Rookie differential question

[jreyes]

Hi have a 9" rear end in my 72 F100, and a friend has a 8.8" rear end from 1990 Grand Marquis with posi trac for sale..

Question is if I can use the posi track in my 9" rear end with out changing my axels bold pattern?

if not, what will be batter to look for with out spending much money?

thanks

[tnab1970]

You cannot use the 8.8 differentials parts on the 9" .

[Grease Monkey]

As tnab1970 said, you can't use parts from the 8.8 in your 9".
As for your other question- if you can set up gears, the cheapest way to go would be to find a used 9" posi and set it up in your third member.
If you would have to pay a shop to set the gears up- then the cheapest/easiest route would probably be to find a complete third member with posi and gears that will work for your application.

[ultraranger]

Finding a 28-spline Traction-Lok 9-inch 3rd member from a donor truck can be difficult to locate. The 31-spline Traction-Lok 9-inch differential is more abundant in the '73-'79 model F100s and F150s. The main problem is the axles.

Dentside axles are longer than Bumpside axles and won't work in a Bumpside 9-inch rear end housing.

I have a Ford 9-inch Daytona N-case with a 31-spline Traction-Lok differential that I pulled from a '75 F150 4x4. I got a pair of 31-spline axles from a '71 Ford F100, that had a Ford 9-3/8" rear end, to install the 31-spline N-case in the 9-inch rear end housing under my '69 F100.

My stock F100 28-spline 9-inch axle on the left. '71 F100 31-spline 9-3/8" axle on the right.



My stock '69 C7AW-E single vertical rib 9-inch 3rd member with single track 28-spline differential/3.50 gears on the left. '75 Daytona N-case 9-inch 3rd member with 4-pinion Traction-Lok differential/3.50 gears on the right.







'71 F100 31-spline (9-3/8") axle installed in the '69 F100 rear end housing with the '69 F100 brake drum installed.



Some Bumpside F100's from '68-'72 had 9-3/8" rear ends. These rear ends had two different 31-spline axles. If the 9-3/8" axle came from a truck with a single track differential, the axle bearing will measure 3-9/64" O.D. x 1-17/32" I.D.(3,300# rating). If the axle comes from a 9-3/8" rear end with a Traction-Lok differential, the 31-spline axles will have axle bearings with a 3-9/64" I.D x 1-5/8" I.D. (3,600# rating).

A 9-3/8" 3rd member will look like this (two vertical ribs widely spaced with a top horizontal rib that curves downward on the left side).