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.

Bolt Transmission 13/34

11 Bolting the transmission mount into place. This is where it started to get fun: Not only was the downward-facing mounting bracket on the trans too wide for the mating surface on the crossmember, but with the height of the standard mount, the trans was pushing up against the underside of the transmission tunnel.

Trans 14/34

12 The trans crossmember, after being heavily modified to fit the T56 Magnum. Installing a new mounting surface, and then boxing it back in for strength, created a pocket for the trans mount to sit in. It also lowered the trans, obviating the need to remove fiberglass from the trans tunnel.

T56 Trans Installed Modified 15/34

13 The T56 as installed in the heavily modified factory crossmember. Note how deeply into the crossmember the trans actually sits.

Driveshaft And Flange Style 16/34

14 The driveshaft and flange-style yoke. Since the ’shaft came separately from the trans-installation kit, we needed to measure the length from the rear of the trans to the differential and send in that measurement. A driveshaft with the correct dimensions was then shipped out.

Dragvette Safety 17/34

15 Our Dragvette safety loop worked fine with the original trans but mounted too close to the new T56. As a result, we had to cut off the old mounting bracket and fabricate a new pair. Walden mocked these up in place and then welded them to the loop.

Install 18/34

16 The driveshaft was installed next, and the LS3 was now officially connected to the rear wheels. Since the loop mounted to the bolts that originally held the safety cable for the seat belts, we welded on an extra set of mounts for the cable and bolted it into place on them.

Remove Pedal 19/34

17 The pedal housing has to be removed in order to add the clutch pedal. After dropping the steering column and surrounding parts, we undid the four external bolts that hold the housing in place and removed it. Once out and disassembled, we bead-blasted it and painted it with some dull-metallic-gray Cast Blast paint.

Brake Pedal Return Weld 20/34

18 One advantage to having the housing out was the ability to weld back on this bracket for the brake-pedal return spring, which had broken off some time before. Without the spring, pressure from the brake system will keep the pedal up, though not enough to prevent the brake lights from burning all the time. Prior to this fix, I made do with a long coil spring zip-tied in place.

Stock Clutch 21/34

19 While the stock clutch pedal uses a stud on the side to operate the clutch, we needed a hole to mount the heim-jointed operating rod for our Wilwood master cylinder. I used a wheelie cutter to cut the welds holding the stud on the pedal, then dressed down the cut and painted it black to protect it from rust. Now, the new stud can be bolted into place once the pedal has been installed.

Reassemble Pedal 22/34

20 When reassembling the pedal housing, it’s important to reinstall the rubber bumpers that cushion the pedals’ impact with the housing when released. It’s the same part for the clutch or brake.

Master Cylinder 23/34

21 The master-cylinder kit for the hydraulic clutch assembly includes a reservoir, the Wilwood master cylinder and its pivoting bracket, and all the required mounting hardware. Even so, we wound up having to do a little fabrication to place the reservoir where we wanted it.

Mount Master Cylinder 24/34

22 After locating the holes on the firewall, we used a step drill to open them up so we could mount the master-cylinder bracket. We then used the four bolts to attach it to the firewall.

Install Threaded 25/34

23 The threaded rod that connects the master cylinder to the clutch pedal can now be installed by adjusting the rod to the correct length, then installing the Heim-jointed stud through its hole in the side of the pedal and bolting it into place there.

Reservoir Bracket 26/34

24 Next, we fabricated a bracket to mount the reservoir to the side of our Hydratech Hydroboost brake booster. Part of the hood-latch assembly would ordinarily fit in here, but since we’ll switching to a pinned hood, the linkage has been removed.

White Lightning 27/34

25 Our trans came with American Powertrain’s White Lightning shifter, which uses an offset arm to locate the shifter handle in the proper location in the console. In our case, we wound up having to use a longer offset than the one that came installed.

Tape Over Mounting 28/34

26 After removing the shifter before installing the trans, we taped over the mounting hole to keep out debris. As it turns out, simply peeling back the tape revealed enough of the mounting surface to bolt the shifter in place, without cutting any fiberglass. (We did later trim a strip of ’glass off the passenger side to ensure adequate clearance.)

Factory Style Rubber Shift 29/34

27 While we had ordered a factory-style rubber shift boot (right) to help seal the shifter opening in the trans tunnel, it turned out to be far too small for our purposes. Help arrived in the form of a universal Mr. Gasket shift boot, which we bolted and bent into place.

Screw In Block Off 30/34

28 Removing the rubber seal that the automatic shifter cable passed through left another hole in the trans tunnel. Using a bandsaw, drill, and press brake, we made a block-off plate and screwed it in place, sealing it up with a little black RTV for good measure.

Corvette America Updated 31/34

29 Since the factory data plate was no longer accurate, we ordered a blank plate from Corvette America and had Street Shop’s Ted Whitney engrave it with the correct information for our LS3.

Paragon Corvette Reproductions New Console 32/34

30 We also ordered a new console plate from Paragon Corvette Reproductions, which came with a four-speed shift map that would have been correct for a stock manual ’72. I carefully pried off that map, cleaned the double-sided tape off the plate, and reinstalled the custom-machined six-speed shift map that matches our T56.

Leather Shift 33/34

31 The final piece required to finish off our new console plate was the leather shift boot, which we ordered from Corvette America. We needed to remove the chrome-ball shift knob and Reverse-lockout T-handle to slip the shifter through the boot when installing the console plate.

Shifter Installation 34/34

32 With the correct shift map and data plate in place on our Paragon console plate, we installed the latter item over the bare shift handle, then reinstalled the ball shift knob and T-handle. All that remains now is to get the motor plumbed, wired, and running again so we can start rowing through the gears.

Sources

American Powertrain
(931) 646-4836
www.americanpowertrain.com
Dragvette Products
(877) 422-0943
http://www.dragvette.com
Paragon Corvette Reproductions
Swartz Creek, MI
https://www.paragoncorvette.com/
Corvette America
Reedsville, PA
www.corvetteamerica.com
Street Shop, Inc.
Athens, AL 35611
256-233-5809
www.streetshopinc.com
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