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Old 09-28-2017, 06:34 PM   #1
CampaS75
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TH400 rebuild ?

I have a '75 GMC motorhome that had the transmission fail badly so removed it. Bought a rebuild th400 that came from an '87 Chev truck not realizing it was 27 spline and my original is 32 spline. I didn't take the rebuilder's tape off until I removed old from motorhome.
So, is it possible to swap old 32 shaft to the rebuild th400 ?
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Old 09-28-2017, 08:09 PM   #2
mongocanfly
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Re: TH400 rebuild ?

clinebarger can answer that but I'd assume the trans would have to be disassembled completely to do so...if its possible...wouldn't it be easier to just change the yoke?
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Old 09-28-2017, 08:20 PM   #3
CampaS75
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Re: TH400 rebuild ?

It's the tougher 32 spline shaft I'm trying to keep because it's pushing 10000 GVW and don't want to risk it snapping on a smaller 27 spline. I realize both transmissions would have to be disassembled to do so but I don't know if they're simply swappable once all apart.
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Old 09-28-2017, 10:51 PM   #4
Warrens69GMC
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Re: TH400 rebuild ?

You want to also see if it needs the lockup wired in to work.
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Old 09-28-2017, 11:15 PM   #5
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Re: TH400 rebuild ?

The TH400 trans from a large motorhome such as that should actually be a TH475. Does it have the drum parking brake mounted to the back of the trans?
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Old 09-29-2017, 12:19 PM   #6
DieselSJ
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Re: TH400 rebuild ?

Quote:
Originally Posted by Warrens69GMC View Post
You want to also see if it needs the lockup wired in to work.
TH400 lockup?


I wasn't aware of such a thing.
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Old 09-29-2017, 02:11 PM   #7
kwmech
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Re: TH400 rebuild ?

27 spline 400 in an 87? not likely. Sure it's a 400 and not a 700? Anyways, both will have to come completely apart to swap outputs. The output is held into the rear planetary drum with a snap ring.
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Old 09-29-2017, 02:12 PM   #8
kwmech
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Re: TH400 rebuild ?

Quote:
Originally Posted by DieselSJ View Post
TH400 lockup?


I wasn't aware of such a thing.
There isn't for a 400. if it was it's called a 4L80
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Old 09-29-2017, 06:42 PM   #9
clinebarger
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Re: TH400 rebuild ?

Sounds like a TH375 out of a 1/2 ton truck.....

There will be fewer frictions compared to a TH400/475.
You will need to compare Boost Valve sizes.
The Governor will be wrong for the application.

The 27 spline output shaft just might hold up.....But the drive shaft & slip yoke changes probably negate any savings here.

The Unit has to be tore all the way down to change the output, At the same time you need to add a friction to each clutch pack using the Forward & Direct Pistons along with the Intermediate Pressure Plate out of your old TH475.

There are some more upgrades that I would recommend if you decide to open it up.
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Old 09-29-2017, 06:57 PM   #10
CampaS75
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Re: TH400 rebuild ?

Well I just got back from looking at several transmissions and none match mine without adjusting yoke n driveshaft.

So I think now i might use the rebuild and get yoke n driveshaft altered.

Didn't know it'd be this tough to find a match !
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Old 10-09-2017, 08:27 AM   #11
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Re: TH400 rebuild ?

The best thing to do is rebuild the trans that came out of the MH.... replacing the original with a lesser TH375 could present problems down the road. 10K GVW on a TH375, how long you think it will last ? If you like R&Ring the transmission on the floor on your MH use the TH375 but rebuilding the original transmission in my opinion would be a better way to go.... sell the TH375, and use the cash to rebuild the original.
What should the lesson be learned here ? Be aware of what you are buying before you toss the cash down....its not a daily driver so time is not an issue...rebuild the original to make sure transmission is designed for the application....there are always upgrades for every application but never replace the original with a lesser application....GOOD LUCK !!!!!!
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