The Stovebolt Site Forums The Shop Area Driveline 1957 3100 Rear End Swap

I been looking to swap out the stock rear end on my 57 pickup for a more modern rear end so i can drive my truck occasionally on the freeway, as of now its really slow and i doubt it goes over 50 mph. I found one at a good price its a 10bolt 6 lug 3.73 gears open diff off a 88-98 chevy 4x4. My question is will this rear work for the swap? Will the ebrake cables work and well as the brake lines? I have a chevy 235 with factory hydramatic transmission. Thanks

you'd need to modify a number of things to get it all to work, and 3.73 isn't a big gain over 3.90, you'd be better off to look at the 3.38 gearset offered by Patricks and others .... or change out the hydramatic for an OD 3 speed

Bill

Im new at all this so i have no clue what the numbers mean. Is there anyway around keeping the transmission and just go about swapping the rear end with the least modifications?

hi i changed mine to 3.38 ratio easy from Toms diffs idaho cruises well now speedo wrong thats all, use gps to work out speeds no hasstle with handbrake etc you change crown wheel and pinion

the stock rear gears are 3.90:1, for just gaining 0.17, the 3.73:1 you suggest would likely not make a noticeable improvement - there are 3.38:1 gears available [scroll down about half way] to install in the stock rear, they were a 'mileage minder' option originally - it's more expensive, but requires no change in rear end mounting, e-brake setup or driveshaft - I don't know if any later rear with as good a ratio could be just bolted in, otherwise folks use car rears with 5 bolt patterns for ratios in the low 3's

Bill

Another issue that will arise using the '88-'98 4x4 rearend is the width. You'll find that the later rear is considerably wider than the stock '57 rear.

Sounds slow. A 235 can spin 3200 rpm all day long. 3200 rpm with 3.9 gears and 28 inch tall tires is 68 mph.

I believe HydraMatic equipped 1/2 ton trucks came with a 3.38:1 ratio. That would give 79 mph as a top speed.

Find out what ratio you have first. 3.73 gears may end up being a step backwards for you.

according to the '57 shop and operators manuals, the rear was 3.90 for all transmissions except the OD, which was 4.11 - Im pretty sure the 3.38's started in '58 after gas prices took a jump

Bill

Il look into finding out what gears are currently in the truck. I would love to be cruising at 79 mph on the stock rear end.

I used a '59 GMC 3.07 rear end under my '56. It was a direct bolt in deal with the exception of the bracket for the brake hose, which had to be moved to the opposite side. This is a Spicer rear end and if you go that route you need the complete assembly, including brake parts.

GM 12 bolt truck axles from the 60s will also work... the backing plates are the same as the eaton HO33 axle. The barke cables will interchange. You will have to have new spring perches welded to the axle.

The HydraMatic is a 4 speed automatic. The linkages are finicky on them. Is your transmission shifting through all the gears? If not, lower speed could be expected.

How are you calculating speed? Stock speedometer? After 50 years a number of factors can combine to make the factory speedometer inaccurate. Taller or shorter tires can add to the inaccuracy. In the 21st century, gps units can be used to get a reasonable idea of how fast you are traveling. There are other meathods that can be used as well such as the measured mile and the stopwatch.

I'm sure the transmission goes through all gears. I just leave it in I-4 and goes through all gears with no problems(unless thats not the correct way of shifting). I'm also running smaller tires 640 15s so im sure that's a big factor in mph. Il look into those Gm 12 bolt axles vs buying 3.38 gears Not cheap.