8 3/4 rearend problem and questions.

[Root2812]

hi guys. So I have a 74 Roadrunner with a factory 360 and 727. The last owner told me he had a bad pinion bearing in the factory sure grip 489 cased center section so he pulled it and stuck in an open diff 741 he had laying around. That is what was in the car when I got it and is still in there.

I bought the car off the Internet to get a mostly rustfree car since I'm in Minnesota and everything rusts here. When I got the car it was as described but with an unexpected problem in the read end. It feels like something is binding or dragging. I can actually feel it slow the car down on the highway. I pulled both axles and checked the axle bearings and those are good and with the car on the lift I can turn the wheels and feel the clunkyness in the rearend.

Any idea what this could be?

Second thing is that I can't afford a new unit so I was going to used if I have to replace it. My car is a 74 and I found a 73 roadrunner complete rearend that is also 8 3/4 on Craigslist. Is there anything I need to look for to ensure I can take the center out of that one and swap it into my car? Like spline counts or anything? Also what can I do to tell if it's any good or not.

Thanks

 

[Darthomas]

Only thing I could imagine is large yoke vs small yoke for u-joints.
Never had one those years, but hard to imagine a switch between 73-4 on anything there.
I thought everything had gone to 8 1/4 by that vintage.
I ran into the 10-spline vs 21 spline pinion issue in mid 60's 8 3/4"s.

 

[hangn0ut]

He said it was laying around. Now its obvious why it was laying around. Its no good. Beg,borrow or steal another one. Cause that one is getting ready to leave you stranded somewhere. And when they go,they stop. Might not even get to the side of the road. And I would think you would have a pretty good idea of how to tell a good one from a bad one. Smooth baby smooooth.

 

[Root2812]

Thanks. I will find one that is smooth. I'll try craigslist. It seems these are pretty cheap on there. I'll look for smooth.

 

[Root2812]

Do you guys think I could possibly rebuild this thing myself? I have no experience and none of the special tools though. I am pricing new and rebuilt units and its all out of my price range.

 

[Darthomas]

If I recall you need to buy a pack of selective spacers, and without the tools, I don't think you'll ever be able to set the pinion depth correctly.
Ring gear maybe.
Sorry.

 

[Root2812]

Thanks, That's the response I expected. I am really hoping to find a way to get this thing back on the road before summer is half gone. If worse comes to worse I do see a few open diffs with 2.76 gears on CL. I can grab one of those to get me by for a while.

 

[Darthomas]

Thanks, That's the response I expected. I am really hoping to find a way to get this thing back on the road before summer is half gone. If worse comes to worse I do see a few open diffs with 2.76 gears on CL. I can grab one of those to get me by for a while.

Acceleration will suffer, but top end is like, wow, man.

 

[Root2812]

Acceleration will suffer, but top end is like, wow, man.

How bad would the acceleration suffer? I will do what I have to do to get the car back going so I can enjoy it but I would hate to lose all the pep that makes it so fun. The 3.55's really set me back in the seat with the power of the 360. Would the 2.76 make it a dog?

 

[GTX MATT]

Yes you will hate it with the 2.76s... Clunkyness in the rear? Did you try to spin it with the axles out? I really cant think of anything that would cause this in a rear that was good at any point unless one of the stoppers for the adjusters is missing and it backed off. Even if the bearings were shot it wouldn't be "clunky."

 

[Root2812]

I didn't spin it with axles out. I feel it at highway speeds so I pulled the axles to check the axle bearings and they seemed fine. I stuck the axles back in and turned a wheel and felt the clunkyness and it felt like it was coming from the 3rd member. What are the adjusters you mention? Is that inside the 3rd member?

 

[Darthomas]

Chunks missing from spider gears, sounds like. Not nearly as bad if that's all, but even if you had a worn case, that's something you can replace at a lot lower cost, reusing your good ring and pinion.
The pinion can stay in, and you probably can set the ring gear properly yourself.

 

[GTX MATT]

I would agree, it sounds like maybe the problem is in the spider gears/case if its clunky.

The adjusters I'm talking about thread into the main caps, and are for adjusting backlash/bearing pre load.

I would remove the 3rd member and inspect it. If the problem is in the spiders then you just need to change the case out for one with good spiders, or change the spiders. You can do this fairly easily, you just need a dial indicator and stand to set the backlash and a spanner wrench (they're like 20-30 bucks from Dr Diff) to turn the adjusters. You can change the case by removing it, R&Ring the ring gear to the good case, and resetting the backlash the same as it was before. I have a good, open case you can have if you need it but it'd probably be expensive to ship from CT.

You could install a sure grip case too if you can find one in your budget. Speaking of which, are you SURE you don't have a sure grip already? If you don't have the correct additive it could cause problems also...

 

[Darthomas]

But the weird part is noticing it going straight down the road, when the spider gears should not be moving......

 

[Darthomas]

When I had chunky spiders in a 7 1/4", and replaced just the spider gears, the replacements got chunked, too.
The case had spread or worn so the next time, I put two cupped bushings under the case- side spiders. Seemed smooth and OK, but chunked again.
Then I found me an 8 3/4", because I was tired of fooling around with a weak rear end, when I wanted to drive like a cowboy.

 

[Root2812]

This is all great information. I don't have a sure grip now. The car came factory with a 489 cased sure grip with 3.55 gears. The guy I bought it from told me after I had the car bought and shipped to Minnesota that he had pulled that 3rd member and put in this 741 open unit because the old one had a bad pinion bearing. He then offered to sell me the original 3rd member for $600 plus shipping. Needless to say it's still in Southern California.

My dad is hounding me to get my car out of his way so I am going to have to rent a trailer and drag it home so I can tear into it to inspect things. The car is 4 hours away at his place so all I could do was the little bit I did.

I would like to upgrade to a sure grip if I can swing the price.

 

[datblackchallenger]

hi guys. So I have a 74 Roadrunner with a factory 360 and 727. The last owner told me he had a bad pinion bearing in the factory sure grip 489 cased center section so he pulled it and stuck in an open diff 741 he had laying around. That is what was in the car when I got it and is still in there.
I they went with 489 rears only in 73 and after.

I bought the car off the Internet to get a mostly rustfree car since I'm in Minnesota and everything rusts here. When I got the car it was as described but with an unexpected problem in the read end. It feels like something is binding or dragging. I can actually feel it slow the car down on the highway. I pulled both axles and checked the axle bearings and those are good and with the car on the lift I can turn the wheels and feel the clunkyness in the rearend.

Any idea what this could be?

Second thing is that I can't afford a new unit so I was going to used if I have to replace it. My car is a 74 and I found a 73 roadrunner complete rearend that is also 8 3/4 on Craigslist. Is there anything I need to look for to ensure I can take the center out of that one and swap it into my car? Like spline counts or anything? Also what can I do to tell if it's any good or not.

Thanks

 

[Cornpatch MO]

hi guys. So I have a 74 Roadrunner with a factory 360 and 727. The last owner told me he had a bad pinion bearing in the factory sure grip 489 cased center section so he pulled it and stuck in an open diff 741 he had laying around. That is what was in the car when I got it and is still in there.

I bought the car off the Internet to get a mostly rustfree car since I'm in Minnesota and everything rusts here. When I got the car it was as described but with an unexpected problem in the read end. It feels like something is binding or dragging. I can actually feel it slow the car down on the highway. I pulled both axles and checked the axle bearings and those are good and with the car on the lift I can turn the wheels and feel the clunkyness in the rearend.

Any idea what this could be?

Second thing is that I can't afford a new unit so I was going to used if I have to replace it. My car is a 74 and I found a 73 roadrunner complete rearend that is also 8 3/4 on Craigslist. Is there anything I need to look for to ensure I can take the center out of that one and swap it into my car? Like spline counts or anything? Also what can I do to tell if it's any good or not.

Thanks

Go to Ebay Motor, search for Mopar 8 3/4 There usually are several listed. problem tho would be truthful listing or amature re-build. I bought a complete 742 case 3:55 sure grip-totally re-built with new parts from a guy that sells on there regularly. I think he was in Michigan ...Very happy with it . Cost under $600 a couple years ago................................MO