I will guess that you have an 8.8" rear end and here are my notes on that rear end. The socket is 1 1/16".

Things I have learned about 8.8 Rear Ends that were set up by the FORD FACTORY and not touched by anybody.

1. All Differential Cases measure 7 23/32” OD from Bearing Cup to Bearing Cup outside.
2. All 8.8 31 spline Differential Cases will interchange in the Rear Axle Housing.
3. All F150s and E150s have 31 splines on their rear ends.
4. If you use the same Ring and Pinion you do not have to change shim sizes when replacing bearings.
5. The Differential Bearing Shims are sized for the Ring Gear and Rear Axle Housing to set backlash.
6. You can use the same Differential Bearing Shims that came with the Rear Axle Housing when changing the Differential Case as long as you use the same Ring Gear that came with Rear Axle Housing.
7. You must put the same Differential Bearing Shim back in on the same side that it came out of.
8. You must put the same Clutch Pack Shim back in with the same clutches.
9. You can not mix Clutches from one Clutch Pack with the other.
10. You can not use any air tools while putting a Rear End back to together.
11. You can drive the Ring Gear off with a punch using the bolt holes alternately from side to side.
12. You can drive the Pinion out through the Companion Flange with a Big Hammer and a brass punch.
13. Ford Traction-Lok, L/S and posi are all one and the same.
14. All F150s have 108 teeth on their Speed Sensor Ring.
15. You can recalibrate the PSOM on 92-96 F150s.
16. Cover plate boltholes do not go through to the inside.
17. The clutch plate next to the gear looks like it is worn out. Normal condition.
18. The clutch pack Ears are square from 1983 (and before) through at least 1989.
19. The clutch pack Ears are rounded from before 1995 to at least 2001.
20. The Differential Case has square slots from 1983 (and before) through at least 1995.
21. The 2001 Differential Case has drilled holes in the end for Short Rounded Ears clutch packs and will not take square Ear clutch packs or Rounded Ears clutch packs from 1995.
22. A roar while accelerating and under load but not there while coasting or slowing is a bad rear pinion bearing (inner).
23. The socket size for the Lock Bolt is 8mm 6 point and use Ό” drive breakover wrench and socket.
24. Socket size is ½” for cover plate bolts.
25. Socket size is Ύ” for Bearing Cap Bolts.
26. Socket size is Ύ” for Ring Gear Bolts.
27. Socket size is 12 point 12 mm for Companion Flange Bolts.
28. Socket size is 1-1/16” for the Pinion Nut.
29. To drive the “S” Spring out use a piece of wood 1 3/8” square 5” long
30. To drive the “S” Spring in use 2 pieces of steel, 1/8” x 1 ½” x 5” clamped on both sides of the “S” Spring with two “C” clamps. Tighten so the 1 3/8” sq. piece of wood just clears the 1/8” bars and drive the “S” Spring in until it hits the Pinion Shaft, then pull Pinion Shaft out until it just clears the spring and then drive the spring on in. Push the Pinion Shaft all the way in and hold it with the Lock Bolt.

Note the Ring Gear was off for 22 & 23 and the Pinion Shaft was used to hold the gears in position while driving the spring. This was done on the bench. The Pinion Shaft will have to be pulled out later after the Differential Case is installed for Axle “U”-Washers.

What I think is true but can not confirm yet.

1. Synthetic Oil will make all bearings and gears last longer.
2. Synthetic Oil has nothing to do with the clutches or clutch packs, its for bearings and gears.
3. You can buy Synthetic Oil that has LS additive already in it and you do not have to add any more LS additive.
4. LS additive makes the clutches more slippery so they won’t chatter.
5. If you put in too much LS additive in the Rear End you will not have an LS action.



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3L55 88 99H06

Second part is the axle ratio: There should be a "3L" preceding the 55 if it's a Limited Slip. If it's open, it would just say '355' .
4th is the ring gear diameter, in your case an 8.8 inch.
5,6,and 7 are the build year, month, and day, in this case, April 21st, of '99. (9=1999, H=8th month [and there is no 'I' in the sequence]) and '06' is the 6th.