[biscuitville]

The rear end of my 73 vette is starting to shudder during slow turns and I am guessing it is about time to rebuild the rear-end. Having never done this before, I was hoping to get some advice. First, has anyone viewed the Richmond Gears DVD and in particular, do they discuss the differential as well as the ring and pinion setup? Second, I want to switch from a 3.55 to a 3.90 ratio. Will this new set still fit in the orignal case? To the best of my limited knowledge, I believe this to be the original 12 bolt case. Finally, are there ring-and-pinion sets to avoid? I am wondering if I need to spend the top dollars since my engine only puts out around 425 ponies. Any other advice for a novice would be appreciated.

[big gear head]

The Vette rear end isn't the same as the 12 bolt. It's a smaller rear end. If you change the eoil and use a bottle of GM oil additive the shudder will probably go away. Be sure that the rear tires are the same diameter and the air pressure is the same. You probably don't need to rebuild the rear end. The shudder is coming from the clutches.

[ericnova72]

That "shuddering" while turning a corner is chatter in the clutch packs of the positraction unit. It means that you need to replace the gear lube and the positraction friction modifier additive in the gear lube. Just like any oil it breaks down with time.

At the worst case the posi unit might need new clutches but try the oil change first as it is a very common problem and fix.

There is likely nothing wrong with the rest of the rear end. Do the oil change first. The friction modifier comes in a small 8 oz bottle, Kendall was the last I used.

Corvette is not a 12-bolt case, it is a variation of the 8.2" 10-bolt but takes Corvette IRS specific gears.

Tom's Differentials offers a 12-bolt upgrade for guys racing the IRS rear, but it is very $$$$$ and requires some machining of the gear case.

[biscuitville]

Eric and BGH
Thanks guys - I was counting what I thought to be the bolt holes in the gasket but it turns out they are not all used. I recently removed the rear-end to simply drain the old oil and replaced it with standard gear oil (no additives). The tires are new and I keep on top of the pressure. So I am guessing that the worn-out clutches suggestion is probably spot-on. I will start with an additive since that is the easiest to try.

[ericnova72]

If you recently replaced the rear fluid and didn't add the additive, this is almost 99% sure the reason for the shudder now. Nothing is likely wrong, just add the friction modifier, drive a little to mix/work it in and all should be ok.

I've seen GM Eaton posi clutches with 100,000+ miles on them with very low wear. If it had good fluid servicing in its life they will last a long time.