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Installing a 700-R4 in a '77-'81 Corvette

Later (And Better) Transmission For Older 'Vettes

Andy Bolig Apr 1, 2001

Step By Step

This is why we narrowed this swap to cover only these years. A TH350 was used between ’77 and ’81, and earlier cars used a TH400. The mounts will be different for the TH400 cars.

We removed the transmission mount from the crossmember by using a cutoff wheel to remove the welds.

You could also use a small grinder.

With the mount for the transmission and the emergency brake out of the way, we were able to mark out where we wanted to cut out the crossmember for clearance.

The front of the cutout will be 3/8 inch lower than the back. We used the hole as a measuring point to make everything consistent.

A pneumatic cutoff wheel works best for removing the section of the crossmember. Remember to wear safety glasses!

We trial-fit the transmission and the emergency-brake mounts, and tack-welded them in place along with a piece of metal to fill in the gap we cut out of the crossmember. Always tack-weld and then check everything before you finalize all of your welding. This makes it easier if any of the parts need tweaking later. Once you’re sure that everything will fit properly, go ahead and finish welding all of the mounts.

Once we were sure that EVERYTHING (including the emergency brake) was going to work properly, we painted the crossmember.

Here you can see the new mount with the transmission installed. Also, don’t forget to fill the torque converter before you install the transmission. Since you already have the transmission out of the car, take the time to install a new transmission rubber mount; there really is no better time. You can see the added bracing we installed on the bottom side of the crossmember mount. Take your time and fit things properly—you’ll have a finished product that’s strong and looks great.

Pay particular attention to the emergency-brake cable coming down from the floorboard. Make sure there’s enough clearance that the driveshaft won’t interfere with the cable. Also make sure that the mount for the cable wheel does not make contact with the floorboard of the car.

The reason for having to cut out some of the crossmember is to make clearance for the 700-R4’s larger speedometer gear.

We told Don Marcone at Fourth Gear what rear gear ratio we were running, and he installed the proper gears for our application.

The speedometer cable connected right up to the transmission. You can see how much larger the 700-R4’s housing is compared to the old TH350.

Make sure your crossmember has enough clearance so that these parts are serviceable should the need ever arise.

We had to shorten our driveshaft 1-1/2 inches to compensate for the longer tailshaft on the 700-R4, and had it balanced again.

Once again, make sure you have plenty of clearance for the emergency-brake cable.

We finished up at the rear of the transmission by installing the emergency-brake cables.

We put new transmission-fluid cooler lines on the car. These little sections that go into the radiator are often overlooked or neglected. We ordered the correct pieces from Corvette Central rather than use straight pieces bent to fit.

The shifter had to be modified because of the extra detent needed for Fourth gear. This is done by drilling out the welds that hold the shifter gate in place and bolting in the new gate supplied in the shifter-upgrade kit we got from Fourth Gear.

Along with the shifter upgrade, Fourth Gear supplied us with a new gear-shift cable.

Bolting up the new cable was a snap using the supplied brackets.

All we had to do was feed it through the floorboard.

Fourth Gear supplies a T.V. cable with every transmission they build. The function of the cable is to control line pressure, shift points, shift feel, part-throttle downshifts, and detent downshifts.

It’s an adjustable unit that simply slips over the pin that protrudes from the transmission.

Make sure you install the seal that goes between the T.V. cable and the transmission, or you’ll have a leak.

One of the improvements that every automatic-equipped Corvette since 1982 has over the others is the 700-R4 transmission, which contains an Overdrive (Fourth gear) and a torque converter clutch providing for better fuel mileage and more flexibility when deciding on a differential gear ratio. Many owners of the earlier sharks would like to take advantage of the newer technology, but updating to a more modern 700-R4 will take some modification. The Corvette Clinic was doing an upgrade to a 700-R4, and we rode along to show our readers just what Chris Petris does to address several issues that occur when doing such a swap.


Corvette Central
Sawyer, MI 49125
Corvette Clinic
Sanford, FL
Fourth Gear Inc.
Apopka, FL

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