28 or 31 spline...

[keith1562]

My 1972 Mach 1 has a 351cj, 4 spd. and a factory traction loc differental. It came with 3.25 gears but a previous owner unpgraded to 3.73. Do mustangs of this era all have 28 spline axels? I spoke to the previous owner who did the switch, he said he used the whole center section out of an early 70's bronco. So I also wonder if bronco of this era all had 28 spline axels. Is their anyway to check, short of pulling an axel shaft? Later this spring or early summer I intend to rebuild the rear end (it currently has the crush sleeve adjusted to the max, but still whines) and possibly switch to 3.91 gears. Speaking of which all I ever see listed by Jegs or Summit is 3.89 not 3.91, what gives?
Thanks for any input... positive, negative or neutral!
Keith1562

[JTS71 Mach1]

The method I found refers to the end of the axle shaft itself. And applies to 1971 and earlier axles. It says after 71 that some 31 spline axles come with the rectangle. If the center looks like a rectangle with rounded ends, it's a 28 spline, if it has 3 holes drilled in a row, it is a 31 spline axle. They also say the only way to know for sure is to pull an axle. Sorry best I could find. JTS 71 Mach1

[turtle5353]

I had several early broncos. 67, 70, 71 and a 76 and they were all 28. Not saying that they dont have 31 but the ones i owned were 28.

[keith1562]

Thanks JTS71 Mach1 that's the info. I was looking for. Thanks to turtle5353 as well, every bit of info. helps!
Keith1562

[1969jimmach]

I don't mean to be contrary but was under the impression all 4x4 fords were 31 splines

[turtle5353]

I don't mean to be contrary but was under the impression all 4x4 fords were 31 splines

The trucks may have been but definitely not the 66-76 broncos. All of the broncos came with 28 spline axles, but did have 2 different housings during those years that had different size wheel bearings. They had a small bearing axle and big bearing axle, but both used the 28 spline axles. I have heard but have never seen it confirmed that 76-77 broncos could be ordered with a heavy duty rear that was big bearing and had 31 splines axles. BUT I have never seen anywhere that this was confirmed. I owned and built quite a few of these broncos over the years and have owned both small and big bearing axles but all were 28 spline. I had a 89 bronco II that I swapped the running drivetrain from a 76 bronco into and the 28 spline axles held up for years to 4.11 gear with locker, 325hp and 35 inch tires.

[gxr02190]

It's unlikely that your 9-inch is equipped with 3.73 gears as that ratio is predominately used by G.M. Your differential is probably equipped with a 3.70 ratio that was commonly used in light duty trucks. The most commonly listed nine-inch "ford" ratios are : 2.75, 2.79, 3.00, 3.25, 3.50, 3.70, 3.89 (before 1970), 3.91(after 1970), 4.11, and 4.30. Ford does, however, offer the 3.73 ratio in later-model E & F Series trucks, and also later-model cars such as the mustang, but those are not of the nine inch ring gear variety. The 3.89 ratio was more widely used in ford cars than the 3.91 - starting around 1957 and ending sometime around 1970. The 3.91 was only used a few years in mustangs and cougars. GXR

[keith1562]

It's unlikely that your 9-inch is equipped with 3.73 gears as that ratio is predominately used by G.M. Your differential is probably equipped with a 3.70 ratio that was commonly used in light duty trucks. The most commonly listed nine-inch "ford" ratios are : 2.75, 2.79, 3.00, 3.25, 3.50, 3.70, 3.89 (before 1970), 3.91(after 1970), 4.11, and 4.30. Ford does, however, offer the 3.73 ratio in later-model E & F Series trucks, and also later-model cars such as the mustang, but those are not of the nine inch ring gear variety. The 3.89 ratio was more widely used in ford cars than the 3.91 - starting around 1957 and ending sometime around 1970. The 3.91 was only used a few years in mustangs and cougars. GXR

Thanks for the insight regarding rear end ratios. It confused me that drag pak cars and some Boss 302s are said to have 3.91 heavy duty (n)case 9" rears. In the aftermarket it's hard to find a 3.91 they list 3.89 instead. You are probably right about the current gears being 3.70s not 3.73. Either way when I rebuilt the rearend I intend to go with 3.89 gears. I'm hoping for a little more low end "grunt". While on the subject, what makes the "Daytona" pinion support special?
Thanks,
Keith

[gxr02190]

As far as I know, it's supposedly a stronger casting (or CNC'd) piece that houses a larger bearing. I believe it's primarily used for circle track racing where there are more extreme loads on the pinion shaft due to being on-and-off the throttle. GXR