8.75s break teeth because the ring gear moves away from the pinion, because of flex. This has been pretty well proven and also that you CAN'T fix it. Yep, you can brace the housing, install better caps and anything else you can think of, but it will STILL flex the case itself and shear teeth. That, you can NOT stop.

A Dana does not do that simply because of sheer mass and heaviness of the case. A Ford does not do it because the pinion is supported on both ends. Other rears, 12 bolts, 10 bolts, 8.8s, Olds, etc, break for the same reason the 8.75s do........CASE FLEX. Look at it from a logical and engineering standpoint. The ring gear wants to roll, NOT go back. The REASON it tries to move back is you stack teeth and close the gap. At this point the teeth are trying to roll over themselves and the ring gear is trying to make space between itself and the pinion. It tries to push back at THIS point and breaks the cap. Yes, you can put a better cap on it, but I have also seen rears with good caps push the whole web out of the case. Something, somewhere, HAS to give unless it just knocks the teeth off clean and puts them in the bottom of the case

exactly, what Monte said is spot on. there is nothing you can do to an 8-3/4 to prevent it. you can prolong it a little but it will spit out teeth at some point.

this should be put in the archives/best of forever

Thought that was your new signature Slo-twig!

What Monte said is the real engineering answer. WHENEVER torque is applied through gears, there is a component of the force that is trying to separate the gears. It is impossible to design a gear tooth that does not do that.

The more rigid the housing, the more torque the gearset can transmit. Most pinions are cantilevered, only supported on one end. When the gears start to push apart, the pinion is moving away but also bending away a little. The Ford 9" and other differentials that use a pinion support bearing on the inside are stiffer because it's harder for that bending to occur. Check out big truck differentials, many of them have the pinion support.

Unfortunately for the street, the pinion support takes up room that could otherwise have been used for a beefier limited slip unit. For the track, a spool is a better choice as it is shaped to not run into the pinion support.

There is no magic inside a race rear end. The billet caps can help as they are 2x stiffer and also stronger than the cast iron caps, but that is not going to prevent the pinion from moving away and the pinion shaft from bending away from the ring gear. As that happens, the tooth loading point moves farther away from the root of the tooth, and the tooth breaks. Just as simple as that. No magic involved.

The only solution is bigger/stiffer or a design more appropriate to the use, which in the Ford 9" is the pinion support.

R.

Weight KILLS... also a stick car doesnt have any
cushion like a conv does.. I've been happy with
my 8 3/4... I ran a stock one for 600 passes and
used a trans brake all the time... it finally broke
a cap.. that was with a back brace.. I swapped over
to a alum housing that has the threw bolts... its
been in there for years now... the car is light..
but if I had a heavy car I wouldnt use a 8 3/4...
my Rampage has a 9".. thats what I had when I built it
and that has a back brace also
EDIT
the 60' has been 1.19 with this rear end.. never
slower than a 1.26

Aftermarket ring and pinions are ALL made from exactly the same stuff. Yet some rears shell teeth off like chiclets and some hardly ever break. What, you think 8.75 gears are made weaker than Danas, or that slight increase in ring gear size "fixes" the issue...........hell no. It's ALL about stuff moving that is not supposed to move. It's been pointed out several times, that 8.75s and Danas, as well as Fords have roughly the same diameter pinion shaft and there is little difference in ring gear diameters..............so, if you look at it from something resembling a rational thought process, that leaves only the CASE as the inherent problem. Most all cast rears have cast caps. Why don't the rest of them spit caps off?

The problem I have with the answer given, Is the fact that I DID stop mine from breaking teeth. The problem was after doing that, I started shearing off the pinion shafts. No broken teeth, just clean breaks at the yolk.
I am not advocating the use of an 8 3/4.
I will say, that using the gearing I did compared to most others, combined with the car weight and trans brake, would put much more strain on the pinion shaft, and with more(numerical) gear, would have most likely run back into the tooth failure zone. However, that is not the "case" (pun intended)
I mean no disrespect. Healthy debate. I just go by my experience. Yours may vary...

Not sure why, if nobody uses the search function.

For example, 6? years ago:

https://board.moparts.org/ubbthreads/ubbt...html#Post565526

And the MP Alum 742 case is supposed to be stiffer, and I also suspect a spool supports the ring gear better, all in harmony of keeping the gears properly meshed. OP should just get a dana and move on.

Ding Ding Ding - winner winner chicken dinner - that's what happens exactly - you can try the elusive POS aluminum mo-par 742 case - and guess what - pretty much the same thing happens with shelling the teeth on the gear - only it looks real pretty under the car.............

My alum has been doing great for years now.....
DING DING

No chicken for you!

only real difference is the heat treat of that same material, as for the 8.75 they are a good diff for their intended purpose, but I changed to a D-60 after grenadeing my 8.75 and never looked back.

I have seen quite a few 9" launch the pinion right out the front when guys tried to get away with just a stock non-nodular case. Even with a 9" you can't cut corners and expect long life.

Mike, just how does your car relate to the OP's B body? Other than the fact they both came from Ma Mopar, nothing. Hell a 7 1/4 would probably live in your car.

simple... mine is much lighter... I know a 7 1/4 wont work...
I tried one in my old car(used it for a mock up but tried
it).. didnt live 2 easy passes
Why do you think they use to run 8 3/4 in the old dragsters
they worked because they were light... they had dana back
then... why didnt they run the dana... didnt need
it because it was light... if any of them ran a big
diff it was the Oldsmobile

How many hits on the P-body in the last five years?

about 75... I pretty much quit racing after I
got my license pulled.. I just didnt feel like
running 10.0... before that it had about 750 hits

It's NOT because of the case though.....it's the pinion support. The stock pinion supports are very weak, unless it is the Daytona support like came under some Mustangs, Torinos, etc. A factory Ford case, nodular or not, is still better than a factory 8.75 case. Just needs a decent support.

Everyone wants to generalize.........that being stock Fords are junk. No, they are NOT. A center that came under a Cobra-Jet Mustang was VERY good. One from a 6 cylinder Torino was not. Same with an 8.75. One under an early 6 cylinder A-body was weak as water. The hi-po units were much better. Danas are good because they are massive, not because of some superior engineering design.

Also talking about 8.75s in flyweight tube frame cars, roadsters and front engine dragsters has no bearing on 3000+lb door cars.

I lost count of the number of 8.75s my dad or me broke over the years and have tried every band aid there is. You will NEVER "fix" it...........that simple

Same deal with Thumpers car..... once in a great while the car sees a track, yet every discussion on this topic the same two chime in on how it doesn't not happen to them. Mike you constantly chime in on this.... are you telling the OP to use the 8.75 over the D60??? Heavy big block stroker with a 4 speed. Your chit, as noted above, is apples to oranges. What a coincidence the factory put the D60 behind performance big block 4 speed cars..... guess they ran low on the mighty 8.75 and figured the D60 would do in a pinch.