28 or 31 spline

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alarmrick

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I have a 71 mach1 with a 351 4v, c6 and 9 inch rear. I am looking to replace the drivers side axle. Would the axle be a 28 spline or 31? Any suggestions on where to get one at a decent price would be appreciated. Thanks Rick
 

Doug's 73 Mach1

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31 splines were typically used with 3.50:1 ratio rear ends and up and included a "N" case "pumpkin". Check your rear axle code on your door data plate. That code can be used to decipher what gear ratio the car came with from the factory.
 

Ron Tanzi

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1972 Ford Mustang Mach 1 bright red, standard black interior,pb,ps
gauge package.rim blow steering wheel,15" magnum 500s,351 C 2bbl running a 4bbl Holley 600,MSD 6a with stock distributor and Pertronix,Comp cam mild grind, The Engine(rebuilt by me in 2002 for the second time the first rebuild I did was stock in 1995) made 305hp and 358 lb ft on the dyno. 9" 2.75 rear end,Global west sub frame connectors,620 front coils.Rear defroster. My parents bought my car brand new the year before I was born. I cherish it like my child.
Another option when in doubt is to just pull an axle and count the splines. On the bright side pulling a Ford 9" or 8" axle is way easier to do than on a junk GM 12 bolt diff among many others.

Ron
 

boilermaster

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3.50 and up and Nodular case might be 31 spline, but my parts car does not follow that path.
1973 Q code, 3.50 gearing, nodular center section, Daytona pinion support, 4 pinion and one wheel peel and 28 spline axles .
just guessing it had the Daytona and nodular case because it was a toploader car.
Boilermaster
 
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My '71 M-code came with a 3.50 traction-lok and has 28 spline axles. The only cars that came with 31 spline were with high performance engines, such as Boss 351s and 429s.

The link I provided above tells you how to determine 31 or 28 splines without pulling the axles.

West Coast Classic Cougar has 28 spline axles for $110 each

 
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3.50 and up and Nodular case might be 31 spline, but my parts car does not follow that path.
1973 Q code, 3.50 gearing, nodular center section, Daytona pinion support, 4 pinion and one wheel peel and 28 spline axles .
just guessing it had the Daytona and nodular case because it was a toploader car.
Boilermaster
Me too - 73 Q-code, 4 speed, 3.50 gears, staggered shocks, one wheeler from the factory, has N-case and Daytona support, all original and 'numbers matching', but 28 spline.

Changed to a Trac Lok with the original N-case now ;)
 

machattack

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Doug's 73 Mach1

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Another way to tell, is 28 spline axles have a deep recessed double "D" shape at the center of the hub the wheel fits over on outboard ends which can be seen with the hub caps off and/or wheels removed. 31 spline type do not, but look essentially flat on the face on the hub the wheels fits over .
 
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Another way to tell, is 28 spline axles have a deep recessed double "D" shape at the center of the hub the wheel fits over on outboard ends which can be seen with the hub caps off and/or wheels removed. 31 spline type do not, but look essentially flat on the face on the hub the wheels fits over .
Careful with that, I have encountered what looked like 31 spline but they were in fact 28 once the axles were pulled and checked.
 

Doug's 73 Mach1

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Photo, 31 spline and 28 spline photo from 429 Mega site

1652230294365.png 1652230294365.png
 
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I believe that after 1972 this method of ID'ing the axles can be incorrect. From what I have seen Ford after 1971 did make some 31 spline axles with the concave hole, so it does not always hold true for cars built after 1971.
I've had personal experience with this and can say that there are 28 spline applications that use the the 3 hole pattern. The application I saw for myself, was a pickup truck axle.
 
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