Gunfreak25

I have a 1954 GMC 2 1/2 Ton truck that was repowered with a 455 Olds and Turbo 400 out of a 1970 Delta 88. AS FAR AS I KNOW, the tranny has never been rebuilt and is stock. It is marked OB on the case if that means anything. The truck weighs 12,000 empty but on the odd occasion I decide to do some heavy hauling it may get up to 22,000. But to be honest I don't see myself loading it full of anything anytime in the near future. I know the Th400 is a strong transmission and I have seen other guys put them in deuce and a half's with BBC. The biggest issue is no OD in this big truck, but I don't plan to drive faster than 45 or 50 in it. I do have tranny cooler lines running through the radiator, which by the way is a 1967 Harrison down flow, "small" type. I sure would appreciate everyone's $.02 on this.

Chesrown 67 OAI

Back in 1970 we owned a 56 GMC 2-1/2 Ton(22.5"wheels) car hauler ramp truck that had an original 316" Pontiac motor with a Hydra-matic and a 2-speed rearend and we re-powered it with a 1970 (new at the time) 455 w/TH400 engine with long tail shaft (ala Delta 88). We even hooked up the trans linkage up to the factory Hydramatic shifter on the steering column! We 'cruised" on the highway at 70-80 MPH easily with one car on the ramptruck and another car on a trailer behind with nothing other than a HD radiator and an auxilary trans cooler. We were back and forth across the country several times with it and it never had a minutes trouble. So I say -- Go ahead with your project - you will be quite happy. P.S. - We set the 2-speed rearend on the hi setting and never changed it!! Good luck --- will toss in a pic of ours too.

TripDeuces

The T400 has been used behind many a big truck or heavy vehicle. It will be fine behind yours but I would recommend an additional trans cooler, the bigger the better. Heat is the number one killer of transmissions. Keep it at 160 degrees and it will last forever.

Automatic transmission oil can provide up to 100,000 miles of service before oxidation requires replacement, at an operating temperature of 175° F. Above this temperature, the oil oxidation rate doubles with each 20° degrees F increase in oil temperature.
Transmission oil and filter should always be changed more frequently when driving with heavy loads (RVs), typically at least every 25,000 mile intervals, since converter outlet temperatures then normally far exceed the ideal 175° F.

joe_padavano

X2. The GM medium-duty trucks used the TH400 until the 4L80E came along. The only alternative would be an Allison as used in the heavier trucks.

panos

I personaly think that it also depends on gearing,a high numerical gear allows the engine\transmission to "push" the vehicle easier and with less stress on the components.

joe_padavano

Yes, you are correct. Steeper final drive gears will lessen the load on the transmission, but will limit top speed. The original engine it this truck was some version of the GMC inline six, so it likely has steep gears already.

Gunfreak25

I really appreciate the input guys, as you surely know more about this kind of repower stuff than I do. To be a little more specific the original motor for it was GM's 302 I6 with the original 4 speed Hydramatic which had the low range on the rear of it (low was 15:1). Obviously with the tranny swap I no longer have low range but i'm okay with that. The transfer case is 1:1 and the axles are 6.17:1 riding on 9.00:20 tires (39 inches).

I was told that I need to swap torque converters and get a stall speed converter with a super low stall speed? Any input on this? Were there any internal differences between the tranny's that GM put big trucks compared to the cars? I'll need the tranny cooler for sure, and I am not sure if I need a kickdown switch or not.

Thanks again for the help guys, here's a couple pics of it by the way. A few weeks old, it's now painted and I have gotten a lot more done under the hood.

TripDeuces

The kickdown switch on a T400 is electrically operated off a switch on the throttle. Do you need one? I doubt it. If need be you can down shift it manually.

matt69olds

The TH400 trans that was installed in heavy duty trucks had straight cut gears and different valve body calibration. I have seen a couple of them. I believe they are refereed to as TH475. I think you will be fine, install the biggest trans cooler you can find. A Trans-go shift kit installed with the tow calibration would be a big help also. The Trans-go shift kit is the only shift kit that will "dual feed" the direct clutches. It can be easily done internally, but requires trans removal and disassembly.