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

Help Support 7173Mustangs.com:

Animal Lawyer

Well-known member
Joined
Sep 3, 2020
Messages
222
Reaction score
27
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
 
Joined
Sep 12, 2015
Messages
5,892
Reaction score
961
Location
SW Ontario
My Car
1971 Mustang Mach 1, M code, 4 speed.
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

Well-known member
Joined
Apr 25, 2020
Messages
115
Reaction score
50
Location
NJ
My Car
73 Mach 1, Q code, 4 speed, fully optioned
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.
 
Joined
Jan 2, 2019
Messages
233
Reaction score
155
Location
Wisconsin
My Car
1972 Mustang convertible, 351C 2V with FMX (#'s matching)
Currently equipped with 351C 4V 4BM, .060 over, roller cam, Sanderson block huggers, -AC
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! 😂
 
Joined
Jul 27, 2021
Messages
734
Reaction score
533
Location
East Texas
My Car
1971 Mustang Mach 1 M code 351 4 speed
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

Well-known member
Joined
Jul 26, 2019
Messages
68
Reaction score
36
Location
Michigan
My Car
71 Mach 1 M Code
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
 
Joined
Sep 12, 2015
Messages
5,892
Reaction score
961
Location
SW Ontario
My Car
1971 Mustang Mach 1, M code, 4 speed.
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.
 
Joined
Apr 3, 2022
Messages
7
Reaction score
2
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

Well-known member
Joined
Sep 3, 2020
Messages
222
Reaction score
27
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?
 
Joined
Jan 1, 2012
Messages
8,173
Reaction score
931
Location
Springfield, OR
My Car
1971 Mustang Sportroof M code
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.
 
Top