I have an 82 blazer with a 6.2/700R4. I am running a 6 " susp. lift with 35 inch tires. The 700R4 doesnt like that too well. I cant even use the over drive. Its just a worn out 700R4. I was wanting to swap it out for a 350 turbo or even a 400. I was wondering what all i would have to do.. are the flywheels the same in a 700R4 as they would be in a 350 or a 400? I assume the both with bolt up fine behind the 6.2. Do i have the change starters or anything? Do you think one would be better than the other... a 350 or the 400? Thanks.

The flywheel/Flexplate is different. (bolt pattern for the convertor) Either trans will bolt up to the back of the engine.

Mods for the AT350 include a vacuum regulator valve to operate the modulator (mounts on the IP) and different geometry on the downshift cable and linkage. The case length is different so you will need an adaptor kit in order for the driveline(s) to fit. Cross member and trans mount will bolt up (with use of the adaptor kit) without modification. Install a shift kit and HD torque convertor if you intend to use it hard. Use the correct filler tube and dipstick for the application. Most AT350's have a short tube that's hard to reach when installed in a 6.2 truck. Even with these mods, the AT350 is not a heavy duty trans like the AT400.

The AT400 requires driveline modification (length change) as the case is longer. You will need to relocate the cross member as well. The same vacuum regulator valve system will be needed as well. HD six-lug convertor and shift kit are also a good idea. The short shaft AT400 uses two different size output shafts. Measure the 700R4 shaft and get a 400 with the same size shaft or you will need to disassemble the transfer case and replace the input gear of the proper size to match the output shaft size of the trans. Sometimes this is the least expensive way to accomplish the task. (If you already own a trans, change the gear) The 400 is more durable but considerably heavier. Don't forget the strut rods from the flywheel/convertor cover to the motor mount block plates. These prevent flexing and cracking of the trans case.

Good luck, it will work.

If i was to stick with a built 700R4, what would be a good torque converter to run with it?

I have used aftermarket H.D. torque convertors with all rebuilds that I've installed. That includes 350's 400's and 700R4's. My trans parts supplier has several types. I usually let them recommend what's best for my application. My guys are trustworthy. They give me stuff that works. Hope you have a similar situation.

They throw out part numbers like "GM-8" and "AT275". This stuff doesn't mean much to me. Just give me what works! Talk to your trans parts guy and explain you primary usage for the truck. They should know what will do the best job.

I like convertors that have heavy duty sprags, needle bearings, welded anti-balloon plates, brazed torus fins, etc. This stuff is trade talk and just means the convertor is "beefed up" to handle the strain of 4X4 H.D. use.

I also like the H.D. flex plate with the six-lug bolt holes to go with the six-lug H.D. convertor. I like to see six bolts holding the convertor to the flexplate instead of three. It's probably over-kill but has served me well.

Good Luck, Fred

I installed a TH400 in place of a 700r4 about 10 years ago. Overall length of a 400 is the same as a 700r4. The 400 uses a 1 3/8 32 spline output shaft compared to a 27 spline output for the 700r4 and th350. The input gear to the transfercase(np208) will need to be changed. You will need a vacuum switch (mounts to the injection pump) for a 6.2 with th 400. Flexplate will need to be changed. If it was me I would regear your axles to 4.10 or 4.56 and have your 700r4 rebuilt. I really missed overdrive on my old blazer and mileage suffered too. I just finished building a blazer for my daughter and I kept the 700r4 and stuffed 3/4 ton axles with 4.10's to handle her 35 inch tires. It runs good.

I measured all three cases last night. The THM350 is 23" from edge of bell housing to the end of the case where the extension housing bolts on. The 700R4 is 24" and the THM400 is 25 1/4" in length. (depending where you hold the tape)

I took a THM400 out of my suburban and found the front driveline slip yoke worn to point of replacement because of the transfer case setback. The yoke was running about 1" of the splines actually touching the driveline splines. When I jacked up the front end and allowed the axle to drop, only about a 1/2" of spline was still inside the end of the front yoke.

The rear driveline had the reverse problem. When going over bumps and compressing the rear springs, the driveline was hitting the end of the spline on the output shaft of the transfer case. It knocked the soft plug out of the front sliding yoke.

This is why I suggested driveline modifications on the swap. My experience dictated the comments. I now have the original equipment style 700R4 back in the Suburban and the drivelines fit properly. (after replacing both yokes)

The THM400 that I removed had the 27 spline small shaft so I was able to install the 700R4 without changing the input shaft gear in the transfer case.

Interesting, I have never seen a turbo 400 with a 27 spline output shaft. Also when measuring the tranmissions are you measuring from the front of the trans to the end of the adapter? The adapters for the 700 and th400 are different thicknesses making them come out close to the same length. I am talking about factory adapters, (cast iron) I know there are some conversion adapters out there that also might have used a 27 spline output shaft. I am pretty sure gm never offered a 27 spline output for the 4x4 turbo 400. In the conversions I did the 700 was an extremely close swap with the 700r4, but I used all factory components. Drivelines fit perfect.

That's what makes our business so much fun!! The 27 spline shaft looks factory. I don't remember if I've seen them before or not. I've done several 1/2T 400's and I believe they had the 27 spline shaft and small yoke. These were from 2WD's. All the 4WD's with 400's that I have worked on recently, have the 31 spline shaft. I, too, don't remember the 4WD 400's having the small shaft. I do have the one I took out of my Suburban but no history as to where it came from. I suppose it could be aftermarket. Like I said, it was originally a 700R4 vehicle. The shift cable bracket was still on the engine and the quadrant still said "OD, 3, 2, 1". The shift pattern positions for the 400 was off just a little as expected. (reverse wasn't quite in the notch, low was a little short as well)

I did take the NP208 directly off the 400 and put it right back on the 700 without a problem. I'm sure the t-case was all original to the vehicle. Whoever did the swap bought whatever was necessary to make it work. I know the people who purchased it new and also knew the 2nd owner who I bought it from. Both parties would have paid someone to "do the job right".

I mentioned "holding the tape". It was from the bottom edge, as close to straight back from the bell housing surface to the end of the case where the adaptor bolts on. Not including the adaptor. I know there are several different length adaptors available. I was just measuring the overall length of the case only. Like I said, That's what makes this interesting!

I've heard of techs swapping them but when I've done one, I've found the driveline interference problem at the rear and the "not enough spline in the yoke" problem up front. Maybe I'm nit-picking, but mine showed abnormal wear and damage to the sliding yoke.

The last one I did was an '87 3/4T 4X4 8600 gvw 6.2 N.A. pickup. I did have the drivelines modified because of the intended use of the truck. (Farm vehicle, heavy trailer towing, etc.) That vehicle provided the "donor" 700 that I rebuilt for my Suburban.

I'll go look at some of my cores, maybe I've got some "gold" among the "gravel".