I would like to replace the chunk in my rear end with a posi unit. But I realized that I have no idea what size the one I have is. It's a 2002 supercab 4x4 F-150, Are they all the same size? Do I have to take the cover off the rear to measure it or is there a plate on it somewhere. I'll look when I get home (currently at work) but I thought I'd throw this out for all you ford buffs to see if you might know.

 

On the tag on your door pillar should contain your axle code it most likely is 355 or 373

 

Well, I'm not really interested in what gear ratio I have, That is pretty easy to determine. I need to know what size (i.e. 7.5, 8.8 or 9 inch) and how many splines the actual differential is. But thanks for taking the time to answer my post.

 

then it would be an 8.8

 

I thought if you got a 5.4 and/or 4x4 got the 9.75 rear

 

My 97 f-150 4X4 has the 8.8 but it's a 4.6. There should be a tag on the rear end that will tell you what size it is. (and the ratio)

 

There are 3 sizes of rear end found in the '97-'03 F150s. Unless you have a 7700, you won't get the 10.25 semi-floater, so that leaves two possibilities -8.8 or 9.75. The engine size really is only a minor part of the equation. As Tony said, you're better off checking the rear diff tag. On the second row you will have an alpahnumeric code that reads as the ratio, followed by the ring gear size, and then other data like build date (Ex: 3 55 8 8 XXX). The "3" is usually located under the bolt holding the tag on. If you see an "L" next to that bolt you already have a limited slip (posi) unit. Another way to tell if you have factory limited slip is the door tag under "axle." If the two digit code starts with a letter, it's factory LS. H9 is the code for 3.55 LS, 19 for 3.55 open, B6 is 3.73LS and 26 for 3.73 open. Most codes are located via link in the website's tech article section.

Hope that helps.

-Kerry