New Rear End Setup - Looking for Opinions

[CV355]

I was planning on keeping the existing 9" rear end in our '69 Mach 1, and just doing a gear change (4.11 to something more like 3.50 so it can be driven on the highway).  This weekend we went to tear the drum brakes off to start on a disc conversion and ran into a few concerns along the way, now I'm looking at replacing the whole rear with something new and known. 

I'm looking at a Moser M9 with a Detroit Tru-Trac helical differential, 3.50 gears, and 31 spline axles.  I noticed the 35 splines are roughly the same cost.  I tend to overbuild/overspec everything for peace of mind, and since this isn't a drag car I'm not too worried about any minor weight addition.  There won't be any hard dragstrip launches, so 31 spline torque limits calculate well within what I think this car will ever put down. 

Any advice?  Anything else I should consider?  The car has a Cal-trac setup at the moment, which I figured I'd keep as-is. 

[Joe-JDC]

I would stay with the 31 spline unit for street duty.  I raced my '69 Mach I for nearly 25 years with Strange 31 spline axles in the stock rear housing with a spool.  It was a 10 second car at the time.   Most of the new Mustangs have 31 spline axles in the V8 cars.  Joe-JDC

[cjshaker]

31 spline axles would be plenty strong for what you want. I've had Strange 31 spline axles and a Detroit Locker in my car for about 20 years now, with no issues on the street or the strip. That's with 497 HP and a 4 spd trans and CalTracs. They've taken plenty of abuse with 4000-5000 RPM launches and lots of hard street use. That includes plenty of dead hooks on the strip, which is really really hard on stuff. The only advantage my rear has is I used a Currie rear with the stronger tubes that are welded inside the housing. Last checked the axles 2 years ago (obviously no racing last year  ) and they looked just fine with no twisting.

[cattleFEeder]

Here is another option, i have had several built from these guys for use with caltracs and always been pleased.
https://www.quickperformance.com/Ford-Mustang-1967-1970-Complete-9-Rear-End-Package_p_21565.html

[475fetoploader]

Thanks for that link. I’ve never seen that company. I may just order a 3rd member and axles.

[blykins]

Thanks for that link. I’ve never seen that company. I may just order a 3rd member and axles.

They are top notch.  Back when I used to push rearend parts, they were one of my suppliers.

[My427stang]

If you go with a new housing, not an issue, but if you do not, brace the spring pads and clean the axle bearing pocket with a wire wheel.  Pads tear up easy, and modern axles use sealed bearings and the sealed bearings don't go in easily after years of gunk and crud. 

Quick Performance are good guys, they are not that far from me, and I have seen their work at displays at car shows and bought some parts from them over the years. 

I don't know what your car is now, but good time to change to the big yoke on both ends and a stronger driveshaft if you don't have it already

[Falcon67]

I put 31 spline Mark Williams in the Falcon rear back in 1998.  I've run thousands of drag strip runs on slicks, wheels up, etc.  If you pull them out, they look brand new.  31 spline will be fine.  Note on disk conversions - watch your flange to hat measure as all the kits have different offsets.  Most also are for large bearing Ford as opposed to small bearing, which is what I run.  I still run 11" disk front/drum rear, no problem stopping from 90+ at the track.  And haven't updated the shoes in years.  Can't justify the cost of the rear disk swap.  If the car moves into the 6s in the 1/8, then maybe. 

[CV355]

Great info as always, guys!  I appreciate it!

[machoneman]

I put 31 spline Mark Williams in the Falcon rear back in 1998.  I've run thousands of drag strip runs on slicks, wheels up, etc.  If you pull them out, they look brand new.  31 spline will be fine.  Note on disk conversions - watch your flange to hat measure as all the kits have different offsets.  Most also are for large bearing Ford as opposed to small bearing, which is what I run.  I still run 11" disk front/drum rear, no problem stopping from 90+ at the track.  And haven't updated the shoes in years.  Can't justify the cost of the rear disk swap.  If the car moves into the 6s in the 1/8, then maybe.

Chris, I agree that if your drums work, keep 'em. But with lightweight, drilled rotors and aluminum PBR or Wilwood type dual puck calipers, you would lose some pretty good weight, on the order of 30+ lbs. all be it on the wrong end of the car. Stopping would or course be improved for sure.

[dozz302]

31 spline > you don't need any more.