8 3/4 rear 741 / 742 center case question - HELP!

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AUS_Duster

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Hi there. I am currently running a 8 3/4 rear with an open 3:55 in a 741 case. When i got the car it came with a 3:91 sure grip in a 742 case.

What is the difference between the 741 and the 742? Will the 742 centre just bolt in?

I want to install the sure grip but don't want to run the 3:91. As i do mainly cruzing i was considering somthing a fair bit taller so 60mph would equal around 2,500 RPM max. Any suggestions on ratios?

Appreciate any advice.
 
the 742 case is a lot stronger than the 741 case. The Sure Grip unit can be interchanged between the two cases without a problem.
 
3.23 work great for the rpm range you want just order (or trade) for a set in the 742 case
 
The only thing that I know of that won't interchange is the ring & pinion between cases. But you can swap the sure grip into the other carrier. But you have to make sure the yoke takes the same u-joint. There are also 2 different spline counts possible for the yokes & pinions. 10/course and 29(?)/fine.
 
so theroritically I could just pull the 741 out and put the 742 in? Just need to change the ring and pinion on the 742 for the 3.23. Is that right?
 
you first have to order or get a set of 3.23 gears and the ring and pinion can not be interchanged between cases. So the 3.23 gear must come from a 742 and only a 742 case
 
Randys, Moser, Strange, Yukon. I like Moser myself but thats me.
 
Also the guy i bought the car off said the rear had cut and resplined truck axles. Are truck axles a different spline? I cant contatc the guy I bought the car off so I cant ask him these questions.
 
The differences in the cases 741, 742, and 489, is the pinion shaft diameter. The 741 has the smallest at 1.375" , the 742 is the next size up at 1.75" (but is straight rather than tapered) which some people prefer, and the 489 is the largest at 1.875". I`ve seen a lot of power in front of a 741 with no issues. I think you`d be fine with either case especially if you install a cruising gear like a 2.94 or 3.23.
 
There should be no difference, if so they were resplined to fit your rear end
 
the 741, 742 and 489 center section can easily be swapped in and out of any 8 3/4 axle housing, a sure grip or locker unit can be swapped between any center section 741, 742, 489, the pinion gear is specific to the center section and will not interchange, a 741 pinion will only fit a 741 section, same for the 724 section and pinion, or the 489,
 
AUS_Duster said:
Also the guy i bought the car off said the rear had cut and resplined truck axles. Are truck axles a different spline? I cant contatc the guy I bought the car off so I cant ask him these questions.
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He probably used those axles to get the 4.5" bolt circle. Rather than buy aftermarket axles, a lot of people will respline C-body or truck axles in order to get the large bolt pattern and save money. There are very few wheels for the 4" bolt so changing over to the large bolt pattern allows for a much wider variety of wheels to be used.

If you`re just looking for a cruising gear, keep an eye out for a single traction 3rd member with a low numerical gear. You can usually pick them up for a song if they don`t have a Sure-Grip unit and the gear is not 3.55 or higher.
 
OK. That makes sense. Is there anyway to tell the yoke spline count without pulling the centre out first? What is the most common?
 
.......As a rule, but not gospel up to 1971 was the 10 spline course, after that 29 spline .........of course gears could of been changed............kim.............
 
340-4spd said:
What's the theory behind the straight pinion preference?
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I wish I could answer that with authority but I just can`t. It may be because the 489 case used a crush sleeve versus a hard spacer. It could be that the sure-grip unit in the 489 case was a cone-style versus a clutch style in the 742, or it could be simply the failure rate of one or the other at the track. For whatever reason the 742 case seems to appear behind most cars with enough HP to break something. In my experience twisted axles and broken ring/pinion teeth seem to be more of a problem than pinion shafts.
 
My observations so take them for what they are worth.

The pinion thickness/bearing size is a red herring. They all have the same neck down in the weakest area... where the yoke attaches.

The 742 has a real bad stress riser in the pinion, IIRC. the 489 also has a 90* stress riser, the 741 has no stress riser.

The 489 preload crush sleeve is a POS. Anybody building one that wants it to last should replace the sleeve with a solid spacer.

Broken gear teeth is a bigger issue than anything to do with pinion bearing size. More of a cap issue.

741's are plenty tough. I've had 426 Hemi and 440-6 cars with 741 SG's in them.

Pull the sure grip from the 742 case and put it in the 741 case. First , check backlash first on 741, then reset to that reading with sure grip installed.

Too bad you are in Australia. I'd be interested in the 3.91's from the 742 case.
 
Longgone said:
For whatever reason the 742 case seems to appear behind most cars with enough HP to break something.
Click to expand...
True, no doubt. But I suspect the reason there is simply that they were produced by the factory for many more years and installed in the cars that ended up at the track. -So many more of them out there to begin with.

I built my 489 with a solid spacer and Powertrax locker. :thumbrig:
 
"I built my 489 with a solid spacer and Powertrax locker" posted by 340-4speed
I'm looking at doing the same thing for my 4.30 gears.
 
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