28 or 31 spline axles

[Paul Nap]

Finally going to pull the trigger on getting rid of the 3.00 open (single wheel) rear end for my 69 Mach 1 351 4bbl and installing 3.25 or 3.50 trac loc posi.
Question:
IS there a way to find out if I have 28 or 31 spline axle without first having to pull the Yoke to count the splines?
Any recomendations or advice on going with 3.25 or 3.50 gears? I still want to be able to do 65mph on the highway with out screaming the motor.

 

[Wicked One]

Through my research you should have a 28 spline. Only the cj came with 31.
With a 4 sp I would suggest a 331 rear, nice cruising. I will do soke more research and see what I find.

 

[Paul Nap]

Through my research you should have a 28 spline. Only the cj came with 31.
With a 4 sp I would suggest a 331 rear, nice cruising. I will do soke more research and see what I find.

Thanks!

 

[MikeRadMan]

MY 1970 Mach I (351W, FMX) had 28 splines.

 

[Paul Nap]

MY 1970 Mach I (351W, FMX) had 28 splines.

That's what I was thinking but probably better to pull it apart and count. What gears are you running with?

 

[MikeRadMan]

That's what I was thinking but probably better to pull it apart and count. What gears are you running with?

The gears were 3.00, But I have changed the entire rear axle. It's now 2" shorter overall, stuffed with Strange Engineering Hardend axles with 31 splines, Eaton Detroit Tru-Track Limited slip with 3.89 gears. Also I am ditching the FMX for a 5speed Manual TKO600.

 

[ford20]

All 69 and 70 Mach 1's with the 351 and 3 speed came with 28 splines. I mean it is possible that someone that had the car before you decided they wanted to beef up the rear end a little and changed out the axle shafts to one of the 31 splines found in one of the Fairlanes or Galaxies with the 427s. You can count the splines or you can look at them, MOST 28 spline axles are tapered slightly before the beginning of the spline and about 3/4 of the way down the end of the axle shaft while MOST 31 splines aren't tapered.

Do you still have the stock 3 speed in the car or what transmission did you swap it out for if not?

 

[Paul Nap]

All 69 and 70 Mach 1's with the 351 and 3 speed came with 28 splines. I mean it is possible that someone that had the car before you decided they wanted to beef up the rear end a little and changed out the axle shafts to one of the 31 splines found in one of the Fairlanes or Galaxies with the 427s. You can count the splines or you can look at them, MOST 28 spline axles are tapered slightly before the beginning of the spline and about 3/4 of the way down the end of the axle shaft while MOST 31 splines aren't tapered.

Do you still have the stock 3 speed in the car or what transmission did you swap it out for if not?

Still the stock FMX 3 speed AUTOMATIC
THANKS!

 

[ford20]

I just found a picture of what I was talking about.

 

[ford20]

Also while doing some research I found that the 3.25s would be if you expected to do more highway use while the 3.50s are more of a street gear and will give you better throttle response but will rev the engine a lot harder on the highway.

 

[Paul Nap]

Also while doing some research I found that the 3.25s would be if you expected to do more highway use while the 3.50s are more of a street gear and will give you better throttle response but will rev the engine a lot harder on the highway.

Im thinking 3.50's. I will be on the highway for an occasional show here and there but not a lot and only an hopur or so away. Right now I'm just over 3000rpm's @70mph

 

[Frankie Lap]

We gotta give that car some legs. She needs more than an hour drive, it's 31/2 to Carlisle Show, and your car would love it there. Especially with your boy cruising in his behind you.

 

[Mike65]

Paul, I would go with the 3.50 rear gears. I had a modified 68 GT Coupe back in the 80's & I used it for occasional drive in's & cruises. Didn't use it much on the highway & the 3.50 gears really gave it some get up & go from a stop.