Okay, so I have a couple questions on possibly going from my current 3.25:1 9 3/8" rear end to a 9". I have been wanting to go to a 3.70 or 3.73 and I figured getting parts for a 9" would be much easier than trying to find parts or make parts work for a 9 3/8". My main question is: if I pull my (31 spline) axles out of my 9 3/8" rear end housing and rebuild a 9" for my 31 spline axles, will the 9" mount back in place of the 9 3/8"? Also will my axles slide back into the new 9" housing or will they be too short/long?

 

While I've never done it I can tell you both rear ends take the same gasket where the carrier mounts to the housing. So it will mount. But the axles would be a different length.
EDIT. I just found a used drop in 9 3/8" assembly with 3.50:1 gears. Call Jenifer @ Arizona Differential 602-272-4600.

 

So if I got a 9" third member and the correct length axles the 9 3/8" housing would still work? Also, thanks for that number. I'll look into that as well.

 

I had done a Google search and read a couple guys said basically it requires custom axles. One guy cut one shorter and custom length other. One guy just mentioned custom Moser axles after his new locker broke after a month. Another claimed no problems. Moral of the story.....I'd buy the used 9 3/8".

 

Or I guess I could just yank the entire set up out and start from scratch. haha..for some reason I just like the idea of having a nine inch with readily available parts. That way changes can be made easier especially because in the not so far away future I may try to beef things up a little (my truck is never fast enough). I appreciate the feedback, maybe if I find a complete 9" set up I'll make a rebuild and install thread...never done anything like that before.

 

If you're gonna beef it the LAST thing you wanna do is go to the weaker 9". www.randysring&pinion has bearing kits for the 9 3/8".

 

There's more than one type of 9 3/8" rear axle. What's the AXLE code stamped on the Warranty Plate? One type uses the same R&P's, differential and pinion bearings (and etc) as the 9.

 

Awesome, I was not aware that these kits were easy to find. Everyone I've ever told I have a 9 3/8" rear end asked me what that was, haha.



I know that the number on the actual axle tag is WEY-A, which I found on fordification was the 3.25 9 3/8" 31 spline. And as far as the axle code on the warranty plate..I will have to get back to you on that one. I'm 150 miles from my truck right now, but I will be reunited tomorrow for the weekend. lol

 

WEY-A = 3.25 - 9 3/8, rear axle parts list #91.

Notice in parts list #91 that some specific to WEY parts are identified with $

 

So based on the parts list, the 9" would be a better economical choice?

 

The axles should interchange between 9-3/8" and 9", so long as both are 31 spline.

 

This is true, ring gear diameter will not change axle length. You could use 8" axles in a 9" "CAR" diff given the 28 spline and small bearing combo. Obviously trucks did not come with 8" diffs.

 

Do your own Google search. When I did they mentioned axles were different length because of pinion off-set. And I can tell you that while Bill is correct, OP's tag number is a 9 3/8" with a 9" parts list (91) not the usual 9 3/8" parts list, as he also noted some parts in that parts list are specific to his 9 3/8". And importantly here is the Carrier IS 9 3/8" specific. SOOOOOO the pinion off set may be different than a 9" and therefore take different axles if you blend the two together.

 

So from the back and forth of this I am still leaning into just sticking a 9" under there..just buy a complete axle and rebuild it. But if this is any help, the axle code on my warranty plate is "17". Which from Decoding Your 1968 Ford Truck VIN - FORDification.com I found is this type of axle: "3.25 (3.3M); Ford (WDM-H -S); 17". Based on the "WDM-H-S" I found "WDM-H" to be a 3.25 9" 28 spline...so now I'm even more confused on what I have. It seems like the actual axle tag would be the best indicator, though?

 

Where did you come up with this incorrect 28 spline jazz for WDM-H?

17: 1966/69 F100 9" WDM-H or WDM-S / 3.25-1 / No Limited Slip / 3,300 lbs. Rear Axle Capacity / Rear Axle Parts List #91

C8TZ-4234-A .. 28 Spline Axle Shaft ~ WDM-S / C8TZ-4234-B .. 31 Spline Axle Shaft ~ WDM-H & WEY (9 3/8").

Right: Notice the symbols next to the C8TZ-4234-A & B axle shafts and then look at the bottom of the page for what they represent.