Up to my rear end…

[PnutDr]

I’m trying to decide what to do about my rear axles that’s shot. So I have no mechanical ability, so I’m having a restoration shop do the work on my 76 Bronco. Found out yesterday that my current rear end is shot, as the mechanic said it has over an inch of play in it. Not sure how that happened because it was my sister’s truck and she wasn’t rough on vehicles. I don’t know how hard it is to find a used replacement rear end or how much rebuilding one will cost, especially when I’m paying for labor. This 76 was about 95% stock, so I’m pretty sure it came with the 3.50 rear differential. My question is should I go ahead and change to a 4.11 differential regardless whether it’s rebuilt or swapped out? Since I want to go with 33” tires I figure the 4.11 will be more ideal. I know it means changing the front differential too, which adds up.

 

[Yeller]

It all depends. You can get a bolt in ready unit from Wild Horses, they also sell assembled pieces all the way down to individual components. So the sky is the limit and how much labor your shop will charge. The least labor for the shop is a bolt in replacement, but that it completely up to you and your shop.

As for gearing 4.09, 4.10, 4.11 is all the same, just splitting hairs between manufacturers and their individual design. Should be good with 33’s. Will certainly keep the fun factor happy along with the transmission and/or clutch depending on your choice.

 

[Timmy390]

Bit of a rabbit hole.....You can get a Ford 9 ready to drop in from many different sources. How you want it built is the question? Do you want the factory limited slip or upgrade to some type of locker? Heck even an open diff is a choice. Then with 33's now is the time to upgrade axles as you have to build the Ford 9 to fix the axles.

Then you have to move to the front. No quick drop in there due to the design. I was quoted $1000 for a gear change on the front.

Gets spendy quick when paying someone else.

Tim

 

[SteveL]

For just a rear diff you can find a used one on your local Craigslist or message place. To have a shop do it the start about $750 depending on how much you're having done. In your shoe the best option is most likely a full rebuild. You're looking at a new 31 spline 3rd member. Then you decide from open to an arb depending on use. Some 31 spline moly axles with new bearings and seals. Roughly $2k. Then if going to 4:11 front will also need a new carrier to match gears. Goodbye tax check

 

[DirtDonk]

Where is the actual play? Is it rotation of the pinion? Is it in and out for the axle shaft?
Reason I ask, and I don’t want to send you in the wrong direction (down the wrong rabbit hole even!), but I drove my 71 with over half an inch of rotational movement of the pinion, for probably 60,000 miles before I rebuilt the rear end.
How long it had been that way I have no idea. But it was over 120,000 miles before I got the work done.

Now, maybe yours is making noises and having issues that indicate that play is far too much. And in reality, it is far too much, because we’re only supposed to be talking a few thousandths of an inch of play.
Just letting you know that, under the right circumstances, it doesn’t mean the end of the world.

 

[gnpenning]

Congratulations on the Bronco and welcome.

Not enough information. With out knowing how you plan to use the vehicle when done everything is a guess. What motor and transmission combo is important as well. You gave tire size, now fill in the rest of the blanks and we can give better advice.

As mentioned the play is interesting and you should have noticed when driving it. How respected is the shop doing the work?

 

[Broncobowsher]

Inch of play, need a better understanding of this. If the shop usually deals in modern vehicles, they may not know that old trucks with low axle ratios will have more backlash then they normally see. If it is an inch of vertical play at the yoke, I am not sure it is even going to move under it's own power. Or an axle end play, well the axle would fall out on it's own if really an inch. That is just a bearing swap and a new seal.

An Inch of play is not descriptive enough of a problem.

 

[DirtDonk]

Also, if the diff is equipped with a locker, it might be the type that shows excessive play when you rotate the pinion yoke.
Just a thought.

 

[bmbm]

I’m trying to decide what to do about my rear axles that’s shot. So I have no mechanical ability, so I’m having a restoration shop do the work on my 76 Bronco. Found out yesterday that my current rear end is shot, as the mechanic said it has over an inch of play in it. Not sure how that happened because it was my sister’s truck and she wasn’t rough on vehicles. I don’t know how hard it is to find a used replacement rear end or how much rebuilding one will cost, especially when I’m paying for labor. This 76 was about 95% stock, so I’m pretty sure it came with the 3.50 rear differential. My question is should I go ahead and change to a 4.11 differential regardless whether it’s rebuilt or swapped out? Since I want to go with 33” tires I figure the 4.11 will be more ideal. I know it means changing the front differential too, which adds up.

If you did not notice a problem while driving it I would go pick it up from that place and drive it until you come up with a plan and buy all the parts.

 

[ntsqd]

Agree with above, if it is rotational play it may not be anything to be concerned about. Detroit Lockers in particular can have a startling amount of rotational play and be perfectly fine.

 

[PnutDr]

Just circling back. Thanks for all the replies. This truck will be an occasional driver. To the beach maybe once or twice a year, to my place in NC which is about 20 miles from where I live, and back and forth to work (15 min ride one way). I simply like the look of the early Broncos with “some” lift and 12.50 33s. That being said, I want to be able to go off-road when called-for, and I don’t like to wonder if my truck is up to it. It’s the philosophy of “I’d rather have it and not need it than need it and not have it”.

Transmission is going to be a C4. I do plan on switching the back axle to positive traction. The engine is gonna produce between 300-350 hp. The shop I’m using specializes in restoring older vehicles, they have a 69 Bronco in the shop now. I showed up yesterday and he walked me through the different stages my Bronco is currently in. He took me to where a guy was prepping the rear axle and he showed me the difference in movement between the shaft and the yoke. It’s like 1.5” of rotation before it engages in either direction. So we’re gonna price out buying a rebuilt one vs him doing the rebuild. His shop is 5 minutes away and we communicate weekly. Basically I think we’re doing what you guys have been suggesting. The rear axle gear ratio worries me. If it’s a 3:5, the 33” tires are going to be a problem for me. If I go down to a 4:11, then the front axle (which is practically brand new) will need to be changed. I’m learning there’s lots of spaces between rocks and hard places in restorations.

 

[Timmy390]

The rear axle gear ratio worries me. If it’s a 3:5, the 33” tires are going to be a problem for me. If I go down to a 4:11, then the front axle (which is practically brand new) will need to be changed. I’m learning there’s lots of spaces between rocks and hard places in restorations.

The rabbit hole....or maybe snowball.....

You could get away with 3.50's and 33's with a manual transmission. The fact you plan to run a C4 means you can get away with 3.50's and 33's for a while but the trans is not going to like it. It will slip, generate heat and clutch ware. In the end, you will kill it.

I run 33's, 4.11 gears with a manual 5 speed (M5ODR2) and it's perfect. With a C4 I would look more at 4.30's minimum with 4.56's being probably the best choice.

Also, Don't fear the gear.......

Tim

 

[Broncobowsher]

If there is already a detroit locker (or other lunchbox style locker) installed, that play he feels is wrong isn't wrong. Being an old car restoration shop doesn't mean they know what off-road parts may have been swapped in the past. You also need to know what axle ratio you have now. Not just what the tag says it was built with. If a locker has already been installed, you might already have a proper gear ratio. And yes, 4.11 with 33s is a good choice on a C4

 

[PnutDr]

Just circling back. Thanks for all the replies. This truck will be an occasional driver. To the beach maybe once or twice a year, to my place in NC which is about 20 miles from where I live, and back and forth to work (15 min ride one way). I simply like the look of the early Broncos with “some” lift and 12.50 33s. That being said, I want to be able to go off-road when called-for, and I don’t like to wonder if my truck is up to it. It’s the philosophy of “I’d rather have it and not need it than need it and not have it”.

Transmission is going to be a C4. I do plan on switching the back axle to positive traction. The engine is gonna produce between 300-350 hp. The shop I’m using specializes in restoring older vehicles, they have a 69 Bronco in the shop now. I showed up yesterday and he walked me through the different stages my Bronco is currently in. He took me to where a guy was prepping the rear axle and he showed me the difference in movement between the shaft and the yoke. It’s like 1.5” of rotation before it engages in either direction. So we’re gonna price out buying a rebuilt one vs him doing the rebuild.

 

[PnutDr]

If you did not notice a problem while driving it I would go pick it up from that place and drive it until you come up with a plan and buy all the parts.

I may have driven this Bronco once or twice in the late 80s. Had to be transported on rollbacks ever since.

 

[PnutDr]

The rabbit hole....or maybe snowball.....

You could get away with 3.50's and 33's with a manual transmission. The fact you plan to run a C4 means you can get away with 3.50's and 33's for a while but the trans is not going to like it. It will slip, generate heat and clutch ware. In the end, you will kill it.

I run 33's, 4.11 gears with a manual 5 speed (M5ODR2) and it's perfect. With a C4 I would look more at 4.30's minimum with 4.56's being probably the best choice.

Also, Don't fear the gear.......

Tim

I may be able to get a 4.56 rearend from a Bronco that my nephew parked in the woods 20+ years ago. I have little faith that it’s still in good shape. Not sure if the front end is on it, but if it is it may likely be a Dana 30 since it’s a 69. There’s a real good chance that I simply don’t know what I’m talking about.

 

[PnutDr]

Inch of play, need a better understanding of this. If the shop usually deals in modern vehicles, they may not know that old trucks with low axle ratios will have more backlash then they normally see. If it is an inch of vertical play at the yoke, I am not sure it is even going to move under it's own power. Or an axle end play, well the axle would fall out on it's own if really an inch. That is just a bearing swap and a new seal.

An Inch of play is not descriptive enough of a problem.

It’s about 1.5 inches of rotational play. Takes about 1.5 inches to engage by turning the shaft or the yoke.

 

[Broncobowsher]

It’s about 1.5 inches of rotational play. Takes about 1.5 inches to engage by turning the shaft or the yoke.

Yep, that sounds like a Detroit locker. If you feel real carefully you may find the different backlashes. Where the ring and pinion touch, where the side gears touch, even the slop in the axle shaft splines. Your 1½" sounds like everything added up. The key is finding which backlash is which and then you can tell if it is a problem or not. Detroits have a ton of 'normal' backlash.

That much backlash and still be drivable in a 'normal' differential isn't typical. Usually there are a lof of problems before it could possible get that bad.

And low axle gears show more backlash than high ones as well. the clearance is the same, just takes more rotation to get through that backlash

 

[Timmy390]

The Spartan locker in my Samurai has about that much "play".

Tim

 

[ntsqd]

Was always disconcerting when cresting a hill in Patch and have it roll downhill uncontrolled until the backlash slack in the rear Detroit had been taken up. I never got used to that.

Sounds like you need to ask the shop to figure out what ratio it has in it now. That may tell you a lot. If the rear drop-out is out, then post some pictures of the diff and the ring gear. Of interest are both the teeth on the gear and any stamped or scribed marks on the OD of the ring gear.