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73-87chevytrucks.com
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73-87 Chevy _ GMC Trucks
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350 to 400 swap question
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Topic: 350 to 400 swap question (Read 882 times)
Mr. Machanic
Registered Users
Posts: 105
Newbie
350 to 400 swap question
«
on:
January 04, 2024, 09:57:30 PM »
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.
«
Last Edit: January 04, 2024, 10:00:11 PM by Mr. Machanic
»
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1978 Big 10 "Bigg'in"
350/350 auto.
Cordova Brown Scottsdale
Zero pollution krap.
Suck, Squeeze, Bang, Blow
zieg85
Global Moderator
Senior Member
Posts: 7545
Re: 350 to 400 swap question
«
Reply #1 on:
January 04, 2024, 11:04:38 PM »
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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Carl
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/
Mr. Machanic
Registered Users
Posts: 105
Newbie
Re: 350 to 400 swap question
«
Reply #2 on:
January 04, 2024, 11:27:26 PM »
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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1978 Big 10 "Bigg'in"
350/350 auto.
Cordova Brown Scottsdale
Zero pollution krap.
Suck, Squeeze, Bang, Blow
Mr Diesel
Frequent Member
Posts: 336
Aaron
Re: 350 to 400 swap question
«
Reply #3 on:
January 05, 2024, 06:34:28 AM »
Why are you thinking of a swap?
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1976 C20 Crewcab, 6.2L/SM465
1982 K30 Crewcab , 427TD/TH400
1983 C30, 6.2L/TH400
1983 K30 Crewcab 454/700R4
1986 K10 350/400. 1989 K30 cab/chassis 454/SM465
Captkaos
OWNER and Administrator
Administrator
Senior Member
Posts: 18382
Re: 350 to 400 swap question
«
Reply #4 on:
January 05, 2024, 07:23:15 AM »
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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Chris Lucas
73-87chevytrucks
captkaoscustoms
squarebody
Mr. Machanic
Registered Users
Posts: 105
Newbie
Re: 350 to 400 swap question
«
Reply #5 on:
January 05, 2024, 09:07:55 AM »
Thanks Chris.
Great info and article.
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1978 Big 10 "Bigg'in"
350/350 auto.
Cordova Brown Scottsdale
Zero pollution krap.
Suck, Squeeze, Bang, Blow
Chevygold
Registered Users
Posts: 219
Newbie
Re: 350 to 400 swap question
«
Reply #6 on:
January 05, 2024, 12:42:50 PM »
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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Mr. Machanic
Registered Users
Posts: 105
Newbie
Re: 350 to 400 swap question
«
Reply #7 on:
January 05, 2024, 01:40:17 PM »
Thanks Graham,
Nice description. I can relate to the mororcycle brake ligjt switch. I might even still have one...lol
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1978 Big 10 "Bigg'in"
350/350 auto.
Cordova Brown Scottsdale
Zero pollution krap.
Suck, Squeeze, Bang, Blow
Chevygold
Registered Users
Posts: 219
Newbie
Re: 350 to 400 swap question
«
Reply #8 on:
January 05, 2024, 02:03:43 PM »
Living in the UK you have to think outside the box sometimes to get over lifes little hurdles!
Graham
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350 to 400 swap question