Need help with differential ID

[SilverGT3RS]

Hey everyone…I have a 67 LS swap I’ve been messing with the last year or 2. Saw a leak on the driveway, so I put it up on my lift and decided since it was leaking I’d drain it, clean it up and actually find out what rear end I have.

From what I can tell, looks like I have 3:73 GM (ring stamped 41:11) gears but I don’t see any markings on the differential.

Any help appreciated!

[pjmoreland]

There should be a code stamped into the top of the passenger side axle housing tube. There should also be a manufacturing date code cast into the pumpkin. The following thread has information on and photos of the codes:

https://www.67-72chevytrucks.com/vbo...d.php?t=578793

[pjmoreland]

Looks like some sort of aftermarket locker.

[SilverGT3RS]

Originally Posted by pjmoreland

Looks like some sort of aftermarket locker.

Yes, this is definitely not the stock rear end. Gears have little to no wear on them. Previous owner said it had a Detroit Locker and 3:90s installed. Obviously not 3:90s and can’t see any stamping on the differential.

[pjmoreland]

41÷11 = 3.73, so you're right about the gearset. It's great that you've got original GM gears with low wear.

[Mike C]

The case is just an open 12 bolt truck 4 series for 3.73 and higher numeric gears. It has a lunchbox locker in it such as a Spartan or LockRight.

[SilverGT3RS]

Originally Posted by Mike C

The case is just an open 12 bolt truck 4 series for 3.73 and higher numeric gears. It has a lunchbox locker in it such as a Spartan or LockRight.

So in other words, if I want a real posi or locker unit, I need to pull all this out and start with a new gear set/diff?

[pjmoreland]

You wouldn't need a different gearset.

[SilverGT3RS]

Originally Posted by pjmoreland

You wouldn't need a different gearset.

Eh, I’d probably just go all new just for my own OCD. I’ve set up a few rear ends but I’ve never seen this type diff before. Learn something new everyday.

Anyone got any feed back on staying with the stock housing with new gears/diff vs going with a 9” rearend?

[67 twins]

Originally Posted by SilverGT3RS

Eh, I’d probably just go all new just for my own OCD. I’ve set up a few rear ends but I’ve never seen this type diff before. Learn something new everyday.

Anyone got any feed back on staying with the stock housing with new gears/diff vs going with a 9” rearend?

I would never replace a good set of GM gears except to change ratio.

[pjmoreland]

Originally Posted by 67 twins

I would never replace a good set of GM gears except to change ratio.

My thoughts exactly. I installed a new set of gears from Yukon two years ago, and they were noisy. Member 72again4x4 gave me a set of low-mileage original gears that I installed, and they're nice and quiet.

http://67-72chevytrucks.com/vboard/s...d.php?t=835135

[SilverGT3RS]

Originally Posted by pjmoreland

My thoughts exactly. I installed a new set of gears from Yukon two years ago, and they were noisy. Member 72again4x4 gave me a set of low-mileage original gears that I installed, and they're nice and quiet.

http://67-72chevytrucks.com/vboard/s...d.php?t=835135

I’m probably going to change the ratio to 4:10. With the OD trans and 20” wheels it should be perfect for what I do with the truck.

[Mike C]

Originally Posted by SilverGT3RS

So in other words, if I want a real posi or locker unit, I need to pull all this out and start with a new gear set/diff?

The lunchbox is a real locker. Not a limited slip. It is automatic when you are going straight or applying power. When you go around a corner and are off the throttle, the outside wheel speeds up and uncouples from the driver. It then slips by the teeth and you can hear it clicking until you straighten up or get back on the gas.

They are not as strong as a full locker with replaced carrier because the use the large window open diff case which isn’t as strong as the dedicated case. The better ones come with a new spider gear shaft that is hardened because that and the case are what delivers all the power.

Plenty of videos on how they function on YouTube. Search for LockRight. I am not a fan of most of the lunchbox style lockers on the street as they introduce a lot of backlash into the drivetrain. But the Powertrax NoSlip (which yours could be as we don’t know what it is currently, but LockRight the most common also by Powertrax) is advertised to work much better on the street. I have a LockRight in my M38A1 and my MB army Jeeps. They work awesome off road and any ill manners are mostly obscured. I have NoSlips in both ends of my CJ-5 and they are quieter and less jerky with only the occasional bang.

Just make sure you are pointed in the direction you want to go when you get on the gas.

[SilverGT3RS]

Thank you Mike C for the detailed explanation!

After much discussion with my gear head friends and more research it seems like the Lock Rights are intended more for the off-road crowd than typical street use. I’m more of a purist, and combined with the leaking differential and the now discovered axle and pinion leak, the Lunchbox had to go. The rear end also had about .018 of backlash which hasn’t seemed to affect the gears but definitely contributed to the clunking I heard quite often.

I’ve ordered a new Eaton and 4.10s for the truck. Already have the housing out and everything disassembled. Excited to get everything cleaned up and back together.