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Old 05-17-2020, 12:02 AM   #1
markymark77
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1957 Chevy Hydramatic rear end gear change

Hi,

I posted this on another site, but wanted to know if anyone has lots of experience/knowledge with the Hydramatic trans.

Just a background, I have an 1957 Chevy 1/2 ton truck that originally came with a v8 and 4 speed trans. I picked up full running gear from a hydramatic truck and was going to swap those parts to my truck (front and rear suspension, rearend etc (mine was changed to something else from a PO). I have a ton of Hydramatic questions so if anyone wants to share their knowledge im all ears!

I have a 57 Chevy 1/2 ton that has a Hydramatic. I have never ridden in a TF truck with Hydramatic, can any one comment on how it drives? Im debating on changing the rear gears to 3:38's, does anyone have this setup? Right now I have a 3:90 rear, what would the rpm's be at 65mph with the original rear. Im torn on keeping the rear end original, but also want to be able to drive it at "highway" speeds even now and then.

Thanks in advance for the input!
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Old 05-17-2020, 04:30 PM   #2
mr48chev
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Re: 1957 Chevy Hydramatic rear end gear change

I'm just going to say this, unless you are shooting for a show room stock truck that may be a real spendy transmission to have repaired compared to a turbo 350 or 400 or even a 700R4. Plus the old timers who really know how to work on them are getting few and far between.

The Dual range hydromatic rebuild kit runs 650 on Amazon https://www.amazon.com/Range-Hydra-M.../dp/B01HS86HXK and I'd expect having a shop rebuild it would take around a thousand in labor plus the converter.
Those hydromatics worked pretty good when they were in good shape but were a fairly expensive option and were usually sold to people who used them for in town delivery rigs.

To me it has to be that you really want an original hydromatic and not a later model transmission like a lot of guys would use and are willing to pay the extra to have it repaired if it comes to that.
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Old 05-18-2020, 12:13 PM   #3
markymark77
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Re: 1957 Chevy Hydramatic rear end gear change

Hi,

I have thought about putting a more modern transmission in, but I was going to keep it original (minus the potential gear change). I found a person to rebuild it that has done them in the past, it is pricey like you said though.

Thx
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Old 05-18-2020, 12:41 PM   #4
1project2many
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Re: 1957 Chevy Hydramatic rear end gear change

I have owned two Hydramatic trucks although I only got to drive one. They are different from what we're used to in a few ways. The Hydramatic transmission has no torque converter. Instead it uses a simple fluid coupling with no torque multiplication. This results in more slip when the truck is under load and when starting from a standstill. There is not park position so the park brake should be kept operational. If "in gear" parking is desired the transmission should be shifted to reverse then the engine can be shut off once reverse is engaged. There is no ability to select a specific gear other than first. The shifter is designed to limit upshifts. A show truck will not have troubles with this but for some this was problematic in work trucks. A fully loaded truck could also show problems with unexpected upshifts. Sufficient pressure could be generated so a vehicle in a lower gear range might accidentally shift to the next gear causing a substantial reduction in engine braking. Again, a show truck is not likely to have a problem with this.

Was the donor truck a 1/2 ton or was it a larger truck? 1/2 ton transmissions came with a different 1st and 2nd gear ratio.
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Old 05-18-2020, 02:19 PM   #5
markymark77
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Re: 1957 Chevy Hydramatic rear end gear change

Quote:
Originally Posted by 1project2many View Post
I have owned two Hydramatic trucks although I only got to drive one. They are different from what we're used to in a few ways. The Hydramatic transmission has no torque converter. Instead it uses a simple fluid coupling with no torque multiplication. This results in more slip when the truck is under load and when starting from a standstill. There is not park position so the park brake should be kept operational. If "in gear" parking is desired the transmission should be shifted to reverse then the engine can be shut off once reverse is engaged. There is no ability to select a specific gear other than first. The shifter is designed to limit upshifts. A show truck will not have troubles with this but for some this was problematic in work trucks. A fully loaded truck could also show problems with unexpected upshifts. Sufficient pressure could be generated so a vehicle in a lower gear range might accidentally shift to the next gear causing a substantial reduction in engine braking. Again, a show truck is not likely to have a problem with this. Was the donor truck a 1/2 ton or was it a larger truck? 1/2 ton transmissions came with a different 1st and 2nd gear ratio.
Thank you so much for the description. The truck would just be for driving around town and will not be a show truck. It wont be used to haul anything substantial. The whole drive train came from a 1/2 Chevy truck. I was told that the rearend gears were different (higher), but from documents there were only the 3:90 and 4:11 option. I have a 3:90 and was thinking of changing it to a 3:38, would that be wise with this transmission? Do you anticipate any issues?

Thx!
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Old 05-18-2020, 09:16 PM   #6
1project2many
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Re: 1957 Chevy Hydramatic rear end gear change

3:38 might be a bit much. I looked at a number of other vehicles using the Hydramatic and it looks like 3.68 was the lowest (numerically) offered gear ratio. If your engine makes enough torque and you don't mind slower takeoff then it might work for you.
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