Hey, on my 69 Mach 1, I have a 9" diff. Is there another way to find out what spline (28 or 31) I have on mydiffbesides pulling the axle?

 

Generally most cars received the small axle bearings and 28 spline axle shafts. Exceptions to this were the ultra Hi performance Boss 302’s, Boss 429’s, 427’s, 428 CJ/SCJ and the 429 cars which received the 31 spline carriers and axle shafts. So, it would be pretty safe to say that you have the 28 spline axle.

 

Pull the axle and count them. You don't know what the previous owners did, so short of pulling it, you'll be guessing at the spline count. FWIW, my Currie 9" in my '65 fastback has 31 splines. FWIW2, you can now also get custom axles made in 35 spline and maybe 33 spline(?) for the 9".

 

Pro-Stock cars run 40's.

 

Stock 9" will have 28. Pull one axle and check, it's very easy. Take the tire off then the brake assy. Then you will see four nuts holding a plate that holds the bearing in place, use the hole between the studs to get at the nuts.Then pull the whole axle out....simple.

 

You can tell the spline count on OEM 9" axles by lookingat the hub, if it has a recessed oval or D shaped indentation in the end of the axle it is a 28 spline. If it is flat, and has two holes (about 1/2" each) it is a 31 spline.
This is just a general rule, but I have never seen a case when it isn't true on a Mustang 9" (unless it has aftermarket axles)

Peter