The Stovebolt Site Forums The Shop Area Driveline 1942 (BL) 3/4 Ton Rear End Swap

I've put a 1962 235 in my 42 3/4 ton and, I kept the original 4 speed trans. I want to swap out the OE 4.56 rear end for something that will give me a bit more highway speed. I've read the tech article about using the pumpkin from a 55-62 1/2 ton (GM 55T). Unfortunately, I haven't been able to locate a 1/2 ton diff from these years.
If anyone can recommend some rear ends from other years and models, which will bolt in to my 42 3/4 ton, I'd appreciate the input (not really interested in any lockers). I've been told the early 70s passenger car rears will work but, I want to keep my 6 lug wheels. If there are some of the 1/2 ton C10 or Suburban rears with 6 lug axles that will mount in my 42 truck, that's probably the best way to go. I'll most likely need to keep the gear ratio down in the 3.55 to 2.70 range to see any real improvement in top end speed.
Thanks in advance for any help.

Hy Ghost_Rider, I'm afraid you have been misinformed, 1/2 ton diffs are not interchangeable with 3/4 ton diffs. I think 4.1 to 1 is the best gear set for top speed that is available as a bolt in swap, they are rare but were available up to the early 70s I believe, hope that helps.

3B:
Thanks for your reply. I suppose I could have been misled but, the tech article I refer to is posted in the tech tips on this site.
My truck is identical to the 3/4 ton described in the acticle, except it is a 42. This is why I saw no problem in using a 55-62 differential from a 1/2 ton.
I have found a 69 1/2 ton 12 bolt rear, with a 3.73 ratio locally so I'm gonna check that out.
What I have not mentioned is that this truck will be a cruiser/driver, not a show truck. I don't plan on hauling anything heavy, so I think I can change the gear ratio without effecting my intended use negatively. Also, our terrain here in south Jersey is pretty flat, so hill climbing is not an issue.
Any thoughts on using the 69 3.73 12 bolt for my truck will be appreciated.

Hy Ghost_Rider, I just read the tech article that you mentioned, and I can honestly say I learned something. Up to this moment I did not realise that there were two such vastly different "3/4 tons" As to the 69 rear axle, that will probably be a little wider than your rear axle, it will probably be necessary to relocate the leaf spring pads, but I think the swap is a doable one. Sorry I stuck my nose in to something you already had figured out.

Keep looking for the 55-62 differentials. They are pretty available. The gear ratio will be 3.90. I believe your 3/4 ton has 4.11 gears, so there won't be much of an improvement.

IMHO, an OD transmission swap would be a much better setup. If you use a browny box, you can keep your crash box. There are several od transmissions that can be made to work using an adapter plate. Some of the transmissions include, but are not limited to the A833, Nv4500, and of course the T5.

3B:
No need to apologize. You gave your best advice, and that's all anyone could ask. I appreciate your honesty and your input.
The two 3/4 tons were quite different as far as brakes, wheels, and some driveline components.
I will most likely continue my search for a 55-62 GM 55T rear end.
Thank once again for your help.

Thanks for your help.
I really would prefer to go with the 55-62 differential swap as described in the tech tip. Since my OE diff is 4.56, even the 3.90 would be a significant improvement. It sure would be a lot less work than swapping out the entire rear end.
All I want to do is put in some taller gears that will allow me to cruise at 60-65 without making my 62 235 scream it's guts out. This motor has dual Rochesters, Offenhauser intake, and Fenton headers, so it will handle the speed but, I don't want to push it to 65 with the OE 4.56 gear ratio.
I'll also look into the OD trans set up to see if that would be another good upgrade for my truck.
Thanks once again for the tips. While I've done several cars in the past this is my first truck mod, so I really appreciate the input.

BTW, can anyone tell me if the passenger car differentials from 55-62 will also bolt into the 42 3/4 ton 10 bolt housing, like the 1/2 ton pickup diff described in the tech tip?
I have found a couple of 55-62 Impala, Nova, sedan diffs on craigslist but, I'm not sure that these are the same as the 1/2 ton pickup diff (GM 55T) used in the tech tip swap.
Thanks in advance for any input you might have.

Hy Ghost_Rider, now this I DO Know, the 55 - 64 car diff's are not the same as the 1/2 ton trucks from 55B series to 1962, sorry that narrows your search.

I have a 12 bolt with 3.73 gears from a 65 1/2 ton in my 1954 1/2ton. The 3.73 gears are a pretty good compromise between low end and high end performance. I also have a 1955-1962 Chevy 1/2 ton three speed overdrive trans that allows me to really drop the rpm's on the highway. Cannot drive under 50 mph in 3rd gear and overdrive or it lugs down, 55 mph and above is best in 3rd and o/d with 3.73. I did have the 55-62 rear end with the 3.90 gears and overdrive trans in the truck for many years and think that was the best combination for take off torque and highway speeds. The GM engineers knew what they were doing when they designed the 3.90 rear end with the three speed overdrive trans. It takes a little more diligence to find the trans and hook up the electrical on the 3 speed overdrive but is well worth it to have the ability to kick in and out of overdrive with the gas pedal in 2nd and 3rd gear - the 2nd gear take offs when driving around town and the free wheeling at low speeds. Complete operating/driving instructions are in the 1955 and later Chevy shop manuals(available on line). After market (Hurst?) shifter would be needed without existing column shifter and shifter clearance from seat frame might be an issue to resolve.