TH350 to 700R4 Swap

[tadslc]

What is involved with this swap?

Thinking of getting a crate motor and figure I might want to add a gear while I'm at it.

1981 C10 SBC



Thanks,

 

[Preston Tucker]

Not too much changes if i am correct. I have a bunch of 700r4 stuff and th350 stuff and most of it looks the same. I dont have a truck with a 700r4, just parted a few.

 

[SquareRoot]

What is involved with this swap?

Thinking of getting a crate motor and figure I might want to add a gear while I'm at it.

1981 C10 SBC



Thanks,

Get a Nv45000 and add two instead. Time and money. Carry on.

 

[tadslc]

Get a Nv45000 and add two instead. Time and money. Carry on.

Isn't that a manual trans? I'm pretty sure I'm looking for an auto.

 

[Dutch Rutter]

A 700r4 will sacrifice some dependability and simplicity for an OD gear ratio compared to your th350.

An NV4500 is a pretty easy swap and nets best of both worlds but in a 5 speed package.

If wanting to keep an auto most would rather skip the 700r4 and go to a 4l80e. One of those "do it right once/cry once" kind of scenarios.

This is coming from someone who has a 700r4 and has needed it rebuilt 2 times now.. I will be swapping to a nv4500 in the distant and dreamy future.

Edit: There is a other option here, add a gear vendors OD unit on the back of your th350.

 

[75gmck25]

The TH700R4 is not the same length as a TH350, so you will need to shorten your driveshaft. You also need to make sure you know how to install the proper TV cable to replace your current kickdown cable. The TV cable must be installed and adjusted correctly to avoid screwing up your TH700R4.

The later models of the TH700R4's (I think you want one that is about an '87 or later) were more durable, and later they got the 4L60 designation. When they added electronic controls it became the 4L60E. I believe you can get a stand-alone electronic controller and use the E model with a non-computer vehicle, but I'm not positive.

You might also want to take a look at how well your current rear-end ratio will work. If you've got something like a 3.07 axle, the overdrive transmission will drop your highway RPM down really low, and this might make it really boring to drive.

Bruce

 

[Turbo4whl]

I have done this swap years ago. What I remember; The long shaft Turbo 350 is less than an inch as long, as the 700R4. The cross member is a different part than the T350 cross member, but you can make it work if you cut or grind a notch for the 700R4 pan to clear on one side. The cross member needs to be moved back a few inches. There are holes in the frame for the 700R cross member, but if you use the T350 cross member you will have to drill 2 holes.

As stated above, you need to swap the T350 kick down cable for the correct Throttle Valve cable. The carb mount for the cable is also different.

You will need to change the convertor lock up electrical plug. The T350 has a 2 wire plug, the 700R4 has a 4 prong plug. The 700R4 only uses 3 of the 4. The new wire added goes to ground.

Very important, the convertor lockup must work correctly. If the circuit was not working on the T350, no big deal, you just don't have lock up. If it does not work on the 700R4, you will damage the trans in a short time.

One of the best features of the 700R4 not stated in this thread, it has a lower first gear compared to the T350. So lower first and overdrive. The later models will hold up just fine with your 305 engine. If you plan on putting a different engine in with more torque and horse power you may have some issues with durability.

 

[Matt69olds]

Since the 700 trans was common in these trucks later in the production, I would think the parts are easy to find. With the right parts it should be a easy swap.

There is no reason a property built 700 shouldn’t live a long life behing a mild small block. They aren’t nearly as bad as people make them out to be. It’s absolutely required that the TV cable be adjusted correctly. Close enough won’t work.


Look for a core with the ST Louis arch over the servo cover on the passenger side. Have it built with a wide band, upgraded 3-4 clutches, Sonnax drive shell, upgraded input sprag, and a good shift kit.

The 700/4L60E has been around since 1982. Any transmission shop should be very familiar with building one.

 

[peats]

I have that 350/700 setup in my '81 with 3.90 gears and i consider it a very comfortable setup. Taller gears are going to struggle in lockup, particularly with a smaller(305) engine IMHO

 

[enrico]

I did (mechanic shop did) the 350 to 700 swap about a year ago....trans was from a 89 van....bolted right in after moving the crossmember.....driveshaft stayed the same....lower 1st gear is nice, as I am still working with the original, stock, tired 350. The lower rpms on the highway were the primary reason for the swap....I'm at 1500-1750 rpm at 55-65, which makes for a nice ride. I do alot of highway.

HOWEVER....as was mentioned in the previous posts....the biggest problem we ran into was the linkage to the carb.....that took several trips to 2 different shops to get correct......actual transmission shops did not want to take on the swap to begin with....they all told me to leave it alone.

A year later, happy i did it.

 

[tadslc]

Thanks,

I also plan on swapping in a mild GM crate 350 at the same time.

What about the column shift? Been trying to research this for hours but haven't really found any definitive information.

 

[enrico]

column shift lets me get all the gears....they dont line up exactly with PRND21, but you know where they are....my 18 yo daughter is the only one who complains about it when she drives it. actually saw a shift indicator on ebay with D321 on it for the later model squarebody....i may try that this summer when i park the truck

73-87 Chevy Truck Instrument Cluster Shift Indicator PRND21 K5 Blazer GMC OEM | eBay

 

[Bennyt]

We are putting in a 700r4 in my son's truck as part of the build. I didn't have a stock crossmember so I bought a POL one for less than a $100. I didn't have a driveshaft to shorten but found a used one out of a '90's Caprice that was the perfect length for $10. Some junkyards have tons of those Caprice Taxi and Police Cars to grab one out of.

 

[DBinWW]

There is a other option here, add a gear vendors OD unit on the back of your th350.

Is this for a 4WD? I'd for sure forget and rip the gears up...

 

[Dutch Rutter]

Is this for a 4WD? I'd for sure forget and rip the gears up...

They do have them for 4x4 rigs. Apparently, they bolt onto the rear of the Transfer case instead of the rear of the trans. At least that's what they say here.

4x4 - Gearvendors

4-WHEEL DRIVE APPLICATION GUIDE “The Absolute best in 4X4 Engineering” GEAR VENDORS makes specific adaptation kits for each model of transfer case. This is far more costly than other manufacturers who will have you move your transfer case because GM/Dodge/Ford all use the same patterns between...

www.gearvendors.com

Seems similar to the idea of a gear splitter, but instead of low and lower its an overdrive unit.