Turbo 400 to 700R4 swap

I have a 85 k20 6.2 diesel with a turbo 400 behind it. Love the truck but id like to be able to cruise the highway comfortably. While I'd love to have a manual transmission line an nv4500, the 700r4 swap seems much simpler. Any other trannys I should consider? Is the 700r4 as weak as they say? I won't be doing any towing.

I converted mine from a TH350 to 700R4. Got it from Gearstar. Nick from Nick's Transmissions on here would be a good one to ask as far as toughness. My 2cents on it is yes, the 700R4 would be just fine in your application. Have it built well and don't abuse it, and it should be just fine. The main issue on the installation is make sure you set up the TV cable correctly. It isn't a kickdown cable, it sets the line pressure and is mission critical for survival. Also, make sure you keep it cool. Add a trans gauge to monitor. That way, you'll know if you're cooling set up is sufficient. If not, you can add an external cooler.
You may need to modify the driveshafts and the right adapter between the t-case and 700R4. Advanced Adapters may be able to help with matching the spline. eBay may also be a source for the adapter.
FYI, a motorcycle lift from Harbor Freight is a great tool for removing the t-case and transmission together. It makes it a 1-man job and much safer than just a regular jack.
With the weight of that diesel, I'd suggest getting the cast aluminum torque converter cover (eBay) and the support rods that mount to the engine mounts. Helps take some of the stress off of the transmission bellhousing.
Nailing down the shift points is another "tuning" step that can be tricky. While some like using the TV cable to adjust, I've tuned the up/down shift points with the springs/weights in the governor. B&M sells a good kit for this.

A 400 has a 32 spline tailshaft and a 700 is 27 so not only will you tcase adapter be different but so is the tcase input shaft.

A GM4L80E might be a better choice to replace a TH400. What are the torque specifications for the the oil burner 6.2L?

Don't ya haveta have a K case(4WD) for the aluminum cover to fit?

Thanks for the info fellas. I think the 6.2 only puts out 235ish lbs ft of torque brand new, which mine is certainly not. The 4l80 needs a transmission controller correct? Could be worth it if it saves me getting a TCase adapter. I assume I'd need new driveshafts regardless?

Yes, use a "K" case since it is a 4x4. A well-built 700R4 will handle 400 ft-lbs of torque, so it'll do just fine behind the 6.2. Just keep it cool and don't beat on it.

A 4wd 4l80 with the stock 4l80 transfer case adapter would bolt right up and is only 1" longer overall length than the TH400, so it will work with the factory driveshafts too. You can run an LS PCM as a trans controller and the 6.2 sends out tach signal from the alternator and I think has a TPS signal also, which are all you need to run the 4l80 controller.

There are people far more knowledgeable about transfer cases, so I’ll avoid that discussion! I’m guessing either the trans or transfer case will need to be disassembled to swap either an output shaft in the trans, or an input shaft on the transfer case.

While the trans is out, I’d definitely install a TransGo shift kit. Diesel governors are fairly easy to find, or as someone mentioned the calibration kit might be needed. Definitely make sure the TV cable is right.

You'll be aight with a 700r4 behind the 6.2. In fact the 6.2 will love you for it. A better lower 1st gear to help get it moving as well as OD and Lock up to lower the rpms cuz 6.2 don't like rpms either. Everyone saying, don't beat on the 700r4. They're correct, but in this case I say, don't beat on the 6.2 so it'll live and you'll have no worries of hurting the 700r4. As far as reliability of the 700r4 in this configuration is just as in any configuration with a 700r4, make your priority to be the TV cable. Making sure it's set correctly, and making sure it stays that way and making sure it's not stretched or worn to the point it's not working correctly. In a 700r4, the TV cable is responsible for controlling the pressure regulation. Not enough pressure allows clutches to slip, get hot, and cooked. Usually starts with the 3/4 clutch pack being the first to go.

The next thing as far as you install you need to be concerned about, when you find your donor transmission, good chance it'll be a gasoline engine unit. The 6.2 diesel does use a different calibrated governor, and it also uses a different torque converter. Make sure you make these changes or that your donor already has them. Past that, I personally think the 700r4 is a great choice behind the 6.2. Both of them, the 6.2 and the 700r4 have their weaknesses, so they both kind of take care of each other too. Don't beat on the 6.2, and I think you'll be happy with the combo.

Bextreme04 said:

A 4wd 4l80 with the stock 4l80 transfer case adapter would bolt right up and is only 1" longer overall length than the TH400, so it will work with the factory driveshafts too. You can run an LS PCM as a trans controller and the 6.2 sends out tach signal from the alternator and I think has a TPS signal also, which are all you need to run the 4l80 controller.

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I thought the 6.2 had a tach signal from the alternator also, one of our members said no they don't. So IDK O.P should definitely check but if you know for sure that would help him and me. Thanks have a great night.

Ricko1966 said:

I thought the 6.2 had a tach signal from the alternator also, one of our members said no they don't. So IDK O.P should definitely check but if you know for sure that would help him and me. Thanks have a great night.

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I know the later ones do. I'm not 100% sure on the 80's ones, but I know at some point it was there.

Bextreme04 said:

I know the later ones do. I'm not 100% sure on the 80's ones, but I know at some point it was there.

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They would have appeared around '87 in the GMT400 trucks. I'm not aware of any in the square body trucks. Any alternator shop should be able to add a frequency tap for a tach signal tho.

I would go 4L80E as @Bextreme04 pointed out.

You can control it with stock components from whatever donor you pull it from. You don't have to buy a $1,000 standalone controller.

jdomarsh said:

I have a 85 k20 6.2 diesel with a turbo 400 behind it. Love the truck but id like to be able to cruise the highway comfortably. While I'd love to have a manual transmission line an nv4500, the 700r4 swap seems much simpler. Any other trannys I should consider? Is the 700r4 as weak as they say? I won't be doing any towing.

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A properly built 700r4 would be fine. See my 700r4 Information thread on here for more information. I also have a YouTube channel with a big 700r4 playlist with all manner of builds showing all the parts I chose for installation…

GM 700R4