Which one to pick, TH350 or TH400 ?

mtnmankev

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The TH350 in my 83 C-20 was rebuilt by somebody at an unknown time before I bought the truck several years ago.
It still shifts good, but has always leaked ATF (VERY annoying to say the least) plus I ended up installing six helicoils in stripped bolt holes.
Short story, I don't trust it completely and I may soon be towing a 5th wheel camper if I can sell my home.
I have two TH400's I got free (condition unknown), and they have been out in the weather a while.
There's a chance I MIGHT be able to build one good 400 and not have to buy too many parts.
I have heard the TH350 is simpler and has less to go wrong, but on the other hand, the 400 is stronger.
I was told that the TH400 has more parts internally and I will lose some horsepower due to that, but don't know if it's a point of contention.
If the TH350 is the one to stick with, I will just scrap out the 400's and haul the aluminum in along with my bags of crushed Pepsi cans.
 

Frankenchevy

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If they’re both going to end up as stock level builds, the th400 will be stronger and probably last longer. Will you need a new driveshaft (or yoke installed) and torque converter?
 

78C10BigTen

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If it comes to scrapping either trans id sell wichever you dont use. I say this because i had one HELL of a time finding a th350 as everybody scrapped them in recently passed years. Help another person out later by not scrapping it.
 

mtnmankev

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I think the torque converter should bolt right to the 5.7 flex plate.
And I have a friend who owns a driveshaft shop, so that is not an issue if anything needs done in that department.
 

mtnmankev

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If it comes to scrapping either trans id sell wichever you dont use. I say this because i had one HELL of a time finding a th350 as everybody scrapped them in recently passed years. Help another person out later by not scrapping it.
I can appreciate that idea, but I have two problems: I live so remote and off grid, NOBODY will drive here for anything, and I can't post on Craigslist anymore, something went awry in my account and now all my postings get flagged and removed.
 

78C10BigTen

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I can appreciate that idea, but I have two problems: I live so remote and off grid, NOBODY will drive here for anything, and I can't post on Craigslist anymore, something went awry in my account and now all my postings get flagged and removed.
That sucks. I ccw and go crazy places so id come to you but im not everybody lol
 

75gmck25

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My ‘75 K25 Camper Special has a TH350 (factory in ‘75) and it’s survived a lot of abuse over the years. If you were building a high HP engine the TH400 might be worth it, but it doesn’t sound like you need it.

You also need to figure out the leak issues on your TH350 because if you use one of the TH400’s you might have similar problems if it’s a transmission cooler line or other external leak.
 

bucket

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What size and weight of 5th wheel camper are you considering?
 

mtnmankev

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What size and weight of 5th wheel camper are you considering?
I don't know, it will depend on what is available IF and when the house sells and I have some money to work with.
So much still up in the air right now.
I hope whatever I do get can safely be towed with the hitch bolted to the bed.
Drilling the frame and getting the right length spacers to fit between the frame and the bed underside will probably be a headache.
 

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I don't know, it will depend on what is available IF and when the house sells and I have some money to work with.
So much still up in the air right now.
I hope whatever I do get can safely be towed with the hitch bolted to the bed.
Drilling the frame and getting the right length spacers to fit between the frame and the bed underside will probably be a headache.
Negative. Gotta be bolted to the frame
 

mtnmankev

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My biggest worry is the current transmission not holding up to the pull.
Especially if I end up travelling with the weather to stay cool in the summer and warm in the winter.
Got some long, steep highway hills to deal with.
 

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Pretty much any 5ver is a lot of work for an old truck and even more so for one that is, so to speak, not up to original factory standards and reliability. (You know what I mean, not dissing your truck)
Once you make this happen, regardless of size of camper, you’re going to have to severely limit what you take with you anyway. Have you considered something a bit smaller and lighter? Not a 5ver will save the time and expense of installing a 5ver hitch. And shorter/lighter pulls easier.
 

Mark Ugrich

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The TH350 in my 83 C-20 was rebuilt by somebody at an unknown time before I bought the truck several years ago.
It still shifts good, but has always leaked ATF (VERY annoying to say the least) plus I ended up installing six helicoils in stripped bolt holes.
Short story, I don't trust it completely and I may soon be towing a 5th wheel camper if I can sell my home.
I have two TH400's I got free (condition unknown), and they have been out in the weather a while.
There's a chance I MIGHT be able to build one good 400 and not have to buy too many parts.
I have heard the TH350 is simpler and has less to go wrong, but on the other hand, the 400 is stronger.
I was told that the TH400 has more parts internally and I will lose some horsepower due to that, but don't know if it's a point of contention.
If the TH350 is the one to stick with, I will just scrap out the 400's and haul the aluminum in along with my bags of crushed Pepsi cans.
If you’re going to pull a 5th wheel trailer, update the driveline to use the 400 THM. As a former GM tech I’ve rebuilt both , and the 400 is far stronger.Make sure you install a trans cooler no matter which unit you use. Heat is what kills a transmission.A 400 is actually a little easier to rebuild at home.Just take your time and make sure all the parts inside are clean.If you have a transmission shop nearby , have them wash the empty case for you in their wash cabinet.Buy a book on 400’s and follow it.
 

JoeR Jr

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While the TH400 has some more mass to it, there's no real HP losses with running one. Plenty of dyno numbers on youtube to verify if you need feel like watching it. The argument comes up over the TH700 and the 4L80 when guys are LS swapping.
If you're towing, I like the TH400. It's not like the TH350 can't be tough. My buddy had an old Nova with one behind a 502. That car would pull a front wheel up when it launched, and that trans lived forever. However, the TH400 is beefier and will live longer when pulling. As Mark suggests, get a cooler and a temp gauge for whichever trans you go with.

Lastly, some friendly advice. If you're gonna tow a 5th wheel, use another truck. New ones are so much more stout than these old girls. If for no other reason that the better brakes on one, go find yourself a nice 3/4 ton silverado and tow safely.

Joe
 

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