The Stovebolt Site Forums The Shop Area Driveline Automatic to Manual Conversion

I have a 55.1 GMC with a 284 and Hydra-Matic. The transmission needs a rebuild. The previous owner said the 2-3 shift would not work. I would rather have a manual, so I am going to put a T-5 in there. What does this conversion require? I would like to bundle everything related to the automatic and sell it to someone who would appreciate it more than me.

Would that include the cab, the floorboard, several crossmembers, and the steering column and shift linkage? The rear axle has a different ratio than the stick shift models also, I believe. There are numerous differences that would make the conversion difficult, and it's entirely possible you'd never complete the project.
Jerry

The cab? I was not aware there were differences there.
My floor is rusted out, so that is not a concern. I have to replace it anyway.
I expected to loose the transmission crossmember, but not others.
Steering column and linkage = yes.
I would not sell the rear axle.

What are the numerous differences?

Joseph, I've got a 54 Chevy with Hydra-Matic. Getting ready to start the restoration. I was considering swapping out the trans but after further research and the input from the forum members here I'm going to keep it (thanks again to all who weighed in). A rebuild is about $3000 plus shipping to Gene at http://www.restorationhydramatics.com. It isn't cheap but you might wind up with less $$ invested than with what you're thinking of doing. Plus, I hear they are a GREAT trans and really trouble free. Just my .02$ for what it's worth. Best of luck on your project. Would love to see some pics.

54hydramaticFL, what transmission were you going to swap it to? Did you figure out what was required to complete a swap?

If you've got a rustbucket that's going to require major surgery on the sheet metal, that's not as bad as butchering up a rare vehicle that's in good condition. The hole in the firewall is considerably larger on the HydraMatic-equipped trucks for extra bellhousing clearance, there's a solenoid-type starter instead of the stomper, the crankshaft flange bolt pattern on the engine might possibly be different, (not sure about that, though), and it's going to be difficult to adapt a clutch pedal and linkage due to a different-shaped frame around the bellhousing area. There were considerably more hydro-equipped GMC's than Chevvies, but us purists just realy hate to see one that's still in running condition get a hatchet job.
Jerry

Check this out.
http://www.jbhardin-gallery.com/trucks/differences.html

Well here is the truck photos from when I first looked at it. I do not know if this is considered a bad rust bucket or not. It probably isn't.

https://picasaweb.google.com/utasilverado/1955GMCTruck?authkey=Gv1sRgCOPc-ODm5sSK5wE&feat=email

The previous owner says the transmission worked a year ago. He decided to sell it when it quit shifting. I am not sure how much I trust that though. He said it was for sale since around November 2011. His buddy that was storing it for him says it is more like a year or longer.

I wanted a manual GMC, but I could not find one. So I have the broken automatic.

That is why I want to save all the parts. Someone could use them to repair their GMC automatic.

I was afraid of that.

Wow. That really shows the differences in the cab. I was not aware of that.

First, I agree with HL's opinions regarding changing this out.

Nonetheless, I think these are answers/clarifications to HL's valid concerns:

1. "The hole in the firewall is considerably larger on the HydraMatic-equipped trucks for extra bell-housing clearance The hole in the firewall is considerably larger on the HydraMatic-equipped trucks for extra bell-housing clearance, . . . "
-- The hole in the floor/toe-board is bigger because the floorboard hole and cover is bigger. The "geometry" of the floor/toe-board is not different. A special floorboard cover (larger) is needed for the Hydra-Matic because the floor-opening is bigger and thus an accelerator pedal is mounted on the cover, not on the floor.

2. "there's a solenoid-type starter instead of the stomper"
-- Not really a problem. As long as you have the the "snout"/cover at the front of the starter, rebuilt starters are available (Chevs of the 40s). Also, the push-button starter-button for the solenoid starter can be placed anywhere on, or under, the dash.

3. "the crankshaft flange bolt pattern on the engine might possibly be different, (not sure about that, though)"
-- I think that there is no difference in this regard between Hydra-Matic and manual-transmission 235s. Nonetheless, the engine and transmission were "mated" at some time during assembly. I assume there might have been some fine-tuning (machining) of the mating-surface on the 235; but, people have successfully put replacement 235s (or non-standard 261s) up against a 1/2 ton Hydra-Matic transmission (engines that were not originally matched to the Hydra-Matic).

4. "it's going to be difficult to adapt a clutch pedal and linkage due to a different-shaped frame around the bellhousing area."
-- The regular post/pivots/pedals-assemblies for manual-transmission trucks bolt right into the same hole in the frame.

Greg_H is better versed in this - maybe he will post his comments/clarifications/corrections?

If you sell your Hydra-Matic parts, you should be sure to include (if possible):
- engine
- Hydra-Matic transmission
- the open drive-shaft will be a different length compared to the drive shaft on a 55-1st 1/2 ton manual-transmission truck
- all the shift linkages and the steering-column shifter mechanisms
- carburetor accelerator linkage and rods from carb to transmission
- a automatic car-carburetor will be needed (it also has an intake-manifold stove-heated automatic-choke)
- frame crossmember at the rear of the engine (it is a bolt-on)
- frame crossmember at the rear of the transmission (it is a bolt-on)

OK, thanks for clearing up a few things. However- - - -this one's a GMC, not a Chevy, and I assume his "284" is actually a 248 engine. Didn't some of them have a different number of flywheel flange bolts? I don't know if the HM and the stick shift models got involved in that or not. If this one's going to end up as a resto-rod, I hope enough attention is given to preserving as many of the unique hydro parts as possible. They certainly aren't available from any of the Chinese repop vendors!
Jerry

Well I would want to keep the engine.

I have a manual choke knob in the dash. Is there a second automatic choke under the hood?

Correct. I can't seem to type that number correctly. This is not the first time I did that.
That is the plan. I want to be completely stock with few changes. Tremec 5 speed. Power brakes. AC. Maybe power steering. Maybe alternator. Maybe independent front suspension. Everything cosmetic would be stock.
That is why I am asking for help.

Pull the sheet metal off and plant it on an S-10 or some similar modern chassis and running gear, then sell the rolling chassis as a complete unit. You'll have a modern engine, independent suspension and all the creature comforts, and somebody can take your leftovers and build a pretty decent original-appearing truck by dropping another cab and bed onto an unmodified original frame. You'll probably come out money ahead also.
Jerry

The shop in California quoted me 3 thousand plus shipping to rebuild the automatic. That is about what the truck cost me. Wow.

all of this talk makes me think of keeping the hydra-matic. it would be so much easier if there was a shop in texas that could rebuild it. does anybody know of one?

The brake pedal goes through the same hole of the cab regardless of manual or hydra-matic transmission. I honestly think that the clutch pedal is just removed from the pivot shaft that both pedals are mounted on and spaced and spring pressured as needed.

My suggestion would be:
If you are intent on going to a manual trans or a modern automatic, and being that you wanted to put a new floor pan in due to rust issues, I would sell the hydra-matic, linkage, and related parts, I would sell the H-M frame and or trade for an easier to find manual frame, I would sell the trans cover, and hope to offset the cost of the swap part of the project. Another angle to consider would be to look up user "yellosub", He has a business where they build a frame and brackets to put the A-D body on an S-10 frame. He may be interested in your parts in trade in part or whatever for his product line. He gets your H-M stuff, and you end up with the modern driveline and suspension that makes you happy...Just a thought. Check out his site and decide for yourself. http://www.ad-engineering.com/

Hi Joseph. Where you at in Dallas?

What is the difference in the bell housings from the Chevrolet and GMC? Could he use a 3 speed and add the shift mechanism? Could he change out the bell housing, transmission, and shift mechanism from his to a GMC to a Chevrolet? I have all he needs if that is an option.

Could the hydra-matic be used in a Chevrolet?

Thanks to all

Joseph. Now you are talking. Keep the automatic. You will have something unique and of more value than the manual. And it will be so much easier. Even at $3,000 it may end up being cheaper or close to the cost of changing everything out.

Another thought is to take it to a local tranny shop and have them take a look at it. Change the fluids ETC. Maybe there is not much wrong with it.

Kurt

Before you go to the extreme of getting an expensive overhaul, consider doing preventive maintenance and adjustments on it. There are two band adjustments, and throttle pressure linkage that connects a lever on the transmission side cover to the carburetor. Adjustment of all these items is critical to good shifting. The HydraMatic is a 4-speed unit with a fluid coupling, not a torque converter. The 1-2 shift will happen almost as soon as the vehicle starts to move, usually at less than 10 MPH. 2-3 and 3-4 shifts will happen according to throttle opening and road speed, and the throttle linkage adjustment is very critical. Shifts will be firm, and there should not be any RPM flare-up between shifts. Maintenance and repair manuals are usualy available on Ebay for reasonable prices, and it's good to have one. These transmissions were used in Oldsmobile, Cadillac, Pontiac, Chevy and GMC trucks, Nash, Lincoln, and certain models of Rolls-Royce. They were also used in WW II tanks, and US military 2 1/2 ton trucks. Once it's adjusted correctly, I think you'll like how it drives.
Jerry

The '54 Chevrolet Truck Manual is also available on Keith Hardys site for free.

Ok. If I keep this transmission, I will need a shift lever for the column. It is a pair of vice grips right now. Where can I find a replacement shift lever?

Mario at Collin County Truck Parts, McKinney, Texas, has one from his 57 Cameo that has just about zero miles on it but came from behind a 283 Chevy. Don't know if it's the same trans as the GMC but I'm sure that if it's for sale at all he'd sell for half the overhaul price and guarantee it. He didn't care for it's quick hard shifts and replaced with a 700R4. I know the later Buick/Olds/Pontiac/Cadillac had a different bellhousing pattern. Maybe the bellhousings can be switched from one gear box to the other.

The transmission bolts onto different bellhousings with the same bolt pattern, so just about any Hydramatic can be swapped by installing the correct bellhousing for the engine it's used with. That changed for cars in 1956 with the Controlled Coupling Hydramatic, but they were never used in trucks.
Jerry

Ok. I think you folks will be happy. I am going to keep the factory transmission for now. You have convinced me. I reserve the right to change my mind later, but I am going to leave it alone for a while. I will track down a service manual for it and see if I can get it rolling without a a rebuild. I cannot afford 3 grand right now.

Now I need some help locating a part. My shift lever looks like poop because it is a pair of pliers. I have looked at multiple places (LMC, Brothers, etc) and cannot find a replacement shift lever. I can find the knob, but not the lever.

As far as the shift lever put a add in parts wanted.

I just started on a 53 3/4 ton GMC and after pulling the cab off the frame and then stripping it down I started seeing things that were not the same as my 52-3100 Chevy. At frist I just thought it was because it was a GMC.I started looking at the tow board and the trans cover and found out that at some time the truck was "converted" to a manual 4 speed trans but had been shipped from the factory with a auto trans.I'll have to make a new trans cover and mate a 4 speed cover with what is left of the front of the auto trans cover and "fix" the cover that the pedals come thru ,somone just drilled a hole in it for the clutch arm.I wish that the hydro trans and all of the parts were easyer to find but for now I'm going to keep it the way it is maybe in the future I'll come across somone with all the parts to bring it back to factory.


I would take the trans to a local trans shop if you have to get it fixed,You'll save a ton on shipping and once you put it back in if there is a problem you can take it there and let them look at it.


Pete

Joseph, Here's another direction for you, why not go with a modern automatic. This way you don't need to find all that clutch related hardware, you can get your overdrive trans and sell all that rare stuff to another guy that needs it. You can have a floor shift like Lokar etc, works great and looks more like stock.
I'd contact Buffalo Enterprises (360 652-7684)re. Makes the GMC to automatic adaptor set up you'd need plus this guy has a zillion tricks up his sleave for Chev/GMC stuff. ALso Langdon's Stovebolt has an adaptor.
Best of luck
Jim

How much of the shift lever do you need? If you find a similar year car with powerglide, you will have the shifter but the indicator and the detent plate will have wrong markings and detent positions.