Ford 9"

[tonpartain]

I'm going to pull a Ford 9" from my '72 F100 and have it rebuilt. Is there a limited slip diff. that will fit into it? If so, what make and model vehicle can I pull it from as I'm planning on going to the junk yard on Tuesday?

[FORDification]

Virtually all of our trucks have 28-spline axles, so that's the most important thing to watch for, as opposed to the 31-spline axles, like those used on '73-up pickups, for example. (There have been a few sightings of 31-spline axles in some '72 trucks, but I certainly wouldn't hold your breath trying to find such a setup.) I've never seen a definitive listing as to what vehicles had 28-spline vs. those with 31-spline.

Any rearend could have been ordered with either an open or locking unit, so finding one is pretty much a crap shoot. The fastest way to tell is to find the factory ID tag, held on by one of the third-member hold-down nuts. A rearend which has (for example) a locking setup with 3.25:1 ratio gears will have an 'L' in the ratio listing, like this:

Locking: 3L25
Non-locking: 3 25

However, I wouldn't buy one just 'cause the tag might have the 'L' in it...it could've been swapped in at some point. If you can get the tires off the ground, spin one tire clockwise and see which direction the opposite tire spins. If it spins in the same direction then it's probably a locking rearend. If it spins the opposite direction, then it's probably an open (non-locking) unit.

[averagef250]

My '70 F-100 2wd has a big bearing 31 spline 3.50 geared posi rear. It's a real pain to find wheel bearings for. I had to have an axle made for it after losing a wheel bearing once. Called every yard in the NW and got the same answer "Ford didn't use a 31 spline rear in 67-72 trucks". Lucky me then. According to the tag it's factory.

[tonpartain]

Keith,

Is a locking unit the same as a limited slip? Or asked this way, which is better, an open or locking unit?

Chris

[MadMaxetc]

Is a locking unit the same as a limited slip? Or asked this way, which is better, an open or locking unit?

*** Note: Fords "posi" as some say was named "Track-Lock". Posi is a GM term for "Posi-Track".***

(Sorry, just a little pet pev of mine )

I know you asked Keith but...

An "Open" rearend sends power to the wheel with the least amount of resistance. ie... when you do a burn out or get stuck in mud or snow, only one tire gets the power and usualy will spin. In normal conditions both have equal amounts of traction.

A "Locking" rearend is just that, locking. When Locked it is like one axle, so this is only when going straight. When turning it un-locks and becomes open, to allow each wheel to spin at differant speeds. These are usualy a little noisy.

A Limited slip will split the power to both wheels like a Locker, but still allow power to both wheels while turning. It does this by...get this...slipping.

"Posi's" and "Track-Locks" are both Limited slip.

Open = Fine for most trucks
LS = Good for mudding, snow and street-strip use
Locker = Race and mostly show or burn out compititions.

Hope this helps.

[tonpartain]

Thanks, Dan.

I assumed that the Traction-Lok was inherently better than an open diff. Sounds like that may not be the case.

What sort of "upgrade" should I make to improve the axle? The truck will be used for hauling/pulling a car on trailer. The gear ratio is 3.00. This may be fine, but I just want to be certain before I take it to the rebuilder.

Chris

[MadMaxetc]

A LS rear end is "better" from a traction availibility stand point. Both Open and LS will transfer the same power to the ground. ie they have the same amount of friction loss under normal conditions. (I can elaborate more if you want)

More then you ever wanted to know about the Ford 9"

I think 3.00 is a little to high a gear for pulling. You will have a hard time getting going and keeping it there I would think. I have 3.00:1 right now and it is good for no lad highway, but if I put 1000# in the back it cant go 60, even with my big 300.

I would drop down to 3.70 gears and a Track-Lock unit. The Track Lock will help if you are in Gravil or Dirt lots.

3.70:1 is a common gear. I find it good for all around use.

To find the right gear for you, YOU need to use YOUR trucks info.

I find that this method works well for towing. Take your engines Max Torque RPM and subtract 1k RPM to get your aproximate Cruising RPM. Use that RPM, your tire size and your trans final gear ratio (normaly 1:1) and use this formula...HERE.

So lets do a test case and see if it works:
Max Torque engine RPM = 4000
Cruising RPM (should be about) = 3000 (thats a little High i think so lets look at 2800)
Highway speed = 70mph (I am a lead foot)
1:1 Tranny gear.
30" tires.

Rear end gear should be = 3.571:1
Not bad but that might make it a little hare to get rolling, so...at 66mph it should be about 3.78:1. That sounds OK, but they don't make a 3.78, only a 3.70. Close enough.

I would run the numbers and see what you think.

Good Luck!