73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: Mr. Machanic on January 04, 2024, 09:57:30 PM
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I'm guessing this might be a silly question for many, but forgive the newbie...lol
Can a 400 auto be a direct bolt up replacement for the factory 350 3spd auto tranny?
This is in a 78 Big10 with a 350 motor.
Will I need a different d-shaft? Mine is a 1 piece shaft.
Thank you.
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TH350 uses a kick down cable operated by the carb. TH400 uses an electric switch on the accelerator pedal inside the cab. Only speaking of the TH400's I've had been in C20's. All of them had 2-piece drive shaft and a fixed yoke on the transmission. There are slip yoke TH400's. I also know when the TH350C was discontinued the TH400 was the standard automatic transmissions even behind a 4.3L V-6 in the 85 and newer. I have never paid attention if they had slip yokes or not. The attached jpg gives you some measurements of lengths to compare along with cross member mount locations
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Well, by the diagram/sketch, the 400 will not just be a swap out.
Thank you, sir for the sketch.
Those guys on Roadkill surd make it all look so easy...maybe I need to be wearing flip flops and shorts...
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Why are you thinking of a swap?
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The frame is probably already drilled for the different mounting of the TH400, if it doesn't you can just measure and drill new holes the distance differnce back.
It will definitely require a different driveshaft and output (I recommend a slip yoke), and you will definitely want to hook up a kickdown switch.
This goes over several options https://www.onallcylinders.com/2014/12/19/ask-away-hook-turbo-400-kickdown-hint-like-3-speeds/
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Thanks Chris.
Great info and article.
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I did it on my long bed C10 when the original 350 would only drive in reverse! Was fairly painless but I have a 2 piece prop and the 400 had a bolt up yoke which didn't match the drive shaft but this was solved with a 2 size U/J, can't remember the sizes but easy enough to source, the holes were there for the cross member to be moved, did the swap in a day. I used a motorcycle brake light switch on the throttle linkage for the kickdown switch works quite well. I still need to rebuild the TH350 but other projects have taken priority.
Graham
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Thanks Graham,
Nice description. I can relate to the mororcycle brake ligjt switch. I might even still have one...lol
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Living in the UK you have to think outside the box sometimes to get over lifes little hurdles!
Graham