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Old 10-09-2018, 12:14 PM   #1
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Location: hawthorne, nj
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Swapping a 700R4 for a th350...what's needed?

Got a sweet deal on a 700 R4 trans and would like to put it in my '71 C10 2wd with a 305 and th350.
What all do I need to make this transfer....wiring, torque converter, speedo gear, linkage and shift indicator for overdrive trans?
Shorter driveshaft?
Thanks in advance for any info or advice

Last edited by airbrushguy; 10-09-2018 at 12:50 PM.
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Old 10-10-2018, 09:49 AM   #2
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Re: Swapping a 700R4 for a th350...what's needed?

It will bolt to the engine so that part is easy. The torque converter may not bolt to your original flexplate holes, I forget. If not an '83 K20 with 350 had a 700R4 option and was still 2-pc rear seal. The torque converter to flexplate bolts are metric so be sure to get the right ones. You can move your rear crossmember. You will need a TV cable and appropriate carb bracket. If it is a Quadrajet you can use the the carb bracket from any of the 83-87 trucks, vans, Caprice etc that had 700R4 transmissions. Your carburetor may or may not have a place to attach the TV cable, but it is CRITICAL that it be hooked up and adjusted. The TH350 yoke will work, but the driveshaft will have to be shortened. How much kind of depends, so you pretty much have to get it installed and measure. You can get a lockup kit from Painless.
'83 K20-TPI
'79 C10-ex-diesel(SOLD)
'01 Tahoe
'07 Tahoe-DD
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Old 10-11-2018, 11:55 AM   #3
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Re: Swapping a 700R4 for a th350...what's needed?

Thanks, the engine is a warmed-over 1988 305 and the th350 trans in it right now was original to this truck when it had a 6cyl.
Not sure but I think the 700R4 was from a Gen3 Camaro around '91-'92
Carburetor is Edelbrock 4bbl

There is no torque converter so I would need a suggestion for that...daily driver.

Last edited by airbrushguy; 10-11-2018 at 12:04 PM.
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Old 10-14-2018, 05:20 PM   #4
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Re: Swapping a 700R4 for a th350...what's needed?

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Old 10-14-2018, 06:42 PM   #5
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Re: Swapping a 700R4 for a th350...what's needed?

Hi ,

I have done a couple of TH700 conversions, in different vehicles, so much of what I have learned will cross over to your conversion.

If you plan on using the cable driven speedo, you will need to match the drive speed, the cable will fit onto the drive, but the T350 drive won’t fit the 700.

You need to count the driven and drive gears on your existing speedo gears, work out the ratio, then check the drive gear in your th700, then pick the driven gear to match the ratio, once you know the size of the driven gear, then you can get the correct housing, there are 2 sizes, one for gears up to 39 tooth, and one for 40-45 tooth.

The drive yolk is the same on a th350 and 700, but your tail shaft will be too short as the factory TH350 is the short housing, so you will need to have the shaft shortened, best measured once the box is in.

The rear cross member will need to be moved or modified or moved rearwards, the th350 mount CL is 20-3/8” from the bellhousing front face, the TH700 is 22-1/2”. The rear mount is the same.

The shifter should be able to be used, you will need to check the selector length to get the quadrant to line up with the gear selected.

The reverse lights and NSS should all be fine.

The best TC for a heavy street driven car is a large dia, I use 12” with standard stall, I don’t have a big cam, I don race my car.
The cam in your engine will help dictate what size stall you go, but keep the as low as you can, it makes for a better street car when its heavy.

I don’t know what starter you have, but to run a 12” TC you may need to run a 168th flex plate, I am not familiar with the 305, if it’s a one piece rear seal, so do your research there before you buy a flex plate.

The TC lockup can be done with a few kits, but to be honest, if you can run a few wires and read a print, you can do it yourself.

I run a vacuum switch at 8-10in hg, this drop out the lock-up at under 8 in hg, so it can spin up as it should. TCI make one, about $25 at summit
Brake switch, to drop out lock-up while on the brakes, so if you lock up the rears in an emergency stop, the engine wont stall.
I have a master on and off switch, but never use it, it just gets left on.
I have the over temp switch wired through a small dash mounted LED, so if it gets hot, light comes on, never has, factory use it to lock the TC and help with cooling if trans gets hot, it uses the ecu to control it. I just have the light, if it did come on, I can change my driving to allow cooling.-

You will need to change some wiring inside the gearbox (pan off) much easier done with the box on the ground. I will hunt down my wiring diagram if you like.

The cooler fitting into the box should both be NPT (gas) so the will swap out, the lower port is the outlet (hot)

Dip stick is different, I use a later o’ring sealed aftermarket one, just got tired of the leak from the factory rubber grommet ones, your choice.

I don’t know what floor hump you would have, the 700 is a little larger in dia, but not much, so clearance should be ok.

Things to keep in mind, ensure the output shaft is kept at the same height so the driveline angels don’t get messed up if you make a new rear mount.
TH 700 Torque Con bolts are 10mm metric, don’t try and jam a 3/8 unc in there, you will be disappointed.

Just saw the donor trans is from a Comaro (1991-92) that's good, the later boxes are much better, but you make need to change or modify the governor to get it to up shift without revving out to much, the lighter car will accelerate faster and it suits the drive style, if you want more of a cruiser, then an SUV governor, with help get the 1-2 partial throttle up shift a little lower in the rev range. you can also by a tuning kit for the Gov with different springs and weights, I jsu added weight to the bobs on mine, worked perfect.

The TV cable is also important, heaps of information on line, make sure you get it right.

What carby are you running? many have suitable TV cable hook-up spots on them, the Qjet lower hole would be pretty much spot on.


Last edited by Chris_oz; 10-14-2018 at 06:51 PM.
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