search down....auto to manual tranny swap???? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

speesh · 06-19-2003, 10:56 PM

I have to change my old junky 3 speed for an automatic in my 71 half ton suburban..... here is what I think I know about the job.
1. I need a flywheel, starter, torque converter, shifter, and crossmember, is there anything else?
2. I am taking out a three on the tree tranny, I think I can put in a turbo 350 without changing the driveshaft.....
3. I just happen to have a parts truck with a rebuilt tranny.... but its a turbo 400! and a 3/4 ton, while my burb is a 1/2 ton.

Question.... if I use the 400, can I use the driveshaft from the parts truck? It is a 69 longbed. Or would it be easier to try to get a turbo 350? Then I wouldn't have to do all the work of removing the other tranny from the parts truck, but I would have to get all the parts.

Need IIIIIINNNNNPPPPUUUUTTTT!!!!!
I know this has been talked about on here many times, but search is not working so I have to ask. Anyone? Anyone? Bueller???
p.s. here is a picture of my burb overlooking Panamint Valley, this is aout 45 minutes from my house...

iluvmy72 · 06-20-2003, 01:26 AM

You have to do a bunch of things......first you have to change the gearing of your rear end......then you have to change the hump in your sled......then you'll have to get a new driveline made for whatever tranny you decide on considering there are several lengths in the TH350 and 400's......you'll need a cradle as well.....big job man......I'd take what you got and swap with another one with a tranny already here.....broken or not it's easier to replace......but do more research and consult with a tranny shop on this as to you don't want to jump into a project swap like this with info from this board.....don't get me wrong....this board is great but ideas are one thing......but directions on what to do for what you are planning need to be done with the aid of a onsite pro.

*Gee_Emm* · 06-20-2003, 06:15 PM

Why would the starter have to be changed?

crazy longhorn · 06-20-2003, 06:41 PM

your 3spdstick+bell, is the same lenght & yolk spline as a 350 turbo with a 6" tailshaft. The short 400 turbo (4" tail) is approx 11/16" longer, & has a different slip yolk(no biggie), as you can have the shaft shortened pretty cheap. you will need to remove the bellmount crossmember, & install an auto crossmember under the trans. Other than that, pretty much all you have listed, plus a cooler & lines & the auto shifter. You can replace the column, or just add a B&M floor shifter, which i feel would be much easier. The 400 turbo is a d@mn good trans, & personally if you have a rebuilt unit, I would run it

its not a tough swap if you know you way around a "wrench". the starter does need to be replaced, or at least the end changed(stick starter mounts to the bell with 3 bolts, whereas the auto starter mounts to the block with 2 bolts).......dont forget the front hanger strap on the starter, it helps keep things straight & tight. Best of luck, & any trouble, feel free to send a PM........i have swapped several(stick to auto , or auto to stick). crazyL

*Gee_Emm* · 06-20-2003, 07:00 PM

I have a 250 6cyl with a 3spd that I want to make auto, Is there any point going with an overdrive tranny or should I stick with a th350. The truck will just be a daily driver, will the OD get me some extra mileage?

Class of 69 · 06-20-2003, 08:37 PM

The OD trans will help mpg on the highway.

crazy longhorn · 06-20-2003, 08:44 PM

Gee_Emm,I havent tryed an O/D behind a I6, but my bet would be with the 3.73 gears in a 1/2 ton 6 /stick, you could probably pick up a little milage on the hyway. If you are primarily a town driver, I dont know that I would spend the extra cash for the 700 r4. 350 turbos are cheap, & pretty reliable. Good luck,crazyL

BubbaGreen · 06-20-2003, 09:11 PM

You dont need to modify the "hump" in the cab with a TH350 or
a 700R4. I have a small hump and they both fit fine. MIght need to modify with the TH400, not sure on that one.
Get ALL the shifer linkage possible, including that "C-CLIP" piece bolted to the tranny shifter selector...been discountinued from GM.
Its not hard to do, but getting the standard transmission crossbrace out isnt fun, you'll also need an automatic cross-member and mount (with shim). My speedo-cable was really short and barely made it. It was quite a bit of work, but well worth it, as my shift linkage would bind up in third ALL the time.

*Gee_Emm* · 06-20-2003, 10:17 PM

The 400 will fit under a small hump

Class of 69 · 06-20-2003, 10:53 PM

I was led to believe all the standard trans trucks came with the high hump installed.

*Gee_Emm* · 06-20-2003, 10:58 PM

Nope both of mine are low hump

crazy longhorn · 06-20-2003, 11:26 PM

The 2wd 4 spd trucks were a high hump(ch465), as were the 4wd trucks. Any of the auto trans equipped 2wd trucks(I 6 or sb ), or 3 spd stick were low hump. I think from some of the posts ,I heard that the BB 2 wd trucks were a high hump with the auto trans.....I havent been into the BB trucks, so somebody that has been there will have to comment on that . crazyL

06-19-2003, 10:56 PM	#1
speesh Senior Member Join Date: Jul 2001 Location: Ridgecrest, CA Posts: 318	search down....auto to manual tranny swap???? I have to change my old junky 3 speed for an automatic in my 71 half ton suburban..... here is what I think I know about the job. 1. I need a flywheel, starter, torque converter, shifter, and crossmember, is there anything else? 2. I am taking out a three on the tree tranny, I think I can put in a turbo 350 without changing the driveshaft..... 3. I just happen to have a parts truck with a rebuilt tranny.... but its a turbo 400! and a 3/4 ton, while my burb is a 1/2 ton. Question.... if I use the 400, can I use the driveshaft from the parts truck? It is a 69 longbed. Or would it be easier to try to get a turbo 350? Then I wouldn't have to do all the work of removing the other tranny from the parts truck, but I would have to get all the parts. Need IIIIIINNNNNPPPPUUUUTTTT!!!!! I know this has been talked about on here many times, but search is not working so I have to ask. Anyone? Anyone? Bueller??? p.s. here is a picture of my burb overlooking Panamint Valley, this is aout 45 minutes from my house... Attached Images __________________ 57 suburban(sitting in the driveway), 2 69 C20 longbeds (sitting in the field ) and a 2003 8.1 Suburban Endeavor to persevere. "All of a sudden, I'm the old timer!" Some old timer on American Pickers.

06-20-2003, 06:41 PM	#4
crazy longhorn Fabricate till you "puke" Join Date: Jun 2002 Location: Ill Posts: 9,402	your 3spdstick+bell, is the same lenght & yolk spline as a 350 turbo with a 6" tailshaft. The short 400 turbo (4" tail) is approx 11/16" longer, & has a different slip yolk(no biggie), as you can have the shaft shortened pretty cheap. you will need to remove the bellmount crossmember, & install an auto crossmember under the trans. Other than that, pretty much all you have listed, plus a cooler & lines & the auto shifter. You can replace the column, or just add a B&M floor shifter, which i feel would be much easier. The 400 turbo is a d@mn good trans, & personally if you have a rebuilt unit, I would run it its not a tough swap if you know you way around a "wrench". the starter does need to be replaced, or at least the end changed(stick starter mounts to the bell with 3 bolts, whereas the auto starter mounts to the block with 2 bolts).......dont forget the front hanger strap on the starter, it helps keep things straight & tight. Best of luck, & any trouble, feel free to send a PM........i have swapped several(stick to auto , or auto to stick). crazyL __________________ 69 longhorn,4" chop,3/5 drop, 1/2 ton suspension/disc brakes,1 1/2" body drop,steel tilt clip, 5.3/Edelbrock rpm intake/600 carb, Hooker streetrod shorties,2 1/2" exhaust/ H pipe/50's Flows , 6 spd Richmond trans,12 bolt/ 3.40 gears....

06-20-2003, 08:37 PM	#6
Class of 69 Registered User Join Date: Mar 2003 Location: Stillwater MN Posts: 1,166	The OD trans will help mpg on the highway. __________________ 69 GMC K10 ..some restoration required....still..

06-20-2003, 08:44 PM	#7
crazy longhorn Fabricate till you "puke" Join Date: Jun 2002 Location: Ill Posts: 9,402	Gee_Emm,I havent tryed an O/D behind a I6, but my bet would be with the 3.73 gears in a 1/2 ton 6 /stick, you could probably pick up a little milage on the hyway. If you are primarily a town driver, I dont know that I would spend the extra cash for the 700 r4. 350 turbos are cheap, & pretty reliable. Good luck,crazyL __________________ 69 longhorn,4" chop,3/5 drop, 1/2 ton suspension/disc brakes,1 1/2" body drop,steel tilt clip, 5.3/Edelbrock rpm intake/600 carb, Hooker streetrod shorties,2 1/2" exhaust/ H pipe/50's Flows , 6 spd Richmond trans,12 bolt/ 3.40 gears....

06-20-2003, 09:11 PM	#8
BubbaGreen Senior Member Join Date: Mar 2001 Location: Cedar Park, TX Posts: 686	You dont need to modify the "hump" in the cab with a TH350 or a 700R4. I have a small hump and they both fit fine. MIght need to modify with the TH400, not sure on that one. Get ALL the shifer linkage possible, including that "C-CLIP" piece bolted to the tranny shifter selector...been discountinued from GM. Its not hard to do, but getting the standard transmission crossbrace out isnt fun, you'll also need an automatic cross-member and mount (with shim). My speedo-cable was really short and barely made it. It was quite a bit of work, but well worth it, as my shift linkage would bind up in third ALL the time. __________________ 71 Chevy C-10 SWB Fleetside RamJet-350/4L60e & Compushift/3.73Posi See it at www.prismnet.com/~grzanich

06-20-2003, 01:26 AM	#2
iluvmy72 Registered User Join Date: Jan 2002 Location: Springville, UT, USA Posts: 1,010	You have to do a bunch of things......first you have to change the gearing of your rear end......then you have to change the hump in your sled......then you'll have to get a new driveline made for whatever tranny you decide on considering there are several lengths in the TH350 and 400's......you'll need a cradle as well.....big job man......I'd take what you got and swap with another one with a tranny already here.....broken or not it's easier to replace......but do more research and consult with a tranny shop on this as to you don't want to jump into a project swap like this with info from this board.....don't get me wrong....this board is great but ideas are one thing......but directions on what to do for what you are planning need to be done with the aid of a onsite pro.

06-20-2003, 06:15 PM	#3
Gee_Emm Account Suspended Join Date: Sep 2000 Posts: 3,991	Why would the starter have to be changed?

06-20-2003, 07:00 PM	#5
Gee_Emm Account Suspended Join Date: Sep 2000 Posts: 3,991	I have a 250 6cyl with a 3spd that I want to make auto, Is there any point going with an overdrive tranny or should I stick with a th350. The truck will just be a daily driver, will the OD get me some extra mileage?

06-20-2003, 10:17 PM	#9
Gee_Emm Account Suspended Join Date: Sep 2000 Posts: 3,991	The 400 will fit under a small hump

06-20-2003, 10:53 PM	#10
Class of 69 Registered User Join Date: Mar 2003 Location: Stillwater MN Posts: 1,166	I was led to believe all the standard trans trucks came with the high hump installed. __________________ 69 GMC K10 ..some restoration required....still..

06-20-2003, 10:58 PM	#11
Gee_Emm Account Suspended Join Date: Sep 2000 Posts: 3,991	Nope both of mine are low hump

06-20-2003, 11:26 PM	#12
crazy longhorn Fabricate till you "puke" Join Date: Jun 2002 Location: Ill Posts: 9,402	The 2wd 4 spd trucks were a high hump(ch465), as were the 4wd trucks. Any of the auto trans equipped 2wd trucks(I 6 or sb ), or 3 spd stick were low hump. I think from some of the posts ,I heard that the BB 2 wd trucks were a high hump with the auto trans.....I havent been into the BB trucks, so somebody that has been there will have to comment on that . crazyL __________________ 69 longhorn,4" chop,3/5 drop, 1/2 ton suspension/disc brakes,1 1/2" body drop,steel tilt clip, 5.3/Edelbrock rpm intake/600 carb, Hooker streetrod shorties,2 1/2" exhaust/ H pipe/50's Flows , 6 spd Richmond trans,12 bolt/ 3.40 gears....

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