8.75 diff near-death experience

I was starting my first pass at the ANRA finals over the weekend and heard/felt a big bang. I had suddenly gained about 45-degrees in pinion angle and sheared off the pinion shaft. The race was over for me on my first run. I got to thinking about Dana-ifying the car. Then yesterday I took things apart and discovered that the problem was the leaf spring perch welds had broken. The pinion shaft must have broken due to the extreme angle after the axle rotated and the pinion raised up.

So what I'm wondering now is if I should fix up the 8.75 (274 case) or take this opportunity to upgrade to a Dana. I've probably got 40 runs on the car total with this axle, and I haven't had a problem until now. Looking at the spring perches, it's obvious that the problem was poor welds that did not penetrate properly.

The car is a 3200# Duster big-block making about 650 hp and 1.51 60-ft through a 727. If I keep the 8.75, I'd be interested in knowing from you guys what upgrades I should make while I have the diff apart

It sounds more like it was just a perch failure (kinda rare tho) as opposed to the 8&3/4 being inadequate for your app.

Were the welds factory? Mine failed on one side once, didn't break the pinion but put a hell of dent in the floor.

The factory weld had almost zero penetration, its a miracle it lasted the 35years it did before it broke.

Sounds like some guys make them live at your power levels. I gave up on mine and went with a dana. But now is the time to check the spag in the trans.

My 0.02 usd.

Tony

I will add if you are going to replace the perches replace them with the ones from Calvert Racing or make some gussets to make the stock ones stronger.

I have a heavy-duty bolt-in sprag and a billet drum, so hopefully the trans. is OK, but I was also worried about the trans.

I will definitely reinforce the perches. I saw the Calvert ones. Other than being kinda expensive, they look like good pieces

The welds were definitely not factory. The springs were re-located 1.5" inward by the previous owner. I'm guessing they were his welds

Take the transmission apart or start saying your Prayers right now.

Before you decide one way or the other on reusing the current 8 3/4 check the housing for straightness now and then decide
My Duster had a bunch of mid to low 10. second runs on it when it bent the heck out of the housing enough to break the ladder bar mounts welds loose from the housing
Luckily that happened on the starting line and not going into 3rd gear It may have been bent a little to start with after having it narrowed
I have someone help me check the housings by using two carpenter square and measure off of the axle flanges or the rear end bearing ends 14 to 16 inches out from the center of the axle to the front and rear and up and down as much as possible, if it varies more than .125,1/8 inch or more, between the front and rear or top and bottom measurments it is bent more than it should be
Let us know what you find and decide to do

Might want to check the tailshaft for cracks and the tranny mount while you’re under there.

We are talking about 12"? (4x3"?) of weld all loaded in shear.
That would have to be some pretty crappy weld.

Yup. Pretty crappy welds now that I've had a chance to really look at them

post a pic of the welds !!

To Pittsburg's point, Have your sprag checked. The fact that is a bolt in doesn't mean it wasn't stressed.
**taken from a well written post by PC-Charger

"So what do you need to do to minimize the chance of witnessing first hand one of these explosions? If you read and understood the above, you can see that a simple bolt in sprag/roller clutch will do little to nothing to prevent a roller clutch failure of this type. Rarely is the problem of the outer race turning in the case a factor in a drum explosion. Not to say that a bolt in roller clutch is not a good thing, just that it will not help with this type of failure. The stock and most aftermarket roller clutches for the 727 utilize 12 rollers and springs. The super sprags from A&A and Coan have 16 elements. These units may offer some relief from this type of sprag failure simply because with the added elements there is less room for the legs to bend back far enough to allow the rollers to slip or fall out of place. The 16 element clutches are probably a good idea not only for this reason but also their added holding power"

Even if you have a Billet/Steel drum

Thanks for the cautionary information about the trans. I will definitely take it out and have a look inside and for cracks in the tailshaft housing. My sprag is the one referred to as the "Ultimate Sprag" and includes more rollers as was mentioned in the write-up. Hopefully it held.

Back to the diff, I've seen write-ups that say to use a billet cap on the driver's side bearing. The price seems kind of high at $100, but if it gives any insurance against future failure, seems like it would be worth it

Maybe the perch welder was trying to make-do with a 110v unit when a 220v would have been much better?

Then again, I think I could get the job done with a 110 and I'm no great welder!

I didn't have tons of passes on mine when I changed it out a couple of years ago but is was behind a mid 9-second 470 bb at 3100+ lbs and in the 1.33-1.35 60's with stock caps. I went with the nodular from Dr.Diff and it's heavier and has thicker webbing to strengthen it plus forged caps so maybe this will last me another 16+ years.........

All the OEM 8 3/4 third members are made of cast steel according to what was said at the old Mopar drag race seminars back prior to 1980 so adding a billet steel cap to them is a good deal to do instead of counting on the stock cast steel cap with the adjuster bolt hole in the center of it to hold up to constant drag strip abuse
I've used 3rd members with a single steel cap on the drivers side and one with both steel caps on it, neither one of them prevented me from cracking the ring gear teeth at the outer corners though on all of them when you start going fast and pulling 3 to 8 inch wheelies on every run
I ended up going to a Dana 60 after breaking a lot of 8 3/4 parts in that car No more problems ever after that

Me, I'd go to a Dana 60 from Dr. Diff, have the perches braced, and go pound on it for years & years.
I'm not a fan of a rearend pitching the car into the wall, flexing & steering the car, or breaking the trans up.
Yeah, it's heavier, your perches were the cause of your problems - not the 8.75 - I get all that. But for a little extra money, you won't have to worry about the rearend holding up.
Been there done that.

Every broken rear I've ever had has broken something in the trans...Bess be a check'n dat sir.