I consolidated information regarding the conversion of the 1947/55 3600 & 3800 rear ends. This information will help you change the gear ratio to 4:10 from the original lower gears.

AXLE IDENTIFICATION

denny graham how-to photo's




This information should be helpful to those that are looking for the eatons.


From Bill: (Longbox55):

There will be a code stamped on the front face of the carrier. I've posted the codes before, but for sake of saving a ton of searching for them, here they are again:

1967; HW, JU. These are listed as 4.11, may be Spicer rather than Eaton
1968; NA, NB, WI, RN (RN is listed as a 1 ton diff)
1969; NA, NB, OT, OW, OY, TF. There are also 1 ton listings; IE, LJ, LZ, MR, NZ, RN.
1970; TAA, THU, THW, TJU, TLN, TMZ, TNA, TNB, TOT, TTF, TWI.
1 ton; TIE, TIH, TLJ, TLX, TLZ, TMR, TNZ,TRN.
4x4 K20; TPW, TPX
1971; TAH, TAJ, TJA,TJB, TJP, TJS, 2G.
1 ton; TAK, TAL, TAP, TJK, TPZ.
4x4; TJH, TJJ,TKB, TKC, TKK, TKL, TKS, TKT.
1972; RAH, RAJ, RJA, RJB, RKK, RKL, RKS, RKT, RJP, RJS, RKB, RKC.
1 ton; RAK, RAL, RAK, RAP, RJK.
4x4; RJH, RJJ.


I do have listings for a 3.54 ('71 and '72) and a 4.11 ('67-'72). but from what I understand, those are Spicer differentials rather than Eaton, though I have seen a '72 with a 3.54 code on an Eaton differential.
BTW, most, if not all, GMC trucks from the '60s used a Spicer differential in the rear, either a Spicer 60 or 70 (I've seen them both ways).
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