1972 Chevrolet Corvette Scarlett Project Car - Six-Speed Manual

More Gears, More Fun - Installing an American Powertrain six-speed manual in our '72 project car

Jeremy D. Clough Jun 2, 2014 0 Comment(s)
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At this writing, "Scarlett," our '72 coupe project car with pretensions to track-day glory, has been in a fragmented state at Tray Walden's Alabama-based Street Shop, Inc. for about six months. During that time we've been working sporadically, mostly on weekends, to exchange her sweet docility for a Smaug-like ferocity. She was a nice, classy driver, great for road trips and lots of fun during long runs through the mountains, but in the end, I just had to build a race car.

So far, we've stripped her to the firewall inside and out, shoehorned in a 600-plus-horse LS3 416, and bolted up a set of polished-stainless side-mount headers. Now it's time to cover what connects the output shaft of that LS3 to our not-quite-wide-enough rear tires, which means installing the transmission and clutch. This job will be a little more involved than usual, not least because we've laid aside the Turbo-400 automatic in favor of a stick shift. And instead of the three- or four-speed manual you'd expect to find in a chrome-bumper shark, we're upgrading to a fully modern six-speed.

Sourced from American Powertrain, the T56 Magnum is a heavy-duty variant of the T56 six-speed that appears in a number of American performance cars, including '97-'07 manual Corvettes. While the OEM transmissions have the shifter location set up for whatever car they'll be installed in at the factory, the Magnum is designed specifically for the aftermarket, with more variation in shifter location. It's also rated to handle 700 pound-feet of torque, which gives us a comfortable margin of safety over the 543 lb-ft our LS recorded on the engine dyno.

While there are several sources for the trans itself, American Powertrain offers a ProFit conversion package that includes the parts needed to install one in an older Corvette, including an automatic. In our case, to the basic conversion kit we added a massive, steel Quicktime bellhousing, a Science Friction Street Slayer clutch rated to 620 lb-ft, and a hydraulic throwout bearing, which eliminates the need to locate and install a mounting point for the factory-style clutch fork. While the Magnum comes in either close- or wide-ratio form, we ordered the close-ratio version, which features a double overdrive (both Fifth and Sixth gears are overdriven) with a 0.62 final gear.

1972 Chevrolet Corvette 2/34

While the automatic transmission has certainly come a long way, I still prefer a manual transmission in a sports car. For Scarlett, we selected the T56 Magnum, a double-overdrive six-speed that we sourced from American Powertrain.

Performing this conversion at Street Shop meant having ready access to both a wealth of specialized tools and a corps of skilled Corvette techs. While the job is covered in some detail in the accompanying photographs, this article is intended to serve as more of a thorough overview than a step-by-step instruction manual. With that in mind, we strongly recommend having the work done at a shop whose mechanics are experienced in custom drivetrain work, rather than attempting to do it yourself.

Once everything was bolted together, I set the Corbeau driver's seat back in the car to make sure I could reach the shifter comfortably. Andy Berryhill walked over to where I sat firmly strapped into the seat with the five-point harnesses, with my hand on the chrome ball shift knob and slowly working it through the gears.

"You know you're supposed to make engine noises with your mouth, right?" he asked.

1972 Chevrolet Corvette Gearshift Three 3/34

01 Where we started, with the gearshift for the three-speed automatic and the center console in which it sat.

1972 Chevrolet Corvette Turbo 4/34

02 The old Turbo-400, post extraction. No doubt it served me well, but this is a car that always needed a manual transmission.

T56 Magnum Six 5/34

03 Sourced from American Powertrain, the T56 Magnum is a heavy-duty variant of the Borg-Warner T56 six-speed used in Dodge Vipers, GM F-Bodies, and some C5/C6 Corvettes.

Clutch Disc Alignment 6/34

04 Installing the clutch disc using the alignment tool, which slips into place in the pilot bearing, basically lining up the clutch with the crankshaft.

Align 7/34

05 Once the clutch disc has been properly installed against the flywheel and aligned with the crank, the clutch itself is bolted into place on the flywheel.

Quick Time Bell 8/34

06 The heavy steel Quick Time bellhousing, whose 0.75-inch-thick steel gives an added measure of protection against anything that might break loose inside. While the bellhousing is usually bolted to the transmission and then to the block, it’s wise to first install the housing on the motor to check for concentricity, preferably with a dial indicator.

Hydraulic Throwout 9/34

07 Before the trans can be installed on the motor, the hydraulic throwout bearing has to be installed on the transmission input shaft and shimmed to the correct height. Since it’s hydraulic, the throwout does away with the traditional clutch-fork arrangement, and with it the need to locate the mounting point for the fork.

Hydraulic Throwout Braided Steel 10/34

08 Instead of a fork, the hydraulic throwout has two braided steel lines, both of which pass through an opening on the driver side of the bellhousing: One routes down from the master cylinder to the bearing, and the other is used to bleed the system.

Clamp Lower Bleeder 11/34

09 Once the throwout was in place and shimmed properly, and the bellhousing had been bolted back on, Tray Walden bolted a small clamp to the body of the trans to hold the lower bleeder line firmly in place.

Bolt Trans To 12/34

10 With the clutch and throwout installed, and the bellhousing firmly bolted back to the transmission, we used the crane to lift the trans and bolt it to the back of the motor prior to installing them both as a unit. They can go in separately, but I’d much rather have the additional working room provided by doing the job on a bench and putting them in together.

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