How do I know what rear end I have?

[Myasylum]

Shouldn't there be a tag or something hanging from the pumpkin?
I assume this is a 8 1/4? Weren't these still pretty fairly stout in 1972?

 

[Kern Dog]

That is an 8 1/4" axle.
The tags they put on them usually only had gear ratio numbers. THey often get lost when the covers get pulled to drain the oil.
They are great axles for light cars with LA and Magnum V8s. They can live behind a heavier car or one with up to 400 HP but their weak link is the bevel/spider gears in an open differential. Extended one wheel burnouts will drastically shorten its lifespan.

 

[Myasylum]

Well that's a let down. Kind of puny for a heavy car such as these with a 400.
Then again, I guess it's latest 50 years, so it can't be too bad.
Is it going to be recommended that I get a stronger axel or can I get a stronger bevel/spider and just add a sure grip?

 

[Kern Dog]

A good differential will help a lot. The 400 was rated at something like 250 HP in 1972 so the axle was not really going to reach it's breaking point.

 

[Darter6]

I agree with the dog. Behind a automatic and adding a sure grip you will be fine.
A 72 what ? B body ? If you are lucky the performance factory ratio was 3.21.

 

[Myasylum]

Yes. 72 Satellite. And in not real lucky, lol.

 

[BeepBeepRR]

When I installed my 440 into a guys 71 satellite the first thing I told him was to expect the rear end not to last. He thought I was trying to sell him an 8.75 for the hell of it. I'm still waiting on that call when he says I was right. He doesn't drive the car much so maybe it will last for years. The 440 I built was a 500hp engine. I expect if he ever gets into that engine its a matter of time before that rear end grenades.

 

[Kern Dog]

Traction is the variable here. You can have a 500 HP engine but unless you have sticky tires, the axle never sees the entire load of the engine.
My car had an 8 3/4" axle and a Tremec 5 speed rated to 600 ft/lbs. The tires go up in smoke so the axle may hold up for quite awhile.

 

[1STMP]

I'm rollin' the dice on a 8.25. It was free so
I figured I'd try to beef it up by adding a girdle
to bolster the bearing caps. The engine I'm
running doesn't quite make 500 so I'm hoping
it will last awhile. The other weak spot with
these is the c clips that hold the axles in the
housing. There's a kit available to eliminate these,
or you can install disc brakes as they'll keep
the axle from vacating it's home.

 

[Darter6]

Just remember Chevy put 450 Hp LS6 4 speed Chevelle's in front of a 12 bolt rear. Not that much different than the 8 1/4.
Plus the Chevy 10 bolt that is closer to the 8 1/4 was no slouch. They held up to a 350/350 in the El Camino's,Monte Carlo's and the many other GM performance cars.
Had a 8 1/4 behind a 73 340 4 speed Duster. Never a problem.I wouldn't be afraid of one.