^^ This ^^ Not to mention the time pissed away working on it.

It matters in the context of the original post. Ring gear deflection is not dependent on it being a 741,2 or 489.

Does it matter, the diff or spool gets pushed out the back of the case. A billit cap helps but the case fails.

Yes that's it, thanks. Looks like the Mustang and other small tire class are using this. I'll use it with my 23 spline trans behind the 520 Hemi. Just enough slip to live, street strip.

"Rear End Strength."

My wife used to have a lot of it...

1. I don't think small bearing size differences have any bearing (pun intended) on r&p deflection issues being mentioned here.
2. Not sure what you mean exactly on referring to "larger bearing", since I believe as shaft size increases the actual roller bearings get dispersed in a smaller area, since outer race dia remains the same(?), not sure how that has any impact on r&p deflection issue.
3. This means to me a 489 with a larger pinion dia shaft has less capacity bearings then the 742, but not sure that is an issue anyway with "thrust" overload, leading to likely failures

You can debate why until the cows come home, but the fact remains that an 8-3/4 in a heavy high horsepower car is going to break.

What is a "heavy" car exactly?
How about "high horsepower"?


My nearly-complete Dart 451, 4-speed has a back-braced 8-3/4" with a Dr. Diff 3.91 sure-grip pumpkin, and 295-50R15 street tires. We shall see...

Not if it never hooks up.

There is that.

Your 8.75 will thank you for your tire choice. Like said before, if all you do is spin, you'll likely never break it.

In response to the "ring gear is deflecting" train of thought, of course it deflects some. but I believe it's more pinion deflection. Basically the major thing the Ford 9" has over the 8 3/4 or 9 1/4 or 12-bolt is the pinion support. That's what seems to me to be making the difference between 8 3/4 and 9".

My friend with the '63 Corvette broke countless 12 bolts before switching to a Dana 60 and hasn't broken anything since.

R.

I don't know, but..... If ring gear deflection isn't an issue then why do main bearing caps fail?

Almost identical experience. I bought a $1300 clutch type sure-grip equipped 742 3rd member for mr 8 3/4 and killed the sure-grip within the first year. Drag radials, sticky track and manual valvebody all conspired to send the rear packing! I did Dr.Diff axles, spool and put ends off an 8 3/4 on a full floater Dana 60 truck rear. It gets abused regularly. Only problem I had was the screw in studs in my Dr.Diff axles were trash and started folding over. Replaced with Strange and all is good!

Don't ignore the driveshaft, either! I finally had to order a 1350 Strange unit to hold up along with u-bolt u-joint yoke instead of the strap type.

Last year my entire summer was spent fixing driveline issues.

Yep, and the 9" pinion rear support doesn't help when the pinion tries to screw itself out the front of the housing.

Pinion flexes, teeth break and stack up b/t the pinion and ring gear...carrier gets bound up and pushed backwards, cracks the cap.

Well looks like I'm going to revisit this thread. After reading these responses a couple months ago I decided to get a Moser Dana 60 (with moser axles, and a TrueTrac, since this is my daily, too) with MW billet caps. Are there any other things I need to do to strengthen it, or would a Moser 60 and billet caps be more than enough? It's funny reading through this thread how much info I left out. I plan on putting drag radials on (when going to the track), and it is still going to be stick.

Honest 550hp drag radials and a stick in a heavy car....?
I like 8 3.4 rears for street cars. Not that combo though!!! Dana for sure IMO

way long ago in a land before time---SAE engineers rated the 8.75 good at 500 hp and the 9 inch at 450 hp----9 inch flaw-----the pinion is located at the bottom of the ring gear---that is the weakest position----it is strongest at the middle of the ring gear-------yes you can buy alot of stuff for a 9 inch because the aftermarket chased it-----like they chased the sbc....