700-R4 Corvette Transmission

A Loyal Companion...With The Right Parts

Andrew Bolig Feb 12, 2000 0 Comment(s)

Step By Step

First, Don inspects the hard parts of the transmission. All the bushings, clutches, and thrust washers/bearings are replaced, so they're not of terrible importance except if drastic wear has damaged the hard parts they interact with. Check the pump, reverse input drum, gearsets, and sprag and roller clutches for damage and scoring. If they show any signs of wear, replace them.

In the front of the transmission is the pump. This is the heart of the transmission.

Don uses only pump bodies that have casting numbers ending with the numbers 732. They’re used on 700-R4s built after late 1986, and have a small lip cast into the housing to prevent the front bushing from sliding forward and causing a front seal failure.

Check for any signs of scoring or wear on the inside of the pump housing and replace it if any are evident. If your torque converter goes bad, this is where it gets damaged first.

The pressure-regulator valve is in the pump housing. Don would not reveal his sources, but he installs larger diameter valves and a 9-pound spring instead of the 4-1/2-pound spring used by GM. Don's replacement valve also uses an O-ring to help prevent any blow-by and to ensure firmer shifts.

Also inside the pump housing is the rotor. If you already have the 732 housing, you'll have the 10-vane rotor (shown on the right with the fins installed). If you don't have the 732 housing, the 10-vane rotor is another reason to find one. They were used on all GM 700-R4s after 1987. Make sure that the rotor is not cracked or shattered.

Don replaces the rings next to the rotor with aftermarket rings, which are hardened for longevity. They have excellent durability, and this is the time to replace them.

Check the surface of the reverse-input drum carefully for signs of wear. Because of the non-adjustability of the band that contacts the drum, there is no way to resurface the drum without having adjustment problems or having the drum crack at the most inconvenient time. If you see any signs of wear, replace the drum.

GM used several servos in the 700-R4 transmission. The Corvette transmission's band-apply servo (the one shown on the right) is the one that's the most desirable. It will provide cleaner, firmer shifts. You'll notice the inside bore is considerably smaller than the others.

The governor assembly from your Corvette is weighted to provide the shift qualities that you would expect from a Corvette. The weights are lighter, resulting in slightly higher-rpm shifts.

The input sprag should be inspected for wear. If any exists, replace the sprag. The early style sprag is shown on the right and the later style sprag is shown on the left.

These are the planetary gears of choice. They're easily distinguished by the oil slinger seen on the top of the unit. What you can't see is that the gear shafts are cross-drilled for better oiling and durability.

Don replaces the six-clutch pack with a nine-clutch pack for greater durability and less chance for slipping. Clutch plates are thinner to allow for the added number of clutches to increase the friction surface area.

By the early 1980s, Chevrolet knew of the mandates on the horizon that would require more from the vehicles it turned out each year. Fuel-mileage regulations were a concern along with the fact that America was not willing to let the Corvette’s performance slide down the tubes simply to please the emissions bean-counters. Corvette enthusiasts needed a reason to buy a Corvette rather than an econobox (which would cost much less). Chevrolet had to pull a new ace from the hole that would please both sides and continue the heritage that Corvette owners have come to hold so dear. The 700-R4 transmission was part of the overall plan.

From a performance standpoint, the 700-R4 had a lower First gear than a typical TH-350 or 400, which helped Corvette acceleration "get up to speed" with the driver's expectations, and the Overdrive in Fourth gear pleased those who had miles-per-gallon signs in their eyes. The 700-R4 proved equally pleasing to both sides, and horsepower was steadily increased each year after 1982, when the first 700-R4 was introduced. But transferring that added horsepower to the rear wheels was not without a price. Pump failures, leaking seals, and loose bushings were some of the dependability problems that were straining the relationships between these transmissions and their owners. GM designers wouldn't sleep well at night until this transmission became as good as it was intended to be.

Over the years, several upgrades were made to the transmission housing, pump, and internals, which have made the 700-R4 a perfect choice for a long-lived relationship. Corvette Fever asked Don Marcone of Fourth Gear Inc. in Apopka, Florida, what it takes to make the 700-R4 transmission dependable for everyday service.

The Corvette line has always received the best there was to offer, so the first choice for a rebuild-quality transmission is probably already in your Corvette, if it was built in or after 1987. By then the problems that dogged the 700-R4 had been addressed. If your Corvette is earlier than 1987, you can look at the build date stamped on a flat spot at the mating surface of the transmission housing and the pan to see whether your transmission has been replaced with a newer unit. If you find you still have the older transmission, you can use some of the upgrades and benefit from GM's experience, but they will be found in a newer housing that you will have to locate anyway. Items like the improved governor assembly and valve body, along with higher spring pressures, are all benefits of finding (or using) your original Corvette transmission.

Don sells transmissions on an outright basis and can design a 700-R4 transmission to suit your particular needs. The customer receives a new TV cable, fluid tube, and dipstick with every transmission.

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