Rear End?
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New User
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Rear End?
Question? From what year car or truck can i get a posi- or limited slip differential to fit my 1979 F-100? Note: the truck has a 302 stroked 347 and the truck will never pull anything> i just want it to you know burn rubber from both back tires, just for ****s and giggles. and also wanted to know what rear end does the 1978 F-150 XL Ranger 351 have and what the 9" F-150 Rear End have. please help?
Post Fiend
Joined: Mar 2002
Posts: 19,402
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From: Bonita Springs FL
No, they're not all limited slip. Posi is a GM term, we don't use that kind of language around here. 
You can get any 9", as long as you keep the same spline count, which should be 31 in a truck. Trucks came with 31 spline axles, most cars had 28.
If you know your axle is in good shape, you might want to have an aftermarket LS unit installed, like a Powertrax no slip or an Auburn. With your own axle, at least you know you're not working with junkyard junk.
Unless it's been swapped, your rear axle is a 9". All F-150's in the 70's had 9" axles. Not sure what you mean by your last question.
You can get any 9", as long as you keep the same spline count, which should be 31 in a truck. Trucks came with 31 spline axles, most cars had 28.
If you know your axle is in good shape, you might want to have an aftermarket LS unit installed, like a Powertrax no slip or an Auburn. With your own axle, at least you know you're not working with junkyard junk.
Unless it's been swapped, your rear axle is a 9". All F-150's in the 70's had 9" axles. Not sure what you mean by your last question.
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are these 9" differentials really used a lot for racing and why is that? new user here please be patient with the dumb questions, i have another truck also a 1979 F-100 custom, what kind of differential does that have?
Post Fiend
Joined: Mar 2002
Posts: 19,402
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From: Bonita Springs FL
The 9" in your '78 may or may not be LS. Even if it was installed from the factory, it might not be working anymore. The easiest way to know for sure is to jack up both rear wheels and spin one tire by hand. If the other tire spins the opposite way, it's open. If they both spin the same direction, it's LS.
The 9" axles are used in a lot of applications. One, ford made a million of them. They're everywhere. Parts are cheap. Two, they've very, very strong.
Your '79 also has a 9", do the same thing to find out if it's open or not.
No dumb questions, only dumb answers.
The 9" axles are used in a lot of applications. One, ford made a million of them. They're everywhere. Parts are cheap. Two, they've very, very strong.
Your '79 also has a 9", do the same thing to find out if it's open or not.
No dumb questions, only dumb answers.
Posting Guru
Joined: Feb 2002
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From: Cleveland, WI USA
If your differentials still have the tags on them, it will tell you if it's limited slip. The ratio will be on there as either 3.50 or 3L50 (with a 3.50:1 gear ratio). If it says 3L50, it's a limited slip. The most common gear ratios I have seen are 3.50:1 or 3.55:1, but other options are available.
As stated above, most trucks have 31 spline axles, however, some of the f100's with the lower GVWR have been known to have 28 spline axles. If you're going to add a LS of any kind, take it apart and count the splines FIRST.
If you find a LS unit, you will be able to use your ring gear, but the bolts that hold it to the carrier are different lengths on the LS and non-LS. I'm pretty sure the LS bolts are longer.
Good luck.
Kevin K.
As stated above, most trucks have 31 spline axles, however, some of the f100's with the lower GVWR have been known to have 28 spline axles. If you're going to add a LS of any kind, take it apart and count the splines FIRST.
If you find a LS unit, you will be able to use your ring gear, but the bolts that hold it to the carrier are different lengths on the LS and non-LS. I'm pretty sure the LS bolts are longer.
Good luck.
Kevin K.
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Another way to Tell if it's a LS without looking at the rearend tag and if the rearend is still factory. look at the tag on the Drivers side door, Find the axle code. If the axle code starts with a "H" it's a LS. Like my 78 F150 the door axle code is "H9" it is a 4.10LS. If you want a 3.50:1 ratio with LS look for axle code "H2"
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