switching put 3speed for th400 i need help - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Chugger · 01-07-2005, 08:09 AM

i got the aut tranny crossmember i just wanted to know what size bolts i needed to bolt it to the frame? Anyone know? Also i am using just a tranny cooler with a manual radiator with this be sufficient? Also my tranny has a 6in tailshaft will i have to modify the driveshaft? And just one final question Its a 71 what tooth flexplate will i need?
thanks
chris

mrein3 · 01-07-2005, 08:23 AM

Regarding the external tranny cooler only -
That depends on where you live. Up here in Minnesota you need to heat your tranny fluid in the winter. The only way to do that is to run it through the tank on the end of a radiator. If you live down south you MAY be ok with just the tranny cooler.

Tx Firefighter · 01-07-2005, 08:47 AM

You'll need a flexplate and starter both. The original starters on 3 speed trucks bolts to the bellhousing rather that the engine. You'll need a pair of bolts for the new starter too. They're much longer than the bolts from the original starter.

Remove the pilot bushing from the rear of the crankshaft.

Crossmember bolts are 3/8 diameter, about 1 inch long. You'll need 8 of them along with washers and nuts for each.

Th400 takes a different yoke than the 3 speed does. You'll need to buy one.

da-burb · 01-07-2005, 08:50 AM

You will need to change out the first drive shaft. They use a different yolk. A TH350(short tail) would have dropped right in with no changes. Flex plate is the 168 tooth. You will also need a different starter too.

Vince M · 01-07-2005, 10:51 AM

What about a shifter too. If you go with the stock auto steering column you'll have to change it out also along with adding the linkage to the tranny.

Chugger · 01-07-2005, 02:04 PM

i have a b&m z-gate and the th400 has the yoke on it. So i will need a starter and flexplate with misc bolts. what about the length of the driveshaft will it need to be modified? Also the yoke from the th400 will not work?
thanks
chris

Tx Firefighter · 01-07-2005, 02:07 PM

The yoke may be an issue.

There's 2 types of yokes on the turbo 400. There's the slip yoke and there's a bolt on yoke.

The way you said the trans had a yoke on it, it sounds like you may have a bolt on yoke. You'll likely need to remove it and convert to a slip yoke. The shop who modifys your driveshaft should be able to advise you what the best way to do it is.

I'm not giving you a straight answer because I don't know the length of the 3 speed you took out. There are a couple of different lengths of 3 speeds. You'd need to measure the old trans and the new trans and compare overall lengths to determine if the shaft will need modifying. The yoke will be different between a 3 speed and a turbo 400 for sure though.

Johnny05 · 01-07-2005, 02:20 PM

I just swapped out my 3spd manual for a TH350 and a couple of things that I didn't see mentioned is new lines metal/rubber and the flywheel bolts(the auto flywheel uses shorter bolts). Also you'll have to make a provision to run a vacuum line to the modulator. You will also be needing a transmission mount and bolts for it.
I used a flywheel and starter off a 75/76 Chevy and they worked fine. I made a list of all the parts I would need and went shopping!

01-07-2005, 08:09 AM	#1
Chugger Registered User Join Date: Nov 2004 Posts: 18	switching out 3speed for th400 i need help i got the aut tranny crossmember i just wanted to know what size bolts i needed to bolt it to the frame? Anyone know? Also i am using just a tranny cooler with a manual radiator with this be sufficient? Also my tranny has a 6in tailshaft will i have to modify the driveshaft? And just one final question Its a 71 what tooth flexplate will i need? thanks chris Last edited by Chugger; 01-07-2005 at 08:38 AM.

01-07-2005, 08:23 AM	#2
mrein3 Registered User Join Date: Feb 2000 Location: Center City, MN, USA Posts: 3,253	Regarding the external tranny cooler only - That depends on where you live. Up here in Minnesota you need to heat your tranny fluid in the winter. The only way to do that is to run it through the tank on the end of a radiator. If you live down south you MAY be ok with just the tranny cooler. __________________ '70 cab, '71 chassis, 383, TH350, NP205. '71 Malibu convertible '72 Malibu hard top Center City, MN

01-07-2005, 08:47 AM	#3
Tx Firefighter Watch out for your cornhole ! Join Date: Jun 2003 Location: Azle, Texas Posts: 14,162	You'll need a flexplate and starter both. The original starters on 3 speed trucks bolts to the bellhousing rather that the engine. You'll need a pair of bolts for the new starter too. They're much longer than the bolts from the original starter. Remove the pilot bushing from the rear of the crankshaft. Crossmember bolts are 3/8 diameter, about 1 inch long. You'll need 8 of them along with washers and nuts for each. Th400 takes a different yoke than the 3 speed does. You'll need to buy one. __________________ I'm on the Instagram- @Gearhead_Kevin

01-07-2005, 08:50 AM	#4
da-burb "Ochre Ogre" Join Date: Apr 2000 Location: Springfield, MN Posts: 3,558	You will need to change out the first drive shaft. They use a different yolk. A TH350(short tail) would have dropped right in with no changes. Flex plate is the 168 tooth. You will also need a different starter too. __________________ Bowtie Truck Stop Inc. Mid-West GM Truck Restoration Parts Supplier Your Key Parts, Auto Metal Direct, Dynacorn, and Goodmark dealer. like us @ www.facebook.com/BowtieTruckStop 1971 C-10 Suburban (Ochre) 1971 K-10 Suburban (Ochre) 1972 C-10 Suburban (Ochre) 1972 K-20 Suburban (Yellow- that just aint right!) Springfield, Minnesota 56087

01-07-2005, 10:51 AM	#5
Vince M Senior Member Join Date: Apr 2000 Location: Orlando, FL Posts: 1,076	What about a shifter too. If you go with the stock auto steering column you'll have to change it out also along with adding the linkage to the tranny. __________________ ............................................................ 68 Chevy CST SWB w/LS1, carb & 700r4. Golden Anniversary clone with a few improvements.

01-07-2005, 02:04 PM	#6
Chugger Registered User Join Date: Nov 2004 Posts: 18	i have a b&m z-gate and the th400 has the yoke on it. So i will need a starter and flexplate with misc bolts. what about the length of the driveshaft will it need to be modified? Also the yoke from the th400 will not work? thanks chris

01-07-2005, 02:07 PM	#7
Tx Firefighter Watch out for your cornhole ! Join Date: Jun 2003 Location: Azle, Texas Posts: 14,162	The yoke may be an issue. There's 2 types of yokes on the turbo 400. There's the slip yoke and there's a bolt on yoke. The way you said the trans had a yoke on it, it sounds like you may have a bolt on yoke. You'll likely need to remove it and convert to a slip yoke. The shop who modifys your driveshaft should be able to advise you what the best way to do it is. I'm not giving you a straight answer because I don't know the length of the 3 speed you took out. There are a couple of different lengths of 3 speeds. You'd need to measure the old trans and the new trans and compare overall lengths to determine if the shaft will need modifying. The yoke will be different between a 3 speed and a turbo 400 for sure though. __________________ I'm on the Instagram- @Gearhead_Kevin

01-07-2005, 02:20 PM	#8
Johnny05 Registered User Join Date: Oct 2004 Location: Austin, Texas Posts: 73	I just swapped out my 3spd manual for a TH350 and a couple of things that I didn't see mentioned is new lines metal/rubber and the flywheel bolts(the auto flywheel uses shorter bolts). Also you'll have to make a provision to run a vacuum line to the modulator. You will also be needing a transmission mount and bolts for it. I used a flywheel and starter off a 75/76 Chevy and they worked fine. I made a list of all the parts I would need and went shopping!



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