Switch 3-Speed to 4-Speed with O/D?

[Chopper9]

Have a 1970 K-10 (350 Auto), and wanted to switch out the 3-speed automatic to a 4-Speed with overdrive to improve the gas mileage.

Can anyone here chime in with advice like what kind of tranny they would recommend, what kind of gas mileage I can expect, and what the operation might cost here in Ohio?

Thanks for any help.

[1968ratrod]

kind of vague questions but I'll take a stab at it. I would probably put a 700r4 in place of the turbo 350. Mileage depends greatly on too many variables like engine size , rear gears, tire size etc, so no good guess there. If you are having the work done you will probably spend between $2000 and $3000 but if you are able to do some of the work you can cut cost back in several areas. Good luck......

[Hubscrub]

i did my conversion with a 700r4 works pretty good i don't know gas mileage as my speedo don't work at the moment. i bought a rebuilt trans for $1000 here at a local transmission shop.

Transmission- $1000 included converter, tv cable, dipsick, and flexplate cover
I installed it -$0
transmission mount-few $
Dad built me a one piece driveshaft
I built crossmember-$0
Still need to get a converter lockup switch sometime extra $100?
And correct speedo drive gear and driven gear i still need

since you have an auto it shouldn't be that hard at all just BE SURE THE TV CABLE IS ADJUSTED CORRECTLY FOR TRANS. LIFE! i had the rebuilder to come by and adjust it for me and he gave me a 18 month warranty too.

P.S 2004r trans have closer gears, 700r4s have a deep first i like the 2004r the same or better.


EDIT: I TOTALLY MISSED THAT YOU HAVE A 4WD so ya it's a little more to it

[mrein3]

Originally Posted by Chopper9

Have a 1970 K-10 (350 Auto), and wanted to switch out the 3-speed automatic to a 4-Speed with overdrive to improve the gas mileage.

Can anyone here chime in with advice like what kind of tranny they would recommend, what kind of gas mileage I can expect, and what the operation might cost here in Ohio?

Thanks for any help.

A 4x4 is a whole different animal that what folks with 2wd have to deal with when putting in a 700r4.

First of all you need to go to Advanced Adapters and get a new output shaft that will work with a NP205 (first verify if you already have a NP205 for a transfer case).

Take the AA shaft and a core 700r4 to a reputable tranny builder in your area and have him rebuild it and install the new output shaft.

After you get your rebuilt transmission back you have a decision to make. Either you can leave the engine where it sits and move the transfer case back or move the engine forward. The 700r4 is approximately 2 inches longer than the th350.

Leaving the engine where it sits now will require you to have the rear driveshaft shortened and the front lengthened.

What I would do is pull the engine, leaving the current transmission bolted to the transfer case adapter. Then with the engine out of the way I'd pull the transmission.
With room to work in there remove the rivets that hold the engine cradle in place.
Then install your new transmission with the cradle out of the way.

Set the cradle in the frame and figure out where your engine will be sitting, mark where you need to make new holes, get the engine out of the way, mount the cradle, then install the engine.

I've installed a th350/NP205 from the bottom in a well equipped shop. It was a bear. I can't see any other way to do a 700r4 than what I outlined above.

You should be able to do this yourself in your home shop for the cost of the tranny rebuild plus the TV cable stuff. All the rest is labor.

With a NP205 the speedo comes out of the transfer case so if your speedo is on now, it will be on after you're done.

With gas going north of $3.50 a gallon, this swap may actually pay for itself in a couple of years.

[MARKDTN]

Originally Posted by mrein3

A 4x4 is a whole different animal that what folks with 2wd have to deal with when putting in a 700r4.

First of all you need to go to Advanced Adapters and get a new output shaft that will work with a NP205 (first verify if you already have a NP205 for a transfer case).

Take the AA shaft and a core 700r4 to a reputable tranny builder in your area and have him rebuild it and install the new output shaft.

After you get your rebuilt transmission back you have a decision to make. Either you can leave the engine where it sits and move the transfer case back or move the engine forward. The 700r4 is approximately 2 inches longer than the th350.

Leaving the engine where it sits now will require you to have the rear driveshaft shortened and the front lengthened.

What I would do is pull the engine, leaving the current transmission bolted to the transfer case adapter. Then with the engine out of the way I'd pull the transmission.
With room to work in there remove the rivets that hold the engine cradle in place.
Then install your new transmission with the cradle out of the way.

Set the cradle in the frame and figure out where your engine will be sitting, mark where you need to make new holes, get the engine out of the way, mount the cradle, then install the engine.

I've installed a th350/NP205 from the bottom in a well equipped shop. It was a bear. I can't see any other way to do a 700r4 than what I outlined above.

You should be able to do this yourself in your home shop for the cost of the tranny rebuild plus the TV cable stuff. All the rest is labor.

With a NP205 the speedo comes out of the transfer case so if your speedo is on now, it will be on after you're done.

With gas going north of $3.50 a gallon, this swap may actually pay for itself in a couple of years.

I agree with what you are saying, but 700R4 is available in a 4wd application. Can't you just buy a 4x4 core?

[mrein3]

Originally Posted by MARKDTN

I agree with what you are saying, but 700R4 is available in a 4wd application. Can't you just buy a 4x4 core?

The 4x4 ones work with newer transfer cases is what I've been lead to believe.

I think there is a different adapter you can purchase for cheap that has you cut off part of the output shaft of a stock 700r4 with a hacksaw but that isn't something I'd do to a vehicle I want to rely on.

[MARKDTN]

Originally Posted by mrein3

The 4x4 ones work with newer transfer cases is what I've been lead to believe.

I think there is a different adapter you can purchase for cheap that has you cut off part of the output shaft of a stock 700r4 with a hacksaw but that isn't something I'd do to a vehicle I want to rely on.

OK, gotcha

[Chopper9]

Wow, great info guys. Really appreciate it.

Since I'm no gearhead, it looks like its gonna be a lot more expensive than I was figuring.

Isn't that always the case??

[mrein3]

Originally Posted by Chopper9

Wow, great info guys. Really appreciate it.

Since I'm no gearhead, it looks like its gonna be a lot more expensive than I was figuring.

Isn't that always the case??

You don't need to be a gearhead. You just need time and space. This is definately NOT a task I would hire out - other than the actual rebuilding of the transmission.

I should have added earlier that you could hang on to your current tranny incase something really bad happens to the new set-up. You can always go back to no overdrive.

Another thing to consider for such a large undertaking is what gears do you currently have in your axels. If they are 3.07s, you probably don't want overdrive.