it's at the rear.
 

aleccolin

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Think, when the wheel is hopping, it's slipping and gripping rapidly. When it loses traction the pinion will drop, but when it grips gain it's going to slam the pinion upward. The only thing stopping that movement are the mounts and the single shear bolts. The upward force would be many times the downward force. Unless you're doing a burnout in reverse, in which case you're on your own lol
 

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This would probably work if it fits, could be shimmed down or cut down to suit. Should be able to mount it without dropping the diff, if you're handy.

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Something still needs to be done at the rear mounts, obviously.

Edit: So, typically a bolt is most likely to shear at a stress concentrations like where the threads end and the grip begins, so it might be a better idea to just use a piece of high strength allthread through the bushing and into the rear mounting tabs, and a nut or nylock to torque it all down.

Alternative to that, the best idea I have is to take the diff cover off, machine a 16mm diameter recess into the mounting face around the 14mm threaded hole, then insert a 16mm OD/14mm ID sleeve through the bushing and into the diff mount, so that it takes up the shear instead of the bolt. It would probably only need to go maybe 10mm deep, should still be plenty of thread engagement left.

You could use a shoulder bolt for the above, but it would take machining a custom one, because you would want it to still pull down when torqued, and not bottom out the shoulder, but only just.
 
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aleccolin

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Maybe this has been discussed, but why not through-bolt the rear with a larger diameter (16mm) bolt? All you have to do is drill out the threads in the ears on the diff.
 

80FoxCoupe

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Maybe this has been discussed, but why not through-bolt the rear with a larger diameter (16mm) bolt? All you have to do is drill out the threads in the ears on the diff.
A kit as you described is available.
 

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A kit as you described is available.
I’ve see one for the front diff mounts where you drill out the threads in the bushing, but so far haven’t seen anybody drill out the ears on the diff cover and through bolt the rear. Would make sense if it had been done though. I might yank the cover off mine next time it’s in the air and have at it, just need to pick up a good HSS 16mm bit, cover is aluminum it should drill out easily. I’ll check the forward face of the ears for flatness and square with the hole, may need to be cleaned up a bit to mate well with a washer and nut.
 
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