During third-generation Corvette production, a variety of transmissions were offered. They include 3-speed manual, 4-speed manual, 3-speed automatic, and 4-speed automatic.
In 1968, the base price of a Corvette included a 3-speed Saginaw manual transmission. The options for that year included a Muncie 4-speed wide- or close-ratio manual transmission and an M40 automatic transmission. If an automatic was ordered, small-blocks were fitted with the T-350 and big-blocks came with the T-400.
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The 3-speed manual was also included in the base price of a 1969 Corvette, but was replaced with a wide-ratio 4-speed in 1970. At the end of C3 production in 1982 only a 4-speed automatic transmission was available.
Much as engine choices were reduced during the C3 production cycle, transmission choices were slowly eliminated to help meet stricter emissions and fuel mileage requirements.
During production the C3s were equipped with several manual transmissions. This included 3-speeds, 4-speed wide-ratio, and 4-speed close-ratio. They were built by Saginaw, Muncie, and BorgWarner. The only year a manual transmission was unavailable from the factory was 1982.
One of the more unusual transmissions fitted to C3 Corvettes was the Saginaw manual 3-speed transmission (2.85 first, 1.68 second, and 1.00:1 third). They were part of the standard equipment base price for all 1968 and 1969 Corvettes. The 3-speed was installed behind the standard 300-hp 327-ci engine in 1968 and the 300-hp 350-ci engines in 1969. Dealers used this basic equipment package to offer customers rock-bottom Corvette pricing. In 1968 a total of 326 3-speed Corvettes were built; in 1969 the number fell to 252 for this rare shifter.
Muncie 4-Speed Manual
Muncie 4-speeds were available for all Corvettes from 1968 to 1974. They carried the option codes of M20, M21, and M22. These manual transmissions were optional or standard for all of these model year Corvettes. Muncies can be distinguished from other manual transmissions because their reverse lever is mounted in the extension housing instead of the side cover. The main difference between the Muncie and the BorgWarner is that the Muncie has a seven-bolt side cover and the BorgWarner has nine bolts.
The Muncie M20 was a wideratio 4-speed (2.56 first, 1.91 second, 1.48 third, and 1.00:1 fourth) manual transmission. This was the most widely produced transmission installed into early C3s. It was optional in 1968 (10,760 delivered) and 1969 (16,507 delivered). The M20 became standard in 1970 and started being phased out during the 1974 production cycle. It was replaced by the BorgWarner M20 wide-ratio 4-speed.
The Muncie M21 was first offered as an option in 1968 and fitted with close-ratio gearing (2.20 first, 1.64 second, 1.28 third, and 1.00:1 fourth). The M21 close-ratio was available until the end of the 1974 production cycle, and then the BorgWarner close-ratio 4-speed replaced it.
The M22 is a strong and reliable gearbox. Produced in low volume from 1968 to 1971, the M22 earned the nickname “Rock Crusher” because its straight-cut gears made a distinctive noise while driving. It was designed for maximum performance and could usually be found installed behind L88s and other high-torque big-block engines. Only 336 were installed at the factory. It was very popular with road racers and came with the same gear ratios that were installed into the M21 close-ratio transmission.
The BorgWarner ST-10 4-speed featured an aluminum case and was phased into Corvette production in early 1974. By 1975, it replaced the Muncie completely. Called the Super T-10, it was offered as a close-ratio (M21) with a 2.43 first, 1.61 second, 1.23 third, and 1.00:1 fourth. The M20 wide-ratio version had 2.64 first, 1.75 second, 1.34 third, and 1.00:1 fourth.
In 1979, the 4-speed option code was changed to MM4, and it was available in both close- and wideratio versions.
In 1980 and 1981, only the wideratio 4-speed MM4 was available and its ratios were 2.88 first, 1.91 second, 1.33 third, and 1.00:1 fourth. This transmission had wider gear spacing to help increase fuel economy.
The last year the optional L82 engine was available with a 4-speed was 1979. During its last year of production in 1980 the L82 engine was only available with an automatic transmission.
BorgWarner transmissions for Corvettes have a 26-spline input shaft and a 32-spline TH400 output shaft. The 1982 output shaft on the TH350 used a 27-spline output shaft and had a 1-inch countershaft diameter. With the correct bellhousing that matches each transmission’s bolt pattern they are compatible with GM 327, 350, 427, and 454 Corvette engines.
Troubleshooting Muncie and
BorgWarner 4-speeds If you’re having difficulty shifting the 4-speed in your C3, first check to determine whether your engine and transmission mounts are loose.
Raise the car safely in the air and set it on safety stands. Then place a floor jack under the transmission while putting a piece of scrap wood between the jack pad and the transmission. Slowly lift the jack while observing if the engine or transmission moves up or down. Any movement indicates the motor and transmission mounts are loose; you need to tighten all of these bolts. If any of these mounts are separated replace the defective mount.
Next, check the clutch pedal travel. Run the engine at idle with the transmission in neutral and the clutch engaged. Let the clutch out, wait about 10 seconds, engage the clutch, and put the transmission in reverse. A grinding noise indicates the clutch is out of adjustment or some other clutch problem. The shifting forks and synchronizers are prone to wear in these units. This causes gear rattle, difficulty engaging a gear without grinding, and the tendency for the gear to pop out of its position. Faulty clutch and shifter adjustment also impact shifting smoothness.
Always check the fluid level in these transmissions, as low fluid also causes erratic shifting. Shifting rods that are out of adjustment is also a common reason that a transmission is hard to shift.
Aftermarket Overdrive Transmissions
If you are planning on doing a lot of highway cruising in your restored Corvette now might be the right time to consider installing a 5-speed manual transmission, such as the Tremec TKO.
One of the GM Turbo Hydra- Matics is also a good choice. In 1968, Corvette’s automatics underwent a major redesign. The new M40 Turbo Hydra-Matic was introduced in two versions: the TH350 and the TH400. Both carried the same option code. The factory installed the correct transmission type based on the type of engine ordered. The former 2-speed Powerglide was discontinued in favor of this 3-speed unit.
American Powertrain offers its Tremec TKO 5-speed with a .064:1 fifth gear. The ProFit system for the C3 makes installation much easier if your car is already equipped with a manual transmission. You can use the existing bellhousing and clutch with this conversion if they are in good working condition, which saves money.
This conversion requires a new driveshaft, which comes with the ProFit system. The new transmission is longer than the GM 4-speed so a shorter driveshaft is required to align with the rear differential housing. This conversion provides a huge increase in fuel efficiency.
A C3 with a final axle ratio of 3.55:1 turns about 3,000 rpm with the original 4-speed at 70 mph. After the Tremec is installed, the engine operates at 1,900 rpm when traveling 70 mph. This adds life to your engine and greatly improves your fuel mileage. You will also notice how much quieter the car is during cruising.
The TH350 automatic transmission is designed for small-block Chevy and Corvette applications. It weighs 120 pounds and is 21.75 inches long. Its case is cast aluminum and features a 2.52 fi rst, 1.52 second, and 1.00 third. It has a distinctive oil pan that is recessed on the right rear side of the case.
This popular transmission has a good reputation for strength and durability. It can be affordably rebuilt with better components. For example, shifter kits can provide quicker, firmer shifts with less slippage and reduced clutch heat.
You can also change the torque converter to improve your transmission’s horsepower capabilities. Aftermarket transmission rebuilders can provide options on how much your TH350 can handle. Gear Star offers three power-output stages for refurbished TH350 transmissions, ranging from 400 hp/400 ft-lbs of torque to 500 hp/450 ft-lbs of torque to 700 hp/550 ft-lbs of torque.
The TH400 automatic transmission is designed for Corvette big-blocks with engines rated up to 400 hp. It tips the scales at 135 pounds and features a cast-aluminum case with a 2.48 first, 1.48 second, and 1.00:1 third. In 1969 a heavy-duty version of the Turbo 400 became available for the L71 (435 hp), L88 (430 hp), and L89 (L71 with aluminum heads). A Y code in the serial number identifies them.
This transmission continues to be a favorite with drag racers because it is so strong and reliable. Aftermarket suppliers offer refurbished 400 transmissions with three power options: 450 hp/425 ft-lbs of torque, 600 hp/550 ft-lbs of torque, and 1,200 hp/1,000 ft-lbs of torque.
Transmission Inspection and Testing
Step-1: Disassemble Transmission
This TH400 has been completely disassembled by transmission rebuilder Gear Star. Each part is inspected and replaced if it shows any wear or damage. The torque converter can be seen at the left, and the valve body is also on the left. The case that is sitting on the right will be completely disassembled and cleaned of all its grit and grime in a high-pressure washer then pressure tested for any cracks prior to being rebuilt.
Step-2: Clean Case and Parts
Once all of the parts have been removed from a TH 400, Gear Star puts the case into this cleaning basket. The basket spins and hot steam cleans all surfaces until they are completely free of all contaminants. The case is then tested to make sure it has no defects before it is overhauled.
Step-3: Check for Wear or Damage
Any component, such as this forward clutch drum, that shows wear or damage will be replaced. The extra horsepower capability that is built into these refurbished transmissions requires the parts to be capable of handling this extra stress.
The intermediate steel (left) and intermediate friction (right) clutches both show wear and will be replaced. Clutches like these can be found in 350 and 400 transmissions and are very prone to wearing as the miles pile on. It is good practice to replace any parts that show wear.
Step-4: Choose Torque Converter
The stock GM TH400 torque converter is on the right. The smaller, higher pressure Gear Star torque converter is on the left. These smaller converters are designed to hold more pressure and can handle engines that produce up to 1,200 hp.
Step-5: Choose Bell housing
Internally both of these refurbished transmissions are built to the same horsepower specifications. The unit on the left is a Corvette TH400 and the one on the right can be used in a Buick, Oldsmobile, or Pontiac. The giveaway is the different mounting outline on the bell housing.
Step-6: Perform Dyno Test
Here, a hydraulic crane is used to lift and install a TH400 transmission onto the dyno. Without fluid they weigh 135 pounds. Dyno testing is a great way to confirm the transmission’s capabilities under stress.
Gear Star has installed new hydraulic fluid fittings on this transmission. During the dyno test the transmission is filled with the correct amount of fluid to check the mechanical workings of the transmission. The top connector sends fluid to the radiator to be cooled. The bottom connector receives fluid from the radiator that has been cooled. This pumping action is important to the health of these transmissions. Because these units produce a tremendous amount of heat, a good cooling system is vital.
A pair of locking pliers is attached to the transmission shifter to allow the dyno operator to shift the test unit into every gear including reverse. If an abnormalities are discovered the unit is sent back to the shop to correct them.
All of the hydraulic lines have been installed. The dyno test checks shifting, pump pressure, transmission temperature, and gear slippage. A printout of the unit’s performance is reviewed by the engineers for any potential issues before shipping.
This is what a completely refurbished Gear Star TH400 looks like when all of the work has been completed. Each completed transmission is tested on the transmission dyno to ensure it meets the owner’s performance requirements. (Photo Courtesy Gear Star)
In 1982 a new Turbo Hydra- Matic 700R4 was standard equipment on Corvettes. It was the only transmission available and featured a fourth gear overdrive to help improve fuel economy. It featured a 3.06 first, 1.62 second, 1.00 third, and .70:1 fourth. The TH700R4 was strong enough for the 200-hp Cross- Fire 350 engine, but it could not support the higher horsepower levels as did previous automatics installed in earlier Corvettes.
Troubleshooting TH350/TH400/ TH700R4
The first step in the troubleshooting process is to check the fluid level. If it is correct check the downshift cable adjustment. Look for any loose or broken vacuum lines and correct as necessary.
Check the manual linkage to make sure it properly engages the transmission into the correct gear. Inspect the vacuum modulator, which can cause the following: harsh up/downshifts, delayed shifts, and slippage in low, drive, or reverse. If any of these conditions occur remove the modulator vacuum line and probe it with a pipe cleaner to see if fluid is present. If it is, replace the modulator.
Inspect the torque converter, which is a mechanical and fluid coupling that connects the transmission to the engine. It serves the same purpose as a clutch pedal in a manual transmission. When the speed increases, the converter spins and multiplies the torque inside the unit. It is important to understand that a one-way or sprag clutch is used in a torque converter. When the converter pins, it spins faster than the engine turns. When the converter slows as the engine speed decreases, it must “catch” or stop so the engine can drive it again. This sprag clutch design lets this happen. It allows the converter to race in one direction only and catches against the clutch in the opposite direction.
A defective sprag clutch won’t allow your Corvette to go into low or reverse gear. If your transmission is full of clean fluid and is slow going into gear (or not at all), the problem could be the converter.
To check for a bad converter, the transmission must be removed. The converter has engagement tabs and a spline. Use a couple of long screwdrivers and wedge one of them into the spline to hold it stationary. Try to turn the tab with the other screwdriver. It should freely spin in one direction and hold firmly in the other. If it doesn’t, the converter is faulty.
If the transmission does not move at all, also look at the pump pressure. This is rarely the problem because most pumps are very reliable, but a quick check is a good idea. The following is not a precise test because a pressure gauge should be installed, but it gives you the needed information.
Loosen a transmission cooling line (most are at the radiator) and start the engine. Wrap a rag around the fitting and place it into a container to prevent fluid from going everywhere. A sound pump sprays fluid under a fairly high pressure. If you don’t see a gusher of fluid, you have a pump problem. Be very careful when performing this procedure, and only run the engine a few seconds to prevent running the transmission dry.
Before condemning the pump, though, remove the service pan and check the transmission filter. A plugged or dirty filter can cause low pressure problems.
If you decide to investigate further into your transmission I recommend How to Rebuild and Modify GM Turbo 400 Transmissions by Cliff Ruggles.
Written by Walt Thurn and Posted with Permission of CarTechBooks