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Discussion Starter · #1 ·
Ive got a few questions on a turbo 400 i just bought today without looking closely like an idiot. Let me start from the begining. A buddy of mine is hard up for money and told me he would sell me his th400 for $100 and i know its good i drove the RV before i gave him the money and pulled it but theres one problem.....I didnt realize it being a "heavy duty" th400 it has a yoke that bolts to the tranny and the slip yoke on the shaft itself. Is this tranny even practical for my 86 GMC 2wd street truck or did i waste my money. Ive been through 2 th350 already and one was built grant it i have a heavy foot and spend alot of its on the road time seeing full throtle and street racing at night. I understand the 400 takes more power to get goin but is it that much power loss and is it worth it for the extra durability? Will it bolt right in or am i gonna have to move the crossmember or anything like that? Is there anything out there i might find in a junk yard that will have the shaft i need to fit in a short bed or will that have to be made? And last im not positive but im prety sure my 3000 stall out of my 350 will work in the 400....i hope. Sorry to load up so many questions but just wondering if i just wasted my money or not. Thanks.
Matt
 

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Ya did good actually. Toss the bolt on yoke for a passenger car slip yoke; done. This motor home TH400/425 may have straight cut planetary gears, so expect it to be "noisier", that's one of the features that makes it a "HD". You will be happy with this swap. I just did the same thing after 2 TH350 trans over the years. This TH400 is great; good, solid shifts, without any lag between them. I should have done it 2 TH350 trans ago.
 

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Discussion Starter · #3 ·
Its ok to slide a slip yoke into the back of this transmission.....wasnt sure with it already having a bolt on one but i will definatly go that route i hate a 2 piece driveshaft. Thanks.
Matt
 

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Discussion Starter · #4 ·
Not to sound like a total idiot but will the slip yoke shaft from my 350 work on it.....i would just try it but with work i dont have the luxury of goin and working on my truck at a buddys shop where its sitting so im trying to have all my ducks in a row and know exactly what i need to do and what to expect in this swap like movin the crossmember, what driveshaft will be a good one to get to go in the tranny and i understand ill prolly have to have it cut. My buddy has the same truck just about but with a regular turbo 400 would his driveshaft work on this transmission? Thanks for all the input.
Matt
 

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Short_box said:
Not to sound like a total idiot but will the slip yoke shaft from my 350 work on it.....i would just try it but with work i dont have the luxury of goin and working on my truck at a buddys shop where its sitting so im trying to have all my ducks in a row and know exactly what i need to do and what to expect in this swap like movin the crossmember, what driveshaft will be a good one to get to go in the tranny and i understand ill prolly have to have it cut. My buddy has the same truck just about but with a regular turbo 400 would his driveshaft work on this transmission? Thanks for all the input.
Matt
Your 350 yoke will not fit the 400.
 

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Short_box said:
Not to sound like a total idiot but will the slip yoke shaft from my 350 work on it.....i would just try it but with work i dont have the luxury of goin and working on my truck at a buddys shop where its sitting so im trying to have all my ducks in a row and know exactly what i need to do and what to expect in this swap like movin the crossmember, what driveshaft will be a good one to get to go in the tranny and i understand ill prolly have to have it cut. My buddy has the same truck just about but with a regular turbo 400 would his driveshaft work on this transmission? Thanks for all the input.
Matt

If its the same truck, his shaft will fit.
Its probably a 2 pce though.

You will also need the kickdown switch at the gas pedal,
move the crossmember, and not sure about the torque converter from the 350 fitting.
 

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Discussion Starter · #8 ·
I called around today to 2 different local transmission shops and both of them told me that a th400 is a th400 wether its considered HD or not they both use the same output shaft and that the driveshaft from a short bed truck that came factory with a th400 would slide onto the output shaft of the th400HD that i have but it dont. We unbolted the yoke of the back of my trans and went to put his driveshaft on and my outputshaft was a biger diameter than his. Whats the deal with that? Id hate to run a bolt on yoke then a 2 piece driveshaft ive seen them come apart alot dealing with 4x4 and this motor i have now has alot more torque than any of my previous 12 4x4 i have built. I dont know if i have another odd-ball tranny or the shops didnt know what i had goin on. Any more help would be much appreciated im getting frustrated. :smash:
Matt
 
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