Building That Rear

Sometime in 2018.

I’m in the phase with the car where all I seem to be doing is taking it apart, rust holes everywhere and I’m hacking sheet metal out wondering when I’ll hit solid steel so I can start putting pieces back in.

Putting stuff back in is very important to me right now because I take a lot of things apart but rarely do I ever put them back together. I desparately want to break that cycle and I’m afraid I’ll lose interest in the car before I replace a single panel.

To help myself mentally, I decide, out of the blue, I’m going to rebuild that 9 inch rearend.

I have the complete housing. I have the nodular carrier with the Detroit Locker in it and it has the gear ratio I want. I’m gonna do this!

I begin by removing the axles. I get one out. The other refuses.

I always thought that all 9 inch came with 31 splined axles. This one schooled me. It had 28 splines. SMH. The differential I bought takes 31 spline.

I go online to see about some 31 spline axles. They’re referencing big and little bearings. What are they talking about? I do more reading. Oh, depending on model and package of the auto, the 9 inch came with either a large axle bearing at the end of the axle tubes or small. Better yet, the size of the bearings dictated the diameter of the axle shafts. I learn that small bearing rears use 28 spline axles and large bearing rears use 31.

Supposedly there were/are some 31 spline axles that worked with small bearings. I search. I find nothing. Crud.

What do I do?

Keep looking for these mythical axles? Buy a housing that takes big bearings? Look for another posi type differential that uses 28 spline axles?

Before I can make a choice, something else pops into my brain.

My Tremec has a 6th gear that is .50 overdrive. Just what will my engine rpm be at 75 mph with the 3.50:1 gears I have?

I think I calculated it to be around 1700 RPM.

Here I have an engine whose power band doesn’t start until 3500/4000 RPM. When will I ever use 6th gear. I may never use 5th gear either.

More computing and I don’t get into a realistic engine speed until I get into the 4’s. 4.56 seems to be the best, according to the calculations but, really? Who runs 4.56s on the street? 4.11s Maybe

I’ve never changed the gears in a rearend. I do have dial indicators and tools but I’m too deep into the Mustang to be taking on ANOTHER first time project. I was imagining a direct bolt together type of project, not a pinion gear depth setting type project.

I have to do something to get at least a part of the car in an advancing stage and I’m not accomplishing a whole lot looking for combinations of parts to make this rear end combo work.

Screw this, I got way too much on my plate now, I’m going to have the rear built for me as planned. I grab all of the pieces I have, load them up and take a drive to Huntington Beach to J&S gears.

I’ve had these guys build a 12 bolt Chevy rearend for me in the past. They do good work.

I wait my turn and eventually talk to the owner whose name escapes me at the moment. We get to talking and I ask about small bearing 31 spline axles. No such beast. Can he upgrade my housing to large bearings? Yes, they’re known as Torino bearings.

I tell him what I’m building and I get his attention. I ask his advice on the rearend gears. I ask him what he suggests for my combination. I give him the info. Trans final drive, tire height, target speed.

He does the calculations and tells me 4.56:1. I laugh and tell him that’s what I came up with as well but thought it was too much and really, I was thinking, How cool would it be to say that I have 4.56s in the back. To avoid letting my ego make the decision, I’m thinking 3.90 to 4.11. The guy says 4.56 is what I should get but he’ll put in whatever I want. I go with the 4.56s.

I need some axles and I’ve been looking for some Tom’s Axles but they’re hard to find. J&S warns me that Tom is semi retired and the axles are hard to come by. He gives me some other name that begins with an N that I vaguely remember. I’m surprised he isn’t pushing the usual Spicer, Moser or Currie stuff. I say ok to his recommendation.

I ask when it will be ready and I’m told 10 business days.

I check back periodically and we’re waiting on the axles. WTH? I thought these guys used this brand a lot, what’s the hold up?

Three weeks total go by and I can pick up the rear end. It takes me another week to ditch work and get back to J&S.

The rear looks good. The orangy-red differential sitting in a blacked out housing look great. He points out the Axles. They’re Tom’s Axles. Very cool. When I dropped the rear off, J&S dude called Tom’s while I was standing at the counter to check on a set of axles for me. Tom’s doesn’t have any on the shelf, they would have to make them. The alternate axles maker would supposedly have them in stock being a bigger business and all. Turns out when J&S places the order and the axles show up, they’re too long. They don’t have my size in stock and will have to make them. Somehow in the middle of all of that, Tom’s calls back and says they can have the axles earlier than they thought. J&S makes the call and goes with the Tom’s axles.

The housing has the Torino bearings installed and the housing is straight. I had asked about that when I dropped it off.

Pretty straight forward stuff. I now have my cleaned up and rebuilt 1970 9 inch Mustang rear end with Torino bearings, Nodular carrier, Detroit Locker differential with 4.56:1 gear set, Tom’ Kick Ass Axles and 1350 pinion.

For a few years the rear has sat on a cart and has moved from one corner of the shop to another but now it’s bolted into the car being suspended by coil over shocks and a three link control arm system.

I still have to add gear oil but it’s all ready to run..