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1941 3/4-Ton Differential Swap

(04 August 2008)

By Kip "Kip's41" Bonds
Bolter # 9129
1941 Chevy Master 3/4-Ton

They're no fun if ya can't drive 'em !

       My plans are to keep my 1941 3/4-ton Chevy truck as close to stock as practical. This includes sticking with the Babbet-pounding 216 and the un-synchronized 4-speed.

       After completing just enough work on the truck to be able to drive it safely around the block, I quickly discovered that its top speed was somewhere in the neighborhood of 42 MPH. This unfortunately was not going to be very practical for drives of any distance.

       Wanting to keep the stock engine and transmission, the only option left was to go to a higher differential gearing. So I set out to find the easiest and least expensive way to replace the stock 4.56:1 differential gear ratio.

       This project lasted the better part of seven months mostly because I could not find information on my differential. I almost immediately discovered that the 1941 and possibly 1942 Chevy 3/4-ton pickups came in two types: Commercial duty trucks with 14-inch brake drums, 8-bolt wheels and a full floating rear axle; and the slightly lighter duty trucks (like mine) with 11-inch drums, 6-bolt wheels and a non-floating rear axle.

       Both types are different from the 1/2-tons in that they have longer beds, narrower frames in the bed area and open drive shafts.

       The next piece of information took much longer. None of the local gear and axle shops or even the larger national places could help. Most would not believe what I was describing as my stock differential.

       After exchanging a whole lot of notes through “The Bolt,” we began to piece together that the differential on the lighter type is a modified half-ton unit.

       Chevy had modified the half-ton torque-tube differential [ image ] by removing the torque-tube housing, shortening the snout and adding an oil seal and exposed pinion yoke [ image ] . This is totally different from the commercial duty and later 3/4-ton differentials [ image ] .

       After figuring this out, the plan to swap differentials was fairly straight forward. I purchased a 3.90 to 1 ratio differential out of a 1961 half-ton Chevy pickup. Turns out that a 1955 through 1962 1/2-ton units will all fit. Three ratios were available through those years:

  • 4.11 with an overdrive transmission
  • 3.90 with a standard transmission
  • 3.38 with an automatic transmission.

       I could not find a 3.38. My guess is that not many automatics were built in those years.

       Anyways ... back to the actual swap out. It was a “piece of cake” to my delight! Both my stock ’41 and the replacement ’61 are mounted to the axle / banjo housing with 10 bolts and both have the same diameter 17 spline axles. IT BOLTED RIGHT IN. [ Here's a larger view of the image to the left. ]

       There are two things that have to be changed. The ’61 differential is 7/8 inch shorter from its mounting flange to the center of the universal cap. And the ’61 universal joint is smaller than the ’41.

       I have been told that you can purchase universals with two different size cap sets. However, having to lengthen the drive shaft anyway, I elected to have a ’61 yoke installed on the back end of the drive shaft while it was being lengthened.

       So, in the end, it’s a fairly easy swap.

       Thanks to “colettsbro” for the ’61 differential and “pre68dave” who actually had a ‘41 like mine and one of the '55 to ’62 units to compare.

-30-

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