can anyone tell me what years the 400 was made with extra low 1st and second gears? or would it be easier to get a rebuild with custom gears?

 

i have never heard of a th400 with "extra low" gears. i could be mistaken but i thought they were the same. are you sure that it is not an aftermarket modification? the heavy duty ones have straight cut teeth on the planetaries but that is about it.

 

ive been told that there are certain years and certain makes of cars that 400's came with a lower gear ratio. havent confirmed it, just wanted to see if anyone knew of the years, or if its even true.

 

i have been in tranny parts for over 8 years at this point. while i am not saying i'm an expert, i have seen many factory th400's. some of the early ones had a variable pitch pump. some had 350 output splines. some differences in sprag assemblies. but i have never seen anything to indicate a difference in gear ratio. the aftermarket can do amazing things with trannys if you know where to look. i haven't seen any differences.

 

I also have heard about the th350 & 400s coming from factory with different ratios. Never have had any proof though.

we need to call in the mythbusters! lol

 

http://www.amazon.com/exec/obidos/tg...36622?v=glance

This is your bible if you want to do anything with a TH400.

There were many factory modifications and changes to all the TH400's and their variations, TH475 etc, but they all use the same 3 gear ratios.

1 = 2.48
2 = 1.48
3 = 1

Like just about any tranny, there's probably a different gear set available in the aftermarket. Trying for a lower first gear will make the spread between first and second too much and it will lose much of it's WOT performance like a 700R4. You don't need a deep first gear unless you have a huge amount of weight that needs to be moved from a dead stop.

The TH475 has the much sought after straight cut gears. Good luck finding one.

 

There were a VERY FEW 400s with a HIGHER 1st gear; none with a lower first gear than the 2.48. Nobody would want one like that anyway, the ratios are already borderline too wide for performance use. It would make the trans behave like a 6-cyl T-5, where the motor goes from "redline" to "lug" in one easy jump, at every shift. Basically, if you think you need that, you have some other problem with how your car is set up, and a deeper 1st gear isn't the right answer.

The only vehicles I know of that came with the other 1st gear planetaries, which were 2.21, were those Pepsi delivery trucks from the mid-late 70. Not Coke, only Pepsi. Coke used a regular cube truck. Pepsi however used a heavy-duty pickup with a 5th wheel trailer arrangement. The transmisions in those had the higher 1st. The teeth were also straight-cut, rather than the usual helical cut; so they were a bit noisy. That might be what Stephen is calling a 475. I've never heard it called that myself.

From days when I had personal connections to TCI, back when they were in Memphis long before they moved to Ashland MS, I used to have one of those planetaries. I have no clue whether I still do or not though, I might have got rid of it in the 20 years or more that have passed since then. I kind of think I did, a few years ago when I was moving, and didn't want to haul literally tons of car junk across the country.

I haven't kept up with what's out there in the aftermarket, there could be almost anything and I wouldn't know. I'm only talking about stock ones.

There's no such thing as a 400 with 350 output splines. There is however a thing called a 375, which is a 400 with 350 output splines. It REALLY REALLY SUCKS when somebody sells you a 400, and gives you one of those instead. Guess how I know that.

 

yes it is called a 375. i didn't want to confuse the guy.

 

so can a th400 be geared lower? it sounds like you guys find it foolish but i think it would be more fun.

 

Sure a TH400 can be geared lower. You install a 4x4 transfer case and get a 2:1 gear reduction. Other than that, no. The planetary gear set offered for the TH400 is only available in the standard gear ratio. If an aftermarket gear set is available, you'll have to contact the transmission retailers to ask them.

I know I can swap out the factory 1.76 first gears out of my powerglide for some 1.80 straight cut gears. They're only $1200 for the set.

 

errrrrrrrrrrr...... sounds like i just need more POWER. not more gear (already have 4.10's)

 

I was fortunate enough to find a straight-cut gearset out of a TH475 trans I found late last year, and in excellent condition:

I never bothered to check the ratio, since I always believed that the TH400 style trannies only came in a 2.48 ratio. From what I understand, the 475 trannys were only put in certain GM "P" chassis trucks (step vans, motorhomes, school buses). I'm not sure what mine came out of because it was half buried in the dirt in the middle of a bone yard. I only paid $100 for it since it had a broken case - apparently they didn't know what it really was.

There are actually two different 375 trannys, a 375A and a 375B. The "A" is a heavy duty version of a TH350, while the "B" is a light duty TH400. Other than the tailshaft and housing, the only real difference between the 375B and the 400 is less clutches in the packs, which can be easily upgraded.

 

375A is the hard oen to find any info about

i always thought the B was the 400 inside the 350 case(350 tailshaft)


thm series



big casting digits(two or three of them) on the upper bellhousing would help identify one from the other,
as with all other trans models of GM

but i do not know of a list of them

ive got a th350C(normal 2wd truck 9" /700r4 length tail) from behind a diesel oldsmobile 5.7L engine from an 81 pickup that is DX cast bellhousing code, has the 10 bolt bellhousing bolt pattern(all GM division patterns)

i wonder if maybe its not one of the 375's possibly,


hmm...

 

Try this little unit...