manual to automatic swap - The 1947 - Present Chevrolet & GMC Truck Message Board Network

mgraham · 11-24-2005, 11:49 PM

I just spent 30 minutes using the search to try to find threads on tips to swap a 4-speed manual to an automatic in a 71 GMC C20. No luck. I'm new here and I'm sure this topic has been covered.
Help please.
Thanks.
Mike

krue · 11-25-2005, 01:10 AM

I don't think we have a faq on it yet. Some things I know you'll need are the auto tranny crossmember, auto steering column and linkage. You may need a new driveshaft (I'm not sure, it may depend on which auto tranny you go with). Radiator with tranny cooler, fab some cooler lines (can buy prebent or bend your own from parts store brake lines), a vacuum port for a TH350. Starter nosecone that bolts to the engine instead of the bellhousing. That's what I can think of off the top of my head.

mgraham · 11-25-2005, 01:37 AM

Thanks Krue. As for the steering column issue, I was thinking of going with a B&M shifter on the floor.
Mike

krue · 11-25-2005, 02:51 AM

Quote:

Originally Posted by mgraham

Thanks Krue. As for the steering column issue, I was thinking of going with a B&M shifter on the floor.
Mike

Be careful to make sure it's in park or neutral before starting it, cuz you won't have a Neutral Safety Switch.

70GMCer · 11-25-2005, 08:43 AM

I went from a 3spd manual to an 350 turbo. I'm not sure about the 400 turbo, but a 350 turbo with the short tail shaft will bolt right up to your existing drive shaft unless it has been altered in the past. The 350 and the manual measure the same distance from the bell housing to the tail shaft. I'm almost positive that the 4spd measures the same. If you're going to a 200R4 or 700R4, i've been told they measure longer, so you would have to shorten your drive shaft. Your 2 piece drive shaft should bolt right up if you use the 350 turbo w/short tail shaft. I was able to use the existing yoke also.

Going to the auto floor shifter will be the easiest also. But you'll need everything else that "kure" said to do the job. Not really a hard job to do. Took me a weekend and having all the parts ready needed.

good luck.......

krue · 11-25-2005, 08:55 AM

The 200-4r is the same length as the short shaft Th350's. The 700-r4 is longer.

70GMCer · 11-25-2005, 09:07 AM

thanks krue.....wasn't 100% sure on the 200........

crazy longhorn · 11-25-2005, 11:25 AM

On the 400 turbo.....the short one is 11/16" longer than the short 350 turbo, & takes a different slip yolk. otherwise, its a bolt in as far as auto crossmember(altho the crossmember will be in a different set of holes )as compaired to a 350T,or 3spd trans. if you are running a 4spd TRUCK trans, the shaft will need to be shortened for any of these swaps,as the truck trans is 1 short booger......

L

**70STOVEBOLT** · 12-27-2005, 01:51 PM

I am planning on going to an automatic from a 3-on-the-tree. I just picked up a TH400 with everything needed to make the swap except the radiator for $100.00. Will I need to shorten the driveshaft? I have a SWB and the truck this trans came out of was a LWB with 2-pc driveshaft. Any info would be greatly appreciated.
Thanks
STOVEBOLT

**70STOVEBOLT** · 06-21-2006, 04:59 PM

TTT

I am planning to do this swap over the long July 4th weekend (I'm off Monday and Tuesday). Anybody know about the driveshaft issue? Also, will the starter nosecone from a V8 for an auto trans fit on the 250 starter? Thanks
in advance for any help.
Stovebolt

ThatOneBlue67 · 06-21-2006, 05:15 PM

One thing I'd like to add, to save anyone some money.

I went from a 3 speed to a TH350 and I found a lot of decent parts at the yard. Prebent lines and a transmission cooler. Also, you can get tranny coolers that are outside of the radiator and just bolt it up somewhere near the radiator support. Those are a lot cheaper than a whole new radiator. (Yes, yes I may be a little cheap but I could really care less whether or not the cooler is built in or on the outside).

FOR INLINE 6 GUYS

Also, I too put a starter on my inline 6. I just picked up a used one off of a smallblock Chevy that had an automatic in it. Your's may vary (being a slightly later model 250) but I did not have pre-threaded hoels in my block for a starter (mostly because 230 inline 6's had manual transmissions, or so I would assume).

I ended up having to put bolts in with nuts on the other side and shim it for the correct distance from the flywheel

11-24-2005, 11:49 PM	#1
mgraham Registered User Join Date: Oct 2005 Location: Tucson Posts: 13	manual to automatic swap I just spent 30 minutes using the search to try to find threads on tips to swap a 4-speed manual to an automatic in a 71 GMC C20. No luck. I'm new here and I'm sure this topic has been covered. Help please. Thanks. Mike

11-25-2005, 01:10 AM	#2
krue Designated A-hole! Join Date: Mar 2003 Location: South Mississippi Posts: 36,450	Re: manual to automatic swap I don't think we have a faq on it yet. Some things I know you'll need are the auto tranny crossmember, auto steering column and linkage. You may need a new driveshaft (I'm not sure, it may depend on which auto tranny you go with). Radiator with tranny cooler, fab some cooler lines (can buy prebent or bend your own from parts store brake lines), a vacuum port for a TH350. Starter nosecone that bolts to the engine instead of the bellhousing. That's what I can think of off the top of my head. __________________ "If it ain't broke, fix it 'til it is!" Being stupid ain't illegal. We're Still Debt Free Except for the House!!! www.daveramsey.com 70 GMC SWB Stepside project "Green Booger" soon to be 6.0l/4l80e 93 S-10 "Poppaw's Truck"

11-25-2005, 01:37 AM	#3
mgraham Registered User Join Date: Oct 2005 Location: Tucson Posts: 13	Re: manual to automatic swap Thanks Krue. As for the steering column issue, I was thinking of going with a B&M shifter on the floor. Mike

11-25-2005, 08:43 AM	#5
70GMCer Senior Member Join Date: Apr 2005 Location: Joppa, Maryland Posts: 4,408	Re: manual to automatic swap I went from a 3spd manual to an 350 turbo. I'm not sure about the 400 turbo, but a 350 turbo with the short tail shaft will bolt right up to your existing drive shaft unless it has been altered in the past. The 350 and the manual measure the same distance from the bell housing to the tail shaft. I'm almost positive that the 4spd measures the same. If you're going to a 200R4 or 700R4, i've been told they measure longer, so you would have to shorten your drive shaft. Your 2 piece drive shaft should bolt right up if you use the 350 turbo w/short tail shaft. I was able to use the existing yoke also. Going to the auto floor shifter will be the easiest also. But you'll need everything else that "kure" said to do the job. Not really a hard job to do. Took me a weekend and having all the parts ready needed. good luck....... __________________ 1997 GMC Sierra Ext 1965 Mustang

11-25-2005, 08:55 AM	#6
krue Designated A-hole! Join Date: Mar 2003 Location: South Mississippi Posts: 36,450	Re: manual to automatic swap The 200-4r is the same length as the short shaft Th350's. The 700-r4 is longer. __________________ "If it ain't broke, fix it 'til it is!" Being stupid ain't illegal. We're Still Debt Free Except for the House!!! www.daveramsey.com 70 GMC SWB Stepside project "Green Booger" soon to be 6.0l/4l80e 93 S-10 "Poppaw's Truck"

11-25-2005, 09:07 AM	#7
70GMCer Senior Member Join Date: Apr 2005 Location: Joppa, Maryland Posts: 4,408	Re: manual to automatic swap thanks krue.....wasn't 100% sure on the 200........ __________________ 1997 GMC Sierra Ext 1965 Mustang

11-25-2005, 11:25 AM	#8
crazy longhorn Fabricate till you "puke" Join Date: Jun 2002 Location: Ill Posts: 9,403	Re: manual to automatic swap On the 400 turbo.....the short one is 11/16" longer than the short 350 turbo, & takes a different slip yolk. otherwise, its a bolt in as far as auto crossmember(altho the crossmember will be in a different set of holes )as compaired to a 350T,or 3spd trans. if you are running a 4spd TRUCK trans, the shaft will need to be shortened for any of these swaps,as the truck trans is 1 short booger...... L __________________ 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.... Last edited by crazy longhorn; 11-25-2005 at 11:26 AM.

12-27-2005, 01:51 PM	#9
70STOVEBOLT Senior Member Join Date: Jun 2005 Location: Winona Lake, IN Posts: 6,266	Re: manual to automatic swap I am planning on going to an automatic from a 3-on-the-tree. I just picked up a TH400 with everything needed to make the swap except the radiator for $100.00. Will I need to shorten the driveshaft? I have a SWB and the truck this trans came out of was a LWB with 2-pc driveshaft. Any info would be greatly appreciated. Thanks STOVEBOLT

06-21-2006, 04:59 PM	#10
70STOVEBOLT Senior Member Join Date: Jun 2005 Location: Winona Lake, IN Posts: 6,266	Re: manual to automatic swap TTT I am planning to do this swap over the long July 4th weekend (I'm off Monday and Tuesday). Anybody know about the driveshaft issue? Also, will the starter nosecone from a V8 for an auto trans fit on the 250 starter? Thanks in advance for any help. Stovebolt __________________ 70 C/10 SWB 402/TH400/3.73 "The Needy Beast" 200,000 Mile Club Disc Brake Club Owner installed options: Front Sway Bar Power Steering Power Brakes Cigar Lighter Courtesy Lights Deluxe Side Markers Wiper Delay Power windows Power Locks Sniper EFI 2015 Silverado 1500 LS 4.3/6L80/3.23 lowered 2" front & rear

06-21-2006, 05:15 PM	#11
ThatOneBlue67 I survived Dead Man's Curve. Join Date: Apr 2006 Location: The Nine-Five-687 Posts: 389	Re: manual to automatic swap One thing I'd like to add, to save anyone some money. I went from a 3 speed to a TH350 and I found a lot of decent parts at the yard. Prebent lines and a transmission cooler. Also, you can get tranny coolers that are outside of the radiator and just bolt it up somewhere near the radiator support. Those are a lot cheaper than a whole new radiator. (Yes, yes I may be a little cheap but I could really care less whether or not the cooler is built in or on the outside). FOR INLINE 6 GUYS Also, I too put a starter on my inline 6. I just picked up a used one off of a smallblock Chevy that had an automatic in it. Your's may vary (being a slightly later model 250) but I did not have pre-threaded hoels in my block for a starter (mostly because 230 inline 6's had manual transmissions, or so I would assume). I ended up having to put bolts in with nuts on the other side and shim it for the correct distance from the flywheel Last edited by ThatOneBlue67; 06-21-2006 at 05:20 PM.



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