I picked up a used 9" on Craigslist. It was advertised as a 3.73 open diff with 58K miles taken from an auctioned fire dept '84 4WD 351w F150. I also got the entire rear sway bar assembly which I needed. It will eventually go into my '77 F100 to replace a Dana 44 open diff 3.00. I got the 9" as an easy way to move from my 302/C4 to a 351w/4R70W to get the ratio right behind the OD tranny. I also planned to add a trac-loc or other brand limited slip carrier. I used a recommended shop in Baton Rouge (Zimmer Gears) to rebuild an 8" for my '65 Falcon and will probably use the same for this 9". I bought it expecting to have to get it rebuilt.

The seller delivered the 9" and it is pretty clean, but it came without a tag on the third member. The seller says he'll send a picture of the door tag but in the mean time I messed around with the rear end and it kind of acts like it has a limited slip carrier. When I turn the driver side drum the passenger side drum turns in the same direction. When I turn the passenger side drum, the driver side drum moves momentarily in the same direction and then stops.

This kind of looks like it has limited slip (both wheels turning in the same direction when one side is turned), but I'm not sure what the failure for the driver side to keep turning means. Does it indicate that the limited slip is worn or is it normal for one side to just sit till while he other is turned? I wound in both brake adjusters before I did the check to make sure there wasn't any drag.

Also, I tried to confirm the gear ratio and I expected the pinion to spin about 3 and 3/4 turns for each turn of the drum. But it was about half of that. Did I need to lock down one of the drums to do that properly? When I did it I had the rear end supported with both drums off the ground.

Thanks for any help. I've searched and read several posts on what should happen, but I haven't seen anything that sounds like what I'm seeing. I obviously don't know much about differentials, but I'd like to understand what I'm seeing before I haul it off to get it rebuilt.

 

"Posi" is Positraction, a GM term. Ford used other terms, and the designs of the various limited slip diffs varied as well.

3.73 is not a ratio found in the 9er. 3.70 was, but was not common afaik.

I would bet more on 3.50 for that diff.

I take it you have the entire axle, housing and all, and it's all in one piece at the moment?

You will have to take it apart to have it rebuilt anyway, so at that time you can examine it for limited slip and count those gear teeth.

One version of a "Traction Lok":

http://www.fordification.com/tech/im...aclock-9in.jpg

 

Thanks for your response. I should have been more precise in the title because I've seen that using the general term "posi" in referring to a limited slip differential tends to focus the discussion on a reminder that "Positraction" was a GM trademark. As an interesting aside, posi-trac or posi-track (terms that are also used generically for a limited slip rear end) are trademarks for a loan tracking software package and I was surprised that GM would allow that usage. However it could be fun to respond to old Earl down at the gas station when he says "I love the posi-trac in my old Camaro" with a quick "Why Earl, I didn't know you were a loan officer now".

I will be pulling the brakes and anti-sway bar before I run it over to Zimmer Gears, but I was hoping to avoid the mess of pulling the axles and differential to get an idea of what I'm looking at. The additional cost for the full rebuild isn't that great anyway and the shop is better set up to check out the housing for any issues.

I did find a table that Number Dummy posted that seemed to indicate that factory Ford truck rear ends (Ford not Spicer) in '84 came in 2.75, 3.00, and 3.25 but I could be reading it wrong and I don't have the door tag info in any case. And since the tag is missing it could have been rebuilt with something else.

I'll check with Earl, who owns Zimmer Gears not the corner gas station and who doesn't own a Camaro but rather a first generation Corvette (the car, not the warship), tomorrow and see what he thinks.

I was just a little mystified about what I was seeing and hoped to get some insight before that phone call.

Thanks again.

 

If you had nothing connected to the pinion, you should be able to turn either side of an open or LS diff, and both sides should easily turn together, since the drag between the spider/side gears is greater than the drag between the ring and pinon. You can try locking the pinion with something like a crow bar, and turning one of the drums. There should be very little resistance in an open diff, so the other side should turn the opposite direction. With a clutch type LS diff, there should be about 40 lbs-ft of torque resistance, unless the clutches are really worn.

 

That's great info, thanks. I'll run over to my storage and try that out tomorrow.

Much appreciated.