73 q code vert competition suspension 9” posi rear - how do I tell 31 or 28 spline?

[Animal Lawyer]

I am considering swapping the 3.25 posi center section for a 3.73 posi. How do I determine whether it’s a 28 or 31 spline? 73 q code vert competition suspension 9” posi rear

 

[Stanglover]

Just a guess, but I'd say it will be a 28 spline. You could pull an axle shaft and count the spline, but a load of work.
OR just wait till you pull the center and order the center accordingly. You'll be pulling the shafts then anyway.
So, just curious, why a 3.73 and not a more gas friendly 3.50. What brand center are you considering? I hear Eaton TruTrac is a good choice.
This is not an area I have any experience other than having my car's center section upgraded to a TracLock, keeping the 3.25 gears in a 28 spline axle. I'm just curious.

 

[Doug's 73 Mach1]

Look at the end of the axle with the center wheel hub cap removed or the rear whole wheel off. If the center of the axle has a deep recess that is double "D" shaded it is 28 spline. If machined smooth with a small counter sunk recess at the center it is 31 spline. 3.25 gears came with 28 spline axles from the factory.

 

[DAverkamp]

Just a guess, but I'd say it will be a 28 spline. You could pull an axle shaft and count the spline, but a load of work.
OR just wait till you pull the center and order the center accordingly. You'll be pulling the shafts then anyway.
So, just curious, why a 3.73 and not a more gas friendly 3.50. What brand center are you considering? I hear Eaton TruTrac is a good choice.
This is not an area I have any experience other than having my car's center section upgraded to a TracLock, keeping the 3.25 gears in a 28 spline axle. I'm just curious.

I at times wish I had 3.73's from my 3:50 trac loc (from the orig 2:75 open) but he prolly wants to get where he's going in a "bigger" hurry!

 

[Don C]

A 3.73 is a good choice for an overdrive transmission.

 

[71ProjectJunk]

My understanding is that in 1973 the only cars that came with 31 spline axles were the ones with a 351CJ and 3.50 Trac-Loc differential, everything else was supposed to be 28 spline.

 

[ArtBBQ]

Maybe those in the know could help decipher my axle spline. 71 M code Mach 1 that had a 3.0 gear limited slip originally. Currently has a 3.50 gear.
I'd like to go to a trutrac or similar rear end

 

[Stanglover]

Maybe those in the know could help decipher my axle spline. 71 M code Mach 1 that had a 3.0 gear limited slip originally. Currently has a 3.50 gear.
I'd like to go to a trutrac or similar rear end

My "tag book" shows for the 71 351 4v with 3L00 (code O) AUTO trans. tag WFB-D. The "L" indicates limited slip. My M code with 3.25 open IS 28 spline. Check the ends of the axle as Doug describes.

ALSO, there is NO 3.73 listed anywhere. it is either a 3.50:1 or 3.91:1 and they were only in the Boss or HO or 429's (not counting the lesser 3.25 and lower ratios)
Side story:
What does still make me wonder was back in 1980, I bought a 72 351Q code 4 speed, that had had the motor partly replaced with Boss parts. That car would pass anything but a gas pump, get rubber in 4 gears, but back then I knew nothing about codes and tag numbers. I stupidly sold that car in 1991 without knowing exactly what was in it. I do know the motor had been played with, but I wonder if the axle was swapped as well. The guy I bought the car from told me he got the motor from a friend who crashed and totaled his Boss 351. I suspect it had the 3.91 TracLock rear end.

 

[jasonwthompson]

I am considering swapping the 3.25 posi center section for a 3.73 posi. How do I determine whether it’s a 28 or 31 spline? 73 q code vert competition suspension 9” posi rear

If you are replacing the entire third member, I would be interested in the 3.25 28 spline posi third member should you sell it.

 

[Animal Lawyer]

I found a virtually new 3rd member 28 spline with a traklock posi for a decent price (he’s asking $725) problem is it has 4.11 gears. Is the teak lock that much better than the trutrak I have, that it pays to buy it and swap to 3.73 gears or should I just swap gears in mine?

 

[Don C]

If you have a Detroit TrueTrac it is much better than the original Ford TracLok. The TracLok uses clutches for the limited slip that don't have all that long of a life span. TrueTrac uses helical gears that last longer and provide better traction to both wheels.