TH400 behind a 350 OEM, when and why?

[leftybass209]

I've been doing some research about the automatics in these trucks. From what I understand, M49 designates BOTH the Th400 and Th350, and there is nothing specific on the SPID that identifies which transmission was installed.

Short of finding a build sheet, were there any options that could be ordered that support putting a Th400 behind a 350. For instance, a camper package, or optional rear gears?

I ask because i'm going to look at a truck tomorrow that says it has a Th400 behind a 350, and is claimed to be stock; totally possible I understand. I'm interested in WHY this would have been ordered that way, and in what circumstances, if any, it may have been suggested by Chevrolet as a better fit.

[73737373]

Lefty,

Good idea. I'm going to follow this one.

[dell72]

I would think the T-400 would be appropriate in an HD towing application where you be more concerned with transmission withstanding extreme heat and towing weight versus a traditional horsepower rating.

I have no actual facts to support my statement, purely speculation. I do recall a former co-worker owning a 69 GMC 2500 that featured a 350/T-400 combo that was claimed to be OEM.

[davepl]

My Dad's '68 910 (heavy half) was a stock 307 TH400 truck, even worse. And he had it from new, so it was original, but Oshawa-built.

As to the "Why it was ordered that way":

Technically, be bought it from his sister-in-law in 1970, and so she'd had it 18 months, but she wasn't doing any engine swaps (she was a nurse). She ordered that configuration in order to tow horse trailers (steeple chase? whatever the jumping over stuff is). So it was built for trailering with all of the related heavy duty options, trailer mirrors, and so on, but a small motor.

A complete base-level truck, steel door panels, no radio, etc, but with side moulding on the options list. As a kid I remember reading the SPID and it listing the spring packages and rear end, etc, but couldn't tell you today what they were, other than it was a coil spring truck with a half-leaf somewhere.

[tdangle]

Interesting question. My 70 C20 is a 350/350 with dana 60 full floating rear and 4.10 gears. Folks bought it new to haul a camper and pull a horse trailer. I thought only BB trucks or 1 tons got a TH400 trans.

[FirstOwner69]

My 1969 3/4 ton CST with everything needed for the Custom Camper nameplate (but not ordered with the nameplate option) was born in Dec. 1968 with the 350/TH400/Eaton 4.10 combo. It's also not a high hump floor. I am 100% sure it's original.

Yes, both transmissions were option code M49 with no notation on the SPID regarding which transmission.

[TBONE1964]

At DLTruck we just parted a 69 that was a C10 350/TH400 with coil springs in the rear. I don't have the spid but it also has a low hump floor.

It was surprise to see a 400 in it as most in this configuration would have TH350 or a manual trans.

[FirstOwner69]

Originally Posted by TBONE1964

At DLTruck we just parted a 69 that was a C10 350/TH400 with coil springs in the rear. I don't have the spid but it also has a low hump floor.

It was surprise to see a 400 in it as most in this configuration would have TH350 or a manual trans.

Was it a 1/2 ton or 3/4 ton?

In addition to what I posted above, mine also has rear coils plus the aux leaf spring.

[leftybass209]

Very interesting.

So far, it seems there is no rhyme or reason to having a Th400 transmission installed based on other optional equipment, other than by a customer sitting down and checking the box on the order form, most likely for towing or hauling heavy loads.

Loving the info so far. I wonder if Jocko, Special-K, or SS Tim have anything to add???

[Bigdav160]

We know for sure no TH400 in a 4x4
Big Blocks w/M49 would be TH400

I would have thought all automatic C20's would be TH400

The sheets I've download off of the GM heritage site it looks like the TH350 might not have been officially offered until the 1970 model year. I think the TH350 started in the Camaro and once units were available it went into other GM products

[fp1967]

In 1986, I bought a 70 C20. It had a small block with a Turbo 400 and a low hump floor. The engine wasn't original and I don't know if the tranny was or not. I've often wondered how much clearance it had with the low hump floor. If I'm not mistaken, I think the big block motor mounts move the engine back a little bit, which might explain the need for the high hump to clear the tranny. I've had two other C20s of this era with small blocks and Turbo 350s.

[fp1967]

The 70 C20 that I mentioned also had coil springs with the half leaf overloads. 4.10 gears also.

[chanclas90063]

My 1970 C-10 came with a 350/th400, but this is not surprising since nothing on this truck is original.

[richard2717]

The 72 C30 longhorn I just got is a 350/ 400 with low hump

[kwmech]

Originally Posted by richard2717

The 72 C30 longhorn I just got is a 350/ 400 with low hump

MY 68 C30 longhorn has/had a 350/400 small hump also. I converted to a 454 and still have the low hump. Plenty of room, can get my hand all around it to put in the bell housing bolts

[cooperhw]

My 68 C30 "Cab and Chassis" came from the factory with a 327 and turbo 400. Stranger yet, it came with no Park. None. No marking on the column, or park pawl in the transmission (had to have one installed). It went to Open Road where they proceeded to extend the wheelbase from 157" to 175" before they put 2 tons of motorhome on it. So the 400 was definitely needed.

[RedRoad]

My brothers one owner 69 C/20 Custom LWB has the 350/th400 combo. Has the coils and Eaton rear with 4.10s, no overloads or any special edition that I could tell.

I thought the th350 became more common on 70 and later C/20s and the th400 continued service in C/30

[jocko]

Both my 71 and 72 3/4 tons with HD springs, etc + custom campers were TH350 trucks (both small blocks).

[Vintage Windmills]

I have never seen a TH350 in a C20 or C30 from 68 or older. Then again, the OP asked about behind 350's which weren't available in 68 or older trucks. The option of TH350 must have been 69 phase in.

[factorystock]

Most sales literature list power glide or turbo hydramatic. There is no distinction between the 2 turbo hydramatics used between '69-'72. Very odd GM didn't list a light duty 3 speed auto and a heavy duty 3 speed auto as separate options. Most SPIDS list both the TH350 and TH 400 as M 49, but some will list the TH350 as M 38. Was it the salesman or factory who put in the most appropriate 3 speed auto for the engine and GVW? Yes, the TH400 was typical behind a big block but the TH400 was also installed in small block and 6 cylinder trucks, but in a much lower percentage than the TH350's. Remember the 292 six Ryder with the TH 400?

[special-K]

I'm pretty sure all C/30s got TH400.

[hugger6933]

We had this discussion a couple weeks ago when I asked if all 20's and 30's had the TH400. I found that 3/4's before 70 did but in the 3/4 ton in and after 70 it could go either way but pretty much all the 30's were 400 equipped from what i could tell. I know in the square body style in the later ones for true the 3/4's got TH400's, cause I have traded for a bunch of them over the years[and built a TON of 3/4 ton trucks] in the farming country and they all indeed had 400's.

CooperHW, did your truck have the bolt on yoke ,or maybe even the big drum on the back for parking?

[hugger6933]

Oh and all 30's I heard of had the 400's if I didn't state that. I was lookinging int oa 4 speed to 400 swap for my C30.

[NGN]

I know we're talking trucks and I was sure 67 trucks with 327 4 barrels came with only t400 if you wanted a automatic. Funny thing , I know my parents 67 Chevy Caprice had a 327 4 barrel Quadrojet with a turbo 400 automatic from the factory. When did the 350 automatic start?

[TBONE1964]

1969 was the first year for the TH350 so it is very possible that if you had an early production 69 pick up that if it was ordered with a automatic it would have been a TH400.