The Stovebolt Site Forums The Shop Area Driveline 2 speed gear swapping?

I've had my '53 3-ton on the highway and while it do very well, I didn't feel comfortable taking it above 75km/45mph. I want to stick to the old stovebolt engine and not interested in a transmission swap (changing the clutch was enough work). Those old overdrive units are rare and new ones are expensive. This leaves me with only one option. I have heard of people changing the gears in differentials, but never in a 2-speed. Is it possible?

Most gear sets for old 2 speeds give speeds of slow and slower, no higher speed gearsets were ever made that anyone has found out. If your steering and brakes are up to higher speeds a Brownie transmission inserted in your driveline will give higher speeds. A Dana 70 or 80 and higher can give a single speed rear with higher ratios, might have to deal with disc brakes. One can also replace the factory transmission with an OD 5 or 6 speed transmission.

Ed

Hy Alex Arrow, depending on which type of two speed rear axle you have, there may be some gear options available. Gears from a Diesel truck are usually lower numerically and result in higher speeds when installed behind a gas engine. How is your two speed shifted, mechanically, electrically, or vacuum? If you could post a picture of what your differential looks like that would help us identify it. When we talk about getting higher road speed out of our Big Bolt we must discuss the fact that these trucks were never designed to go more than Fifty miles an hour (the common speed limit when they were new), upgrades to brakes, tires, wheels and steering should be made to keep everyone safe at speeds above fifty. Ed has made an excellent suggestion for a newer Dana axle with more ratios available, it will most likely come with better brakes, possibly even disc brakes if you find the correct one, that would be a win win for you and those on the road around you. Hope that helps.

I understand what you mean, but even if I stick to that, a higher gear would have the benefit of lower RPM.

As for the diff, it is vacuum operated. Added some photos. Not sure if it is visible, but the cover has GMC stamped in it.

I found, thanks to an old time rebuilder in Duluth, a two speed set, 4.13 high, to replace my single speed 6.5 rear.

Ed

Hy Alex Arrow, your pictures are of a medium duty Eaton axle I think. It looks like it could be converted to electric shift. I am not sure what ratios were available or if that model was ever used behind a diesel engine?

Alex, if you can figure out what your optimum ratio would be, whether you live in the mountains or plains, frequent loads, etc., what engine you have, or might be able to get, tire size, etc., suggestions might be made from people with similar situations as what might work for you.

Ed

Well, I live on the (mostly) flat-ish prairies, so steep hills would be uncommon (but not rare). Likely wouldn't load it up heavily or max often, if ever. Currently has 20x9 tires with a 1955-57 Chevy 235 and basic 4 speed.

Most likely, I just want to reduce RPMs on the highway, rather than increase speed(a few people expressed their concern about running those engines at high RPM). To do this, would the gear swap or adding another transmision - a brownie - be easier to perform and find parts for?

Seems the gear swap is easier, but only if I can find the needed gear.
The transmission would be easy to find*, but then there is the mounting, shaft attaching, shift stick modifying and shortening the drive shaft.

*I know of an old GM truck manual 3-speed(more like 2, as 1st is marked as Low).

What do you consider to be "high RPM"? Most of the folks here on the Stovebolt forum start getting sweaty palms and heart palpitations if their engines approach 3K RPM, but the things were designed to turn closer to 4K all day every day, especially in the big rigs with the 4 speed trans- - - -2 speed rear end setup. Forcing the engine to run lugged down at the bottom end of the horsepower curve is actually harder on the engine than letting it stay wound up.

Did Chevy ever actually build a 3-ton truck? I thought GMC was the only one that rated them above 2 1/2 tons.
Jerry

I don't actually know - no tachometer. Going by ear.

It is a GMC, but with a Chevy engine.