Correct axle

[onebluemcm]

When I'm looking at late model C3's, how can I be sure the diff is original? I realize it may or may not be hard to tell if gears have been fooled with, but what numbers should I look for to be sure the rear end hasn't been entirely changed?

I guess the next question is: is there a way to determine the diff gear ratio if it's unknown? Or to confirm the gears have or have not been swapped?

Hmmm...

 

[JohnZ]

The ratio and date code are stamped in the machined surface at the bottom of the carrier, between the strut rod bracket and the lower spring mounting plate - usually takes some wire-brushing to clean off the crud so you can read the stamped numbers. There will be two letters (the ratio code) and a date code.

To determine the installed ratio, jack up the rear end (both rear wheels off the ground), mark the rear of the prop shaft with a piece of tape, mark a tire with tape, and turn the tire one full revolution (trans in neutral), and count the number of prop shaft revolutions for one turn of the tire. The number of prop shaft revolutions will tell you the ratio (3-1/2 = 3.55, 3-3/4 = 3.70, etc.). After you jack it up, chock the front wheels and put jackstands under the rear trailing arms to 1) Support the car (don't EVER get under a car with just a jack holding it up) and 2) To load the rear spring so the half-shaft U-joints aren't running beyond their angle limits - they can ground out on the outboard spindle flanges while rotating if they're fully extended.

The opposite side tire should turn with the tire you turn if the Posi is working; if not (or if it's an earlier car without Posi), have a helper turn the opposite side tire exactly the same full revolution as the one you're turning.