8 3/4" third members

[TURBOFIRE]

Okay, I have a couple of questions. I understand that the three main castings have different pinion diameters. I understand that axle shafts are axle shafts. My car has a 742 with 2.76 open ratio. I want to either buy or have a new third member built with 3.55 sure grip. Question #1 - If I have a 742, do I need to build or buy another 742 to replace it or can a buy a 489?
Question #2 - Will the U-joint/drive shaft work fine if I do find another third member that is not a 742?
Question #3 - So if I'm thinking correctly, I only need to know what casting # I have if I intent to rebuild that specific third member or is there something I'm missing? Thanks

 

[has2bmopar]

You can put a limited slip carrier in your 742 with a new ring gear and pinion, or you can get a complete drop out/third member already set up.

You should have the big yolk on your car if it is original which I'm sure it is. If you get a 489 drop out, get it set up with the correct yolk on it and don't think you can just pull yours off and swap it out because the 489s use a crush sleeve to set up the preload on your pinion bearing. The 742s, 741s etc are shimmed.

 

[TURBOFIRE]

I was really wanting to leave the original third member original. Then, get a second third member to build. This is why I was wondering if I still need to get another 742 to build or if I could use a 489, seeing how they seem to be easier to come by. The only question I guess is will the draft shaft mount up to the 489?

 

[68plymouth383]

The only challenge with going to the 489 from the 742 is making sure you have the same pinion diameter. They made 2 ujoint sizes. A 7260 small ujoint, and a 7290 large ujoint.

 

[polara71]

Agree but they do make a U joint that has both sizes on it.

 

[mr. fix it]

Agree but they do make a U joint that has both sizes on it.

I don't think that is available as an option any more...
If you change out then you would need another drive shaft

 

[polara71]

Hmmmm I bet you could locate those u joints if you looked hard enough. It was just 7 years ago I bought them off the shelf at the local drive shaft store

 

[twostick]

They make yokes for what ever U-joint you need for 489. If you 742 has the same spline count as the 489 pinion does you could use that one.

Give Dr Diff a call.

Super duty FORGED STEEL 7290 U-bolt yoke

Kevin

 

[polara71]

Or you could do that.....

 

[TURBOFIRE]

They make yokes for what ever U-joint you need for 489. If you 742 has the same spline count as the 489 pinion does you could use that one.

Give Dr Diff a call.

Super duty FORGED STEEL 7290 U-bolt yoke

Kevin

If this is the case, why couldn't they build me a 742 third member?

 

[twostick]

If this is the case, why couldn't they build me a 742 third member?

They could build you a 741 if you'd like. They all will fit your car.

The 741 case can be had cheap because it is considered inferior to the 742 & 489. This is mostly myth. The biggest downside to the 741 is limited ratio selection.

Kevin