3 speed vs 4 speed auto tranny - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Sstarkey · 08-05-2011, 05:58 PM

Got a '72 K5 CST with a regular 3 speed automatic coupled to a 350 engine. Any easy way to install a 4 speed automatic or something with an overdrive? Tks.

SeventyOne · 08-07-2011, 07:41 PM

"Easy" is a subjective term. It's been done and continues to be a popular swap. Try a search and you'll get a ton of info.

For a quick rundown: 700r4 is your best bet IMHO, you will need to get a new output shaft for the trans to mate up to your tcase, or an adapter, or swap tcases all together. Then due to the extra length of the 4spd vs 3spd you will need to move the engine forward or the tcase back. Both have pros and cons.

Good luck.

Sstarkey · 09-03-2011, 04:50 AM

Thanks for the direction on the swap from a 350 to 700 tranny - got plenty of space to move engine forward but just need to work out the cross-member. Everything else looks pretty.

red71cheyenne · 09-04-2011, 09:56 PM

I don't know about the 4X4 versions, but on the 2wd 700R4's, if you get a short tail shaft version, it will mount in the same place as the TH350. Thats what I did in the 71. Might be able to find the same thing in 4X, but someone with more knowledge will have to chime in.

Firebirdjones · 09-05-2011, 05:47 PM

It's a rather pricey swap on a 4x4.

I went with a 4L60E (electronic 700R4). I needed the shorter output shaft, and an adapter for the stock 203 transfer case.
Actually it was a little more in depth for me, as I needed the coupler and decided on a new transfer case mount as well because I didn't want to modify the original. The stock coupler doesn't work if you are using the ball bearing type. The swap kit is designed for the newer style splined sleeve that GM started using on later models.

All of the necessary parts are sold through Advance Adapters. I spent about $800 for everything to adapt (not including the transmission).

It can be done cheaper, depending on the route you go.

I chose to move the engine forward, as there was more than enough room for it. Simply redrill new holes in the frame for the crossmember.

Sstarkey · 09-05-2011, 07:12 PM

Thanks Firebird, I appreciate it - Did you get the cross member from Advance Adapters as well? I can't seem to find a cross member for the engine, only for transmissions and/or transfer cases. Am I missing something?

Firebirdjones · 09-05-2011, 07:18 PM

Sorry, I was under the impression that you were still using the original 350 engine in the blazer.
In which case you can just use the stock cross member and move it forward with the engine and redrill the frame. I don't believe there would be any clearance issues with the differential on that one. Might have to trial fit though and see. If you have a suspension lift then no worries.

I did an LS swap at the same time, so I used a custom cross member from Pacific Fab.

08-05-2011, 05:58 PM	#1
Sstarkey Registered User Join Date: Mar 2011 Location: Los Angeles, CA Posts: 3	3 speed vs 4 speed auto tranny Got a '72 K5 CST with a regular 3 speed automatic coupled to a 350 engine. Any easy way to install a 4 speed automatic or something with an overdrive? Tks.

08-07-2011, 07:41 PM	#2
SeventyOne Registered User Join Date: Mar 2007 Location: Oakland, Ca Posts: 1,370	Re: 3 speed vs 4 speed auto tranny "Easy" is a subjective term. It's been done and continues to be a popular swap. Try a search and you'll get a ton of info. For a quick rundown: 700r4 is your best bet IMHO, you will need to get a new output shaft for the trans to mate up to your tcase, or an adapter, or swap tcases all together. Then due to the extra length of the 4spd vs 3spd you will need to move the engine forward or the tcase back. Both have pros and cons. Good luck.

09-03-2011, 04:50 AM	#3
Sstarkey Registered User Join Date: Mar 2011 Location: Los Angeles, CA Posts: 3	Re: 3 speed vs 4 speed auto tranny Thanks for the direction on the swap from a 350 to 700 tranny - got plenty of space to move engine forward but just need to work out the cross-member. Everything else looks pretty.

09-04-2011, 09:56 PM	#4
red71cheyenne Back in the sticks Join Date: Jan 2001 Location: Fordland, MO Posts: 3,188	Re: 3 speed vs 4 speed auto tranny I don't know about the 4X4 versions, but on the 2wd 700R4's, if you get a short tail shaft version, it will mount in the same place as the TH350. Thats what I did in the 71. Might be able to find the same thing in 4X, but someone with more knowledge will have to chime in. __________________ 1971 Cheyenne C-10 w/700R4 and Tuned Port Injection 1969 K5 Blazer w/Tuned Port 2010 2SS/RS Flaming Orange Camaro 2011 K1500 Suburban 2014 K1500 Pickup 2008 Nissan Altima? The wifes' hoopty

09-05-2011, 05:47 PM	#5
Firebirdjones Registered User Join Date: Oct 2009 Location: Prescott, Arizona Posts: 2,396	Re: 3 speed vs 4 speed auto tranny It's a rather pricey swap on a 4x4. I went with a 4L60E (electronic 700R4). I needed the shorter output shaft, and an adapter for the stock 203 transfer case. Actually it was a little more in depth for me, as I needed the coupler and decided on a new transfer case mount as well because I didn't want to modify the original. The stock coupler doesn't work if you are using the ball bearing type. The swap kit is designed for the newer style splined sleeve that GM started using on later models. All of the necessary parts are sold through Advance Adapters. I spent about $800 for everything to adapt (not including the transmission). It can be done cheaper, depending on the route you go. I chose to move the engine forward, as there was more than enough room for it. Simply redrill new holes in the frame for the crossmember.

09-05-2011, 07:12 PM	#6
Sstarkey Registered User Join Date: Mar 2011 Location: Los Angeles, CA Posts: 3	Re: 3 speed vs 4 speed auto tranny Thanks Firebird, I appreciate it - Did you get the cross member from Advance Adapters as well? I can't seem to find a cross member for the engine, only for transmissions and/or transfer cases. Am I missing something?

09-05-2011, 07:18 PM	#7
Firebirdjones Registered User Join Date: Oct 2009 Location: Prescott, Arizona Posts: 2,396	Re: 3 speed vs 4 speed auto tranny Sorry, I was under the impression that you were still using the original 350 engine in the blazer. In which case you can just use the stock cross member and move it forward with the engine and redrill the frame. I don't believe there would be any clearance issues with the differential on that one. Might have to trial fit though and see. If you have a suspension lift then no worries. I did an LS swap at the same time, so I used a custom cross member from Pacific Fab.



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