The Stovebolt Site Forums The Shop Area Engine Shop Tranny swap - 3-speed to PowerGlide?

I currently have a 3-speed transmission bolted to my 55 235 with the original torque tube driveshaft in my 51 Chevy 1/2 ton pickup. Can I swap out the 3-speed with a PowerGlide automatic transmission without changing the rear end and driveshaft? Thanks for any/all feedback!

No. If I understand you correctly you want to put the PG in your 51. The PG links up to an open driveshaft. Your 3 speed has a different end on it, one made for a closed (torque tube) set up. You would also need a differential set up for an exposed drive. If memory serves me there was a PG that hooked up to a torque tube but in car.
Getting the rear end changed is not major because a centre section from a later model (56 and later) will bolt right in.

1950-53 cars had a Powerglide with a closed driveline. I doubt that it would be a direct swap, but you might be able to adapt one with enough effort. I really have my doubts that the torque tube would be the right length, and you'd have to fabricate a rear crossmember and modify or eliminate the crossmember at the bellhousing area. The front mounts would have to be eliminated, and a mid-engine mounting system would need to be fabricated. I doubt that your block will have the mounting holes for mid-engine mounts, as only a few 216's had those holes, or even the casting bosses for them.

Then, there's the burning question of "why"? The early Powerglide was a really poorly-designed transmission that had lots of problems, and only had two gear ratios- - - -too low and too high! You would be far better off to adapt a Turbo 350 or a 700R4 to your engine, and find an open-driveline rear end.
Jerry

I agree.

But if you insist on figuring out how to adapt an automatic to the torque tube there are a couple of other options.

The 4-speed Hydra-Matic was offered in torque tube trucks in '53 and '54. It would be a factory parts conversion but you end up with an old design transmission that is expensive to rebuild and does not drive as smoothly as modern automatics like the TH350.

Or you could engineer a torque tube adapter for a modern automatic similar to what Patrick's has done for the Saginaw 4-speed. Do a good enough job and you might find you could make a small fortune selling such a thing provided you started with a large one.

The Hydramatic trucks had lots of factory modifications, including a different-shaped hole in the floorboard, and frame and crossmember modifications. Retro-fitting one to a standard-shift truck would be a PITA, even with a Hydramatic-equipped donor truck.
Jerry

I believe the only difference to the frame and cross members is bellhousing cross member which can be changed with some work. The floor and wiring harness are actually different if you want the truck to look original, though anyone thinking Powerglide isn't doing a serious resto.

Personally I'd rather retrofit a Hydra-Matic than an early Powerglide. But given a choice I'd go open drive and a modern automatic.

Hmmm, the one I'm currently working on I'm going from Hydra-Matic to TH400 with a V-8. That might reveal my bias a little.

I did just this with a donor truck rolling chassis. Frame differences are so that the trans can be removed from under the truck with heavier bolt in crossmembers. The brake line routing is different for the same reason. The trans cover "climbs the firewall" a bit so that bellhpusing bolts can be accessed. There is also the conversion to solenoid starter and a neutral start switch as well as the pushbutton starter switch. You can live without the auto choke but all linkage is different from the gas pedal forward. That and the shift mechanism, a parts truck would really be the only way I would want to tackle the swap. Oh yeah, the driveshaft is a different length.

I've toyed with the idea of converting my Suburban to Hydra-Matic. Its most likely an idle thought that will never develop into any action. Since I have a complete Hydra-Matic truck and parts of another it doesn't seem too scary.

Another big thing to consider is that Chev and GMC use different linkages and shift mechanisms as well as the neutral start switch so you need to make sure that the donor truck is correct for the application.

All of my stuff is GMC so in my case that's not an issue. All mine is '54 or '55 1st too, so it should all be at least close.

The truth though is that I'd be much more likely to adapt a modern automatic than to perform this conversion. I think it would both be less work and would give a better result.

Plus I just found a '53 GMC with a Hydra-Matic and am sure tempted by a first year Hydra-Matic even though I really have no business even thinking about buying another truck right now.