Looking at a possible replacement for my blown rear in my ‘68. This one is out of a ‘74 and is supposed to be a 3.36 ring and pinion. Aside from the stamping on the case “AX” or “LR” which would show what the case originally came with thats no guarantee that its wasn’t changed.
Mopar ring and pinion sets are clearly marked with the ratio on the ring gear. Are the GM factory gears stamped in this way or have a serial number that can be referenced to on a chart. I’ve tried a search for that and I can’t get past the case stamped codes. I’ve read the ring gear on a 3.36 will be 37 teeth while the pinion is 11 but it’d be hard to count the pinion teeth in an assembled unit.
I’d like to find a good rebuildable rear with the factory gear set. The factory 3.08 gears in my original rear were nice and quiet.
Don’t want to the noise modern made gears make.
Also masking the switch from 3.08 to 3.36 to aid drivability in slow traffic while still retaining decent highway cruise ability.

 

Im no expert and doing my first rebuild chnanging from a 3.08 to 3.36. The edge of the ring gear has numbers stamped on it so open the rear cover and spin the gear until you see the number. In the line will be an 11 and a 37 if its a 3.36.

 

Place a mark on the pinion yoke, then count the number of turns the pinion makes through one complete rotation of the axle. A 3:08 ratio will result in just over 3 full turns, a 3:36 3 1/3 turns and a 3:70 just under 3 3/4 turns.

 

Ok, I looked mine over really well then looked up a 3.36 GM gear set on eBay and got my answer.
I knew there’d be stamped markings of some sort.
It’s almost as simple as the Mopar gear sets are. The trick is you have to know the tooth count for a given gear set and that’s what’s stamped on the each gear. Mopar stamps the ratio but for GM it’s the tooth count. 3.08 has a 37 tooth ring gear and a 12 tooth pinion gear. This is clearly stamped on both gears…




Here is a shot of the 3.36 gear on eBay… 37 tooth ring gear and an 11 tooth pinion

 

You called it. Saw this after I did my own looking but thanks.

 

Hi Mike
Nice speaking with you the other day.

As you found out, GM stamped the part number, tooth count, and date code into the ring and pinion.

When looking at any used differential, you need to closely check the posi case for cracks, the web section for grooves from broken ring gear bolts, the left wall and pad of the housing for gouges from broken ring gear bolts. Look at the caps for witness marks and the shim type used.

Do not believe any stories regarding the diff, look at it to see. If someone won't pull the cover, walk away.

If it is represented as rebuilt, as by whom, what was done, and then look to see if it has been done.

These diff's break down a lot of times due to abuse or have been worked on by unqualified people who may have caused any failure for it being out of a car.

A 74 posi case is one the better ones of the C3 run. If they are not cracked or have elongated cross shaft holes, they should be blended, radiused, and polished to help prevent cracks from forming - but nothing is 100% and anyone claiming a vette diff is bulletproof is either a liar or an idiot. They all can be broken, 10, 12, and 9"

The 74 spiders are 10 -17's and what you want. The axles were junk but maybe they have been changed, if so, to what?

Good luck with your search, they're out there and are not rare by any means.

 

Thank you Gary, I agree 100% with your points on inspection and greatly appreciate your input. I’ll be traveling to have a look at that one and fully expect to open the case. If the posi gives any concern I may still buy it at the right price and replace it with a new Eaton unit. I’ll be looking hard at the gears as if they are good being original GM I’d prefer that .
Either way it’ll get rebuilt, steel clutch plates, new timkin bearing, new seals. It won’t be one of your diffs but it’ll be better than a shot in the dark used time bomb of a rear.

 

Mike
I just set these up today- NOS US GEAR 373, the best that was on the market and last made in the USA. Gone now. These dialed in perfectly and the gear mesh is the best I've heard. The new imported gears dialed in good so far, mostly Motive or Yukon but I'll be testing a set of Richmond 390 in an upcoming build.

Unless the used gears you're looking at are obviously chipped or broken you're not going to be able to tell much. If on a stand you can check the lash and run a pattern to see but interpreting a used gear set can be interesting.

New loaded Eatons and Yukon's come with fiber clutches and heavy springs. The Eaton spiders are still usa made, the nothing on the Yukon is. There is a clear appearance difference between the Eaton spiders and the Yukon or DANA/Spicer spiders. The Eaton case is cast and machined in China, assembled in Mexico. The Yukon I believe is all Chinese, I don't use them.

I use a lot of new Eatons and every single one of them I took apart and rebuild, new out of the box.

Good luck in your search, be careful and don't settle for junk.

 

Thanks Gary, checking the lash is beyond my skill set so unless the used set is obviously bad I’ll be putting my faith in the shop and making a calculated roll of the dice again. Not a stranger to that. I’d like to hear how the Richmond gear set turns out. Will it scream like the others from them?
Spoke with the shop today and they have no problem installing the solid steel clutch discs. Don’t have a price quote yet but so far things seem positive.