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8.75 swap help

Jeff Erwin

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71 Dodge Charger 400x6. The previous owner was going to put it on the drag strip back in the late 90's so he put a 4.10 Sure Grip in it, then put it in a warehouse where it sat for many years. I obtained it about 4 years ago. Everything is perfect now except for the 4.10, it has to go. I am going to put a 3.23 set in it by just pulling the existing case and plugging in a different one.

I haven't done this before, so I have some questions. The housing has a large '6' on the side, so does this make it a 489 or a 742, or is it something else? Are the different cases interchangeable? There are a few places where I can order a complete differential, but I have to get it right so the thing fits.

Large yoke? What are the options and how do I tell what I have now?

Spline count? I'm pretty sure these are the splines at the end of the axle that insert into the diff, how do I tell what my axles are other than pulling one and counting?

Dumb questions probably, but I want to make sure what I end up ordering actually fits and works.
 
That a big 9 is for 489. The axle shaft splines are all the same on stock rear axle shafts in the 8.75.

you can put in a 741, 742 or 489.

7290 large yoke is 2 5/8” on the inside of the retainer clips

7260 small yoke is 2 1/8” inside the clips.

The don’t have external retaining clips like a ford or GM has.
 
You can change just the pinion & gear set but you will have to pull out the pinion to replace it with the new one which means you'll need a new crush sleeve or crush sleeve eliminator new bearing and seal i'd take it out and take it to someone who knows what they are doing with preloads not hard but if you don't know it's better to take it to a pro while it's out i'd do all the axle bearings and seals but that's personal preference
 
No need to replace good axle bearings, just clean them and repack with grease.

he said he is replacing the whole theod member. Don’t need to get into setting up gears, no need to take apart a good 4.10 third member, easy to sell it.
 
That a big 9 is for 489. The axle shaft splines are all the same on stock rear axle shafts in the 8.75.

you can put in a 741, 742 or 489.

7290 large yoke is 2 5/8” on the inside of the retainer clips

7260 small yoke is 2 1/8” inside the clips.

The don’t have external retaining clips like a ford or GM has.
So if I have a large yoke I have to replace it with a member that also has a large yoke or are they interchangeable?
 
That a big 9 is for 489. The axle shaft splines are all the same on stock rear axle shafts in the 8.75.

you can put in a 741, 742 or 489.

7290 large yoke is 2 5/8” on the inside of the retainer clips

7260 small yoke is 2 1/8” inside the clips.

The don’t have external retaining clips like a ford or GM has.
Is there any advantage to one vs. another (741 742 489)?
 
So if I have a large yoke I have to replace it with a member that also has a large yoke or are they interchangeable?
You can always use a cross over U-Joint that will allow you to have the larger shaft end with the smaller yoke end.
 
Pull the thing apart and see what diff it has. If it's a factory Sure Grip, the 89 came with the cone type diff and it'll have the large thrust pin inside which is pretty much impossible for it to come out. You have to take the diff apart to get it out. If it has the clutch plate Sure Grip, yes those can break the roll pin that holds them together. That usually happens when someone doesn't know how to set the axle end play....IF it still as the tapered roller bearings on the axles. If it has the 'Green' axle bearings (ball bearing), it's usually better to remove the thrust pin as it's no longer needed with the Green axle bearings. Pull the third member out and see what it has then we can go from there. There are actually 4 different yokes....big and small and fine and course splines.
 
You want the last three numbers of the casting to tell what you have. This one is a 742...
rear end.jpg
 
A 7290 the cups are 1.125" dia. and as mentioned inside C clips. On drivers side the 3 numbers of case are either 741,742 or 489.
 
Pull the thing apart and see what diff it has. If it's a factory Sure Grip, the 89 came with the cone type diff and it'll have the large thrust pin inside which is pretty much impossible for it to come out. You have to take the diff apart to get it out. If it has the clutch plate Sure Grip, yes those can break the roll pin that holds them together. That usually happens when someone doesn't know how to set the axle end play....IF it still as the tapered roller bearings on the axles. If it has the 'Green' axle bearings (ball bearing), it's usually better to remove the thrust pin as it's no longer needed with the Green axle bearings. Pull the third member out and see what it has then we can go from there. There are actually 4 different yokes....big and small and fine and course splines.
Do I need to know all this if I am replacing the entire unit?
 
Do I need to know all this if I am replacing the entire unit?
If your axles have ball bearings (Green Bearings) instead of the factory type tapered rollers, then it's a good idea to know it. Some have run the stock center thrust pin with the Greens but I'm not one of them that will do that....
 
Doesn't hurt. You can download a service manual from link below. You may need to use a 1970 manual the 71 says body service manual. Shows how to set axle end play, you need to do that!
Service Manuals – MyMopar
 
Doesn't hurt. You can download a service manual from link below. You may need to use a 1970 manual the 71 says body service manual. Shows how to set axle end play, you need to do that!
Service Manuals – MyMopar
Yup....needs to set axle end play but only if they have the factory type wheel bearings.....
 
Yup....differentials are not exactly simple and if you get things wrong, it can cost you time and money....
 
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