Rear End Question...

[Joel Talka]

Hello all!

I have a couple of rear end questions.. I finally figured out that my '66 has 2.93 gears in an 8 3/4 axle.. With the 4 speed tranny I am going to need to up that ratio to something more drivable. I tried to get the complete part number off of the unit unfortunately the last two digits were not fully readable. I managed to get 207074 but I can't be sure if the last number is a 2 or a 1... I understand that both part numbers do exist; ending 41 for small pinion shaft and 42 for the large one. Is there any way I can figure out what I actually have without taking the whole thing apart. The plan is to hit Carlisle in July and get a gear set in the mid 3's or a complete differential if I can locate one.

Another question is on strength.. I read that the 8 3/4" rears are good up to about 400 HP.. Is that a realistic number or can I push it a bit higher?

Any input/feedback is greatly appreciated!!

Thanks,

Joel

 

[66383]

What engine & how much horsepower do you have? I had a 3:23 open rear end and recently installed an Eaton Tru Trac with 3:55 gears I have a 383/727 combo. the 8 3/4 will withstand lots more than 400 HP from what I have experienced in the past.

 

[Ray70Chrg]

I believe you can tell by the diameter of the pinion shaft. 741 has a 1 3/8 dia. and 742 has a 1 3/4 shaft. The end of the shafts can be seen at the rear universal joint.

 

[33 IMP]

If you cant tell whether it is a 741 or 742 from the number...you would have to take it apart to find out for sure (unless somebody knows something i dont know)
As far as horsepower level for a 8 3/4? In my opinion, a better question would be be torque output, and shock load. 800 hp, with a stock converter, 3.23 gears, and street tires....it will live forever. 600 ft/lbs, stick shift or trans brake, and 14" slicks.....borrowed time.
I have got away with 550hp with a 742 case, 3.91 gears with a detroit locker,727, and stock axles and 10"slicks. But i am seriously planning an upgrade to a dana, while the 8 3/4 has value, before i break it. No sense buying axles and bracing the housing when that money can be put to a far stronger piece.

 

[33 IMP]

I believe you can tell by the diameter of the pinion shaft. 421 has a 1 3/8 dia. and 422 has a 1 3/4 shaft. The end of the shafts can be seen at the rear universal joint.

Nope. All three pinion sizes are exactly the same at the yoke. The only difference is fine spline or coarse, and a 741 or 742 could be either.

 

[Ray70Chrg]

Nope. All three pinion sizes are exactly the same at the yoke. The only difference is fine spline or coarse, and a 741 or 742 could be either.

Thanks for that info. My only experience is with a 489, so there was no question what size the pinion shaft was.

 

[66chargerpat]

IIRC you can tell by the big letter or number on the side of the 3rd member. Maybe Cranky knows.

 

[R413]

Yes there is a LARGE number cast on the drivers side of the carrier, filler plug on pass side. Look for X and 1 are 741, the a 2, 9, or Yukon has a letter Y there.

How will it hold up? With a 4 speed it depends how hard you are going to shift it and what tires you have. Sticky tires and power shifting will be harder than the banana skin BFG TA’s and granny shifting

Also the front of the case where the bearing goes is largest on a #2 because it has the largest front pinion bearing. So the seal is larger. #1 and 9 use the same pinion seal. So the dust shield on a #2 yoke is bigger than 1 and 9. Ever have a dust shield rub the case on a #2? when swapping yokes? I have.

 

[66Satellite47]

Like 33 said, not so much the HP level, it's the torque & traction. A 8 3/4 can live a good long life behind 500 HP with good traction as long as it is well setup. I preferred the 742 case to the 489 because of the ease of setup. I also preferred the clutch style limited slip diff. If you use the clutch style diff, be SURE that the left hand case bolts are properly torqued with highest strength Loctite, ask me why. LOL
BTW for real high power & traction I'm a Dana 60 guy. Heavy, but don't break.

 

[Joel Talka]

I believe you can tell by the diameter of the pinion shaft. 741 has a 1 3/8 dia. and 742 has a 1 3/4 shaft. The end of the shafts can be seen at the rear universal joint.

Thanks, that will work!

 

[33 IMP]

See! I figured there was somebody here who knew something i didn't know. Thanks, R413!

 

[Joel Talka]

What engine & how much horsepower do you have? I had a 3:23 open rear end and recently installed an Eaton Tru Trac with 3:55 gears I have a 383/727 combo. the 8 3/4 will withstand lots more than 400 HP from what I have experienced in the past.

Right now is is a 318 with a mild cam and Edelbrock AFB.. I have that 440 block and parts which I do want to pop in there so I'm guessing somewhere between 400-500

 

[Joel Talka]

Yes there is a LARGE number cast on the drivers side of the carrier, filler plug on pass side. Look for X and 1 are 741, the a 2, 9, or Yukon has a letter Y there.

How will it hold up? With a 4 speed it depends how hard you are going to shift it and what tires you have. Sticky tires and power shifting will be harder than the banana skin BFG TA’s and granny shifting

Also the front of the case where the bearing goes is largest on a #2 because it has the largest front pinion bearing. So the seal is larger. #1 and 9 use the same pinion seal. So the dust shield on a #2 yoke is bigger than 1 and 9. Ever have a dust shield rub the case on a #2? when swapping holes? I have.
View attachment 1125873

There is undercoating all over the rear but a can see a small "x" on that side of the housing, perhaps an inch or so in size, so looks like this is a 741....

 

[33 IMP]

My 741 case has a x about an inch in size, cast onto the case between the ribs. Pretty sure you are correct, youve got a 741, especially if originally a 318 car.
(And i personally think there is nothing wrong with a 741, an unfair rep for being weak.)

 

[dadsbee]

741 in my Bee. It's a '66 from a Charger. 2.94 would be your ratio.. not 2.93. That is what mine is, even though I have it tagged 3.23 to what OE would have been.

 

[Joel Talka]

Hi, you are correct, it is 2.94.

 

[Kern Dog]

741 in my Bee. It's a '66 from a Charger. 2.94 would be your ratio.. not 2.93. That is what mine is, even though I have it tagged 3.23

You lie to yourself about the ratio?

 

[dadsbee]

Yep, just like all the "OE correct guys" with a #'s block that's been stroked and has stock valve covers on it. LOL