The Stovebolt Site Forums The Parking Lot '60-'66 Trucks 1961 c20 posi rear end

If the little tag on the posi-trac rear end is correct, I have 4-56 gears in my truck. I'm in third gear just hitting the edge of the forty foot driveway. URGGGGG!

I don't need to go really fast, but I would like something a bit more road friendly. I won't be using the truck as my hunting and off-road vehicle.

I called a supplier (can't remember who - found 'em on the internet) and they said that I couldn't buy a different gear ratio for that truck. They could only sell me a new set of 4-56. I would have to change out the rear end, and then change the gears if I wanted.

Does anyone know if this is true? It is more than I wanted to do this year, I was hoping to pop in new gears and call it good. Surely not everybody out there with c20's are grinding around with 4-56's are they??

Help, advice, tips, and opinions would all be appreciated.

4.56 was the standard gear in the 3/4 ton for that year, with 5.13 as an option. The only definite option for better gearing is to find a 4.10 third member from a '68-'72, thous they are getting hard to find. There is a company claiming to have a 3.90 gearset, they still list them on their site (I don't recall the comapny name), but I don't think anyone has been able to confirm if they are for the Eaton diff or a Spicer.
A transmission swap to one with an overdrive gear may be a better solution.
You mention it being a posi, if so, it may not be the original differential, as GM didn't offer a posi in the HO 52/72 axle as far as I know. They did offer a Detroit Locker as an option in the later '60s, though. You migth need to confirm what differential it is (Eaton, Spicer, Corp 14 bolt), as this may open up more gear choices, especially if it's a Spicer.

Bill, there is a tag in the cab above the fuel tank which reads:

"NoSPIN Differential
for greater traction, safety and economy
NoSPINS positively rotate all driven wheels under power
WARNING"
...
"Detroit Automotive Products Corporation
Model # KS10"

The differential is a 10-bolt round pattern. How can I tell if it is an Eaton, Spicer, etc...?

Oh, and the other reason I think it is posi, or locker, or something similar, is at low speeds it turns like something with tracks - a tank maybe?? hehehe

If a tranny swap is the best option, what years would bolt up well?

Ok, that clears things up. No-Spin is the same thing as a Detroit Locker. It is what's causing the issues turning, too. Just from the description, you do have the HO52, which was the oem diff.
For the trans, and MY6 from an early '80s Chevy/GMC would be the easiest bolt in, the NV4500 is another good choice, but will require an adapter.

Thanks, Bill! That helps a lot! I'll see what I can find.

Maybe that's what mine is too. I have a 61 half ton with a 3:90 rear end...at low speeds around corners, it feels like the truck is on tracks instead of wheels. But only when warmed up. Is that normal? I'm happy with the gearing otherwise...it'll still do 65 miles an hour.

Way over my expertise level, Adam61. Hopefully, someone else will know. I just know I wish I had 3:90s. :-) Good luck. Do you have pics of your truck somewhere?

Husker, I do have pics on photobucket. I will have to set up a link to them sometime in the near future.

Adam61, thanks, it would be great to see what others are doing to their trucks. Might get some ideas.

You might have the Positraction diff. There wasn't a No-Spin option on 1/2 tons.

So all half tons are positraction?

No. It was an option. Most were open differential. The Positraction HO33 axle isn't very common, they were only made for 3 years ('60, '61, '62). Parts availabilty is iffy, last time I looed into it, only the clutch packs were still available from GM. Bearins and seals should be the same as the open diff, though.

Bill-
Know if or where I can find a photo of the "no-spin" 4.10 rear end?
I just found a rear end, and the guy says it looks like it's a locker...but it's 4.10 and in a ho52/72 third member.
I don't want to assume it'll fit in my rear end and drive 3 hours just to find out it won't.
Thanks!

Jeremy

Externally, they look the same as the open diff version. You would only really be able to tell if it's a No-Spin by either seeing if both axles turn the same direction when one side is turned, and you can't hold one side still when the other is turned, or by opening it up and pulling the carrier, splitting it, and looking inside.
Here's an exploded veiw of the HO 52/72 differential. The No-Spin components would take the place of the spider gears inside the carrier. The third members interchange from '47-'72.
Here is an exploded veiw of a No-Spin/Detroit Locker.

Thanks for the help! Appreciate it. The guy at the yard pulled the cover off for me and looked for numbers. When he called me back. He couldn't find any. So I directed him. To the numbers on the face of the ring gear. It's a 4.10 and it's in an Eaton so I'm going to go pick it up as soon as I figure out how to get there.
Tip for folks looking for a 4.10 gear set for their 3/4 and 1 ton AD truck... Look at junk yards around farming, ranching communities that are within 200 miles of a metro area... Taking stuff to market, picking up stuff.... Etc.. Farmers and ranchers are smart, frugal,people...

Jeremy

I was given a 62 K20. It has the same tag referred to earlier in this thread.

Would the front diff. also have a locker? Or would that make the steering near impossible?
Looking forward to working on this truck.

I also have a 64 GMC w/the stock V6 and a 48 Stylemaster sedan with a 216.

Go on line check yukon gear they have a 3.38 set of gears and rebuild kits they call it a gm55t there.

No, there was no locker option in the front, and finding one in the rear on a 4x4 would be unusual.

The GM55T is the HO33, which is the 1/2 ton axle. The 3.38 gear they offer will not work in the 3/4 and 1 ton.