Differntial Swap Questions

[Subzero]

I'm replacing my entire differntial on my 1968 F100 due to have a bent axle shaft and bad bearings, along with both wheel cylinders blown out. I've found someone who has a 1966 9in and a 1970 9in rear diff. The 1970 diff is from a truck with a 390 and he said it could be an "N" code differntial which is desirable to me. I don't want to Frankenstein the truck too much so I'm looking for the most direct swap that would use the most common 1968 parts such as 1968 brakes and bearings, although either way I'm remaking the entire axle with all new hardware and brakes. He also has a complete great condition F250 frame that's been painted from around the same age that has a Dana 60 and front disk brakes; which I would like to convert my F100 to one day. If I was able to get the F250 frame I would then swap my entire truck body and power train onto that rolling frame. Also might be looking into doing a trac-lok internals upgrade if that's cost effective. What is your opinion on what would be the best option for me to do? If I was to buy one of the differentials which one would be the better swap to do?

[sargentrs]

My rear end, with 3.25 gears, locked up on me a couple of years after I got her. A friend owns a wrecker service and junk yard and had a slick, I think it was '64-'66 maybe. Pulled the diff and it was 3.89 gears. I was ok with that. Just pulled it and put it in without changing/replacing anything. One little catch to that though. The slicks had the fill plug in the rear of the housing and bumps have the fill plug in the case. After I got it in I realized I had no way to fill it with gear oil Drilled a hole in the upper half of the housing and plugged it with a 5/16" self tapping bolt and copper washer. Those 3.89s really woke her up taking off but she screamed at highway speeds. Ran fine until I started my rebuild project. Going back in with 3.00 gears. While I had the rear end apart, I installed the Richmond PowerTrax No-Slip. It was $500 but was an easy install.

[ultraranger]

Not that the 9-inch 3rd member couldn't be an N-case but, I highly doubt that it is. The N-case is pretty rare. I have two but, it took a good number of years to find them.

It's something you would more likely find in a Dentside truck --particularly if the truck was a 4x4 with an NP435 granny low 4-speed.

The '57-'60 model 9-inch 3rd members look very similar to the ~'64-'86 N-case 3rd members. The N-case 3rd members didn't get the large "N" cast between the two vertical ribs until 1970. '69-earlier N-case will not have this identifier. This is why, at first glance, the old double vertical rib cases look like an N-case.

On the outside, the old gray iron case 3rd members horizontal ribs will have a continuous arch to them and they will not flatten out at the ends. If you don't know what you're looking for, the only sure way to identify an old (weaker) gray iron 3rd member is to look on the inside of the casting (3rd member would have to be removed from the housing).

If you see the casting letters, "WAA," "WAB," or "WAR," on the inside of the 3rd member, it's an old gray iron 3rd member. (best to walk away from it).

WAR casting seen here.



The most common 9-inch 3rd member is the one with the single vertical rib. Casting number for it is also on the inside of the case. The most common casting for this 3rd member is C7AW-E.


My old, stock , 69 F100 C7AW-E single vertical rib 9-inch, single track, 28-spline, 3.50 geared 3rd member on the left. 31-spline, Traction-Lok differential, 3.50 geared, 9-inch Daytona N-case 3rd member on the right. The N-case came from a 1975 Ford F150 4x4 manual 4-spd. truck.





Casting numbers for the N-case can also be found on the inside of the casting. They can be C4AW-B, D0OW-B or D0OZ-B.