1980 Differential Posi

[Frank F.]

I am working on my differential (1980, 4-speed). In particular, the Posi unit. No issues, just thought the time has come for an overhaul.

For the posi unit I'm replacing the internals; clutch packs, thrust washers, and cross shaft/c-locks. All the gears look fine.

I'm a little confused; the posi unit has no internal shims. In particular, shims for the side gears/clutch packs. The service manual for a 1980 indicates no shims.

Question: How do I confirm/verify my posi rebuild was done properly?

Thanks.

[Duke W.]

Re: 1980 Differential Posi

GMPD offered three inside differential case shims (not to be confused with the side shims between the case and carrier), I recall three, five, and seven thou to get the differential gear backlash into the range of (I recall) two to six thou.

I recall your axle has Dana 44 guts inside a unique aluminum carrier that is integral with the cross member mount. Tne only reason I can think of why equivalent internal case shims are not used is that the all machining was precise enough to make them unnecessary to get the proper differential gear backlash and yoke end play.

Speaking of yoke end play none of the GM overhaul literature that I'm aware of specifies a yoke end play range, but the Chevrolet Power Manuals from the seventies recommend zero to five thou.

When I rebuilt my '63 axle back in the seventies I bought a complete "loaded Positraction case" from GM and upon completing axle assembly measured differential gear backlash within the range and yoke end play of three and five thou. I thought about pulling everything apart to see if I could tighten the five to three or less, but decided it wasn't worth it.

I don't know the range of OE yoke end play. Understand that both Positraction clutch wear and side yoke end wear will increase both backlash and yoke end play, and after tens of thousands of miles the axle can begin to feel pretty sloppy with end play of well over ten thou.

If you measured yoke end play before disassembly I would consider that to be about the maximum, and if upon reassembly they're no more than about ten thou, that's probably okay.

Duke