C3 side gear shims

[Tom L.]

I am in the process of rebuilding my differential and I can't seem to find any detailed information in the manuals about shimming the side gears. The sides and spiders are new and of a moderately different design, so I don't want to rely on simply re-using the old shims.

I've found descriptions of the shimming procedure without the use of the factory springs and plates, but I plan to keep them to keep it original. Is anyone familiar with the measuring and shimming process to be followed while still using the factory springs and plates? Thanks in advance.

[Duke W.]

Re: C3 side gear shims

There is no specification in the overhaul manual for side yoke end play, but The Chevrolet Power Manual suggests 0-.005" for racing, however, I think anything under .010" is okay for road use.

The differential case side shim configuration affects both pinion gear backlash and yoke end play, and IIRC the overhaul manual gives a pinion gear backlash specification, which is what you should shoot for. Then make sure yoke end play is reasonably small and equal on both sides.

GM offered three shims, but I don't know if they are still available.

9778347 .005"
9778348 .007"
9778348 .010"

Bill Bradley and I co-authored an article on Positraction axle overhaul a couple of years ago in The Corvette Restorer, and you should read it.

Using new internal components in an old case, I suggest you assemble the case without shims, then measure the pinion gear backlash and yoke end play and select shims based on these measurments. Then disassemble the case and reassemble with the calculated shims and repeat as necessary. AFAIK it's okay to use mutilple shims on either side, if necessary, to get the final assembled clearances you are looking for.

"Blueprinting" a Positraction differential case is a time consuming process.

Duke

[Joe L.]

Re: C3 side gear shims

Duke-----

Shims GM #9778347, 9778348, and 9778349 are GM-discontinued. The GM #9778348 is replaced by GM #15526140 which remains available. I can't find any supercessions for the others but there may well be new numbers.

In any event, these shims are easily available in the aftermarket from sources like Tom's Differential, Randy's Ring and Pinion, and many others.