The impetus for this project was the addition of the OE aluminum cover and a fluid change. While I was in there I decided to see if I could actually get the OE "limited slip" function to work as it has never satisfactorily performed from day one.
The truck is a 2007 F-350 Crew Cab, 6.0 with 74,000 miles so I have the three pinion setup. On a 2 pinion I'm not sure how you'd get away with running as much preload as I did because you don't have the advantage of the 2 piece case. On a three pinion you can use the two case halves to compress the Belleville washers instead of trying to force the 2 pinions in the opening and getting the pin to line up. Anyway, this is about the 3 pinion setup:
I'm not going to get into removing axles, part numbers, tricks to keep the cover from falling into to drain pan, keeping track of your shims, torque specs etc. as that has been covered elsewhere. The one thing I will say is be ready to catch the diff once you get it clear of the housing as it is HEAVY! I'm going to focus on the bench rebuild of the differential itself and subsequent driving impressions.
I picked up the new Ford replacement rebuild kit. It contains: 3, 4 ounce bottles of friction modifier, 10 clutches, 10 steels and two each of shims .030", .035", .040", .045", .050", .055" and .060". It also contains an instruction sheet with all of the torque specs you'll need (except for the differential bearing caps).
I soaked the clutches in my 75-140 synthetic gear lube (Wal-Mart house brand) for over a day but a half hour should suffice.
After removing the 6 case half bolts and splitting the diff the stock clutches were in good shape, no galling on the steels, plenty of clutch material left on the clutches etc.
One thing to note was the original clutches had clutch material on the whole face of the steel backer while on the new replacement clutches the clutch material was variegated in an almost mosaic pattern. I'm not sure why, maybe it helps to evacuate oil and aid in lockup when pressure is applied to the clutch pack. To put it more clearly, the factory clutch material is a solid disc with no grooves or channels and bonded to the steel backer. The replacements are little triangular pieces of friction material in an alternating pattern each bonded separately to the steel backer. The original clutches and steels were arranged correctly. Clutch-steel-clutch-steel etc. It wasn't "wussafied" from the factory with two clutches or two steels stacked together.
The average measurement of each of the used original packs with 5 clutches and 5 steels only was .830" (one was about .827" and the other was about .832"). This is as they were removed from the carrier. The factory used a .050" shim on both sides and of course the Belleville washer that provides the tension. This gives me (.830"+.050")=.880" plus the Belleville washer for the stack height.
The new stacks were about .880" with the steels and clutches. I used the .030" shims (you'll see why below) so my total stack height was about .910". I didn't follow the Ford directions which utilizes a special measuring tool so you can devise the correct shim to use based on the carrier and clutch/steel characteristics. They already did that once at the factory and it didn't work.
Prepare to gasp and flame me on how my rear end will self destruct but the only "enhancement" I did was to use the original Belleville washer IN ADDITION to the new replacement.
So, the stock arrangement was
(Belleville-.050" shim-c-s-c-s-c-s-c-s-c-s-axle gear).
The "new and improved" arrangement is
(old Belleville-new Belleville-.030" shim-c-s-c-s-c-s-c-s-c-s-axle gear)
On my case this left just enough room to physically fit everything in the cavity with zero room to spare when the case halves were torqued back together - no gaps - no room - no "spring" on the Bellevilles, everything collapsed and tight. Just FYI, the Belleville washers are about .060" when flattened so my total compressed stack height would be about an inch.
I tried adding one of my old clutches to the new stack for an arrangement of:
(Belleville-.050" shim-c-s-c-s-c-s-c-s-c-s-C-axle gear).
But this was way too much as the old clutches were just shy of a tenth of an inch. This also pushed the last steel away far enough so it was about half on and half off the splines of the axle gear. It doesn't do any good to add a steel, they just act as shims at that point. If you tried to add a steel towards the outside of the stack such as this:
(Belleville-.050" shim-S-c-s-c-s-c-s-c-s-c-s-axle gear)
it would be totally disengaged from the axle gear splines and acting like another shim.
So, once again, as assembled is:
(old Belleville-new Belleville-.030" shim-c-s-c-s-c-s-c-s-c-s-axle gear)
I put everything back together with my shiny new Ford aluminum OE cover and filled with 75-140 synthetic with NO FRICTION MODIFIER.
With both back tires off the ground I could only get the diff to unlock by violently shaking one tire back and forth several times. I put a wrench on a lugnut (tightened to 150 ft-lbs) and the nut kept getting tighter and tighter until I wasn't sure if I was going to break a wheel stud but the diff didn't spin.
On the first drive and first turn it was noisy and jumpy. I should have noted earlier that the replacement steels ship with a black oxide coating to either keep them from rusting or to aid in "break-in" or for some other reason, I don't know. Anyway, this coating is rather rough and I suspect that once some of it wore off it would settle down which it did.
After several circles in both directions, backwards and forwards it smoothed out considerably. I thought I was going to have to add the friction modifier but I don't, it's settled down that much.
Now, on a turn with no throttle applied the rear end will make almost a frog like "ribbet-ribbet" noise and you can feel the slightest shudder. If you apply a little throttle it will lock up and scrub the inner tire regardless of which direction I turn. I'm running 36" tires and I tried the same thing on a hard packed dirt road. It will still release (mostly) on a turn but the inner tire does leave a little scrub mark.
Long story short - the darn things works like I want it to
now. To the naysayers, I understand that this little trick isn't for the faint of heart but I took a $160 gamble that I could get the factory unit to perform. If it blows up or self destructs - so what? It wasn't working before! If I couldn't get it to work I was going to buy a TrueTrac anyway so the outcome is equal. The most I lost is $160 on a rebuild kit but I potentially gained hundreds and hundreds by not having to purchase a TrueTrac, a bearing kit, set it up etc.
Total estimated time for a shade tree with air tools and half a clue like me; about 2 & 1/2 hours.
Hopefully someone will find this information useful.
The rebuild is still functioning just fine but I decided to add the limited slip additive that came with the rebuild kit. I had the truck up on the lift for brakes all the way around so I decided to add 2 of the 3 additive bottles to the rear. Like I said, it was/is working fine but I got tired of people telling me that there was something wrong with my truck when i went around a corner. After I added the fluid and let it work in it cut the chatter down to be almost inaudible and you can occasionally feel just a little shimmy, not the make-and-break clunks like it was before. It still seems to function almost as well as before effectiveness wise.
So far I've got about 78,000 miles on the truck, 4,000 on the rebuild.