Many faithful readers will recall the ’68 Bel Air we’d enhanced with a 500-plus horsepower, 460-cu.in. Chevy big-block and TH400 that we were hoping would pull off a reasonable recreation of Baldwin-Motion’s legendary “Street Racer Special.” Their bare-knuckled boulevard brawler was cloaked in a lightweight Biscayne body and motivated by a potent 425-horse, 427 big-block backed by a Muncie four-speed.
The imminently popular Chevy was the everyman’s muscle car. Even with a few upgrades to make it an even more intimidating adversary, it still rang up for less than three grand. While they called it the “Street Racer Special,” we called it “Sleep on the Cheap,” and the combination really resonated with us.
Our tribute featured a TH400 automatic, however, and a set of 3.73 cogs in hopeful anticipation of 12-second quarter-mile passes like the original. But the more we thought about it, the more we realized we’d concocted a single-purpose machine, and the restrictions that posed. Our vision of the completed project took an abrupt turn when we decided that we’d also like to hit the highway occasionally to attend automotive events at distant destinations. While the notion was not entirely impossible, the feasibility factor was lacking; concerns about fuel mileage and high rpms on the road became effective deterrents.
We reasoned that the easiest way to transform our Bel Air into a multi-purpose machine would be to jettison the TH400 and swap an overdrive in its place. But not just any overdrive. In order to keep things a bit more authentic to the theme, we added a third pedal and a Tremec TKX five-speed to offset the 3.73 gear that would remain in the 12-bolt out back.
Reliability is a shared desirable trait, whether drag racing or long-distance running. In our more exuberant youthful days, the exhilarating thrill of bangin’ gears and burnin’ gas almost always offset the possibility of disastrous results. Now, however, our main focus is having fun without mechanical mayhem, whether it takes place on the drag strip or driving down the highway.
The final justification for the swap was seeing a 427-urged, ’68 Biscayne prominently featured in a recent issue of HMM . Unabated drooling and heart palpitations soon followed. So, with that incentive etched into our memory banks, we hit the brakes and began procuring parts necessary for the swap.
Although obtaining the correct clutch/brake pedal configuration was a bit of a challenge, we found and restored a set and supplemented a few parts from Classic Industries to make the pedals fully functional. The process was recently recounted in an HMM how-to article.
The rest of the hardware required for the transmission exchange would have to be sourced from various places, but Summit Racing Equipment was a likely place to begin our search, with auctions and online ads providing the remaining essential bits.
Aside from the pedal setup, we also needed a flywheel, clutch, and bellhousing, plus the throw-out bearing and clutch fork. On Summit’s site we found nearly all we needed with Holley’s transmission installation kit (PN 319-502), which included all the items mentioned. The bellhousing, flywheel, and clutch are all from Hays. With decades of developing racing and high-performance driveline systems, their components allow us to channel the engine’s output without apprehension.
Likewise, we wanted a gearbox with similar capabilities, and quickly pulled up the American Powertrain website to tap into their transmission swapping expertise and to order the hardware we needed. American Powertrain offers complete swap kits for a variety of popular applications and knowledgeable tech support to help overcome any difficulties that might be encountered during the process.
The gearbox we chose is the recently introduced and much-heralded Tremec TKX five-speed. Research indicated it would be up to the task of propelling our 3,300-lb Bel Air down the dragstrip without drama; it’s rated to handle 600 lb-ft of torque and allows gear changes up to 7,500 rpm. The previous TKX installation and subsequent testing in our ’88 IROC Camaro certainly gave credibility to those claims. Perhaps the most attractive feature, however, is the physical size of the gearbox, which easily occupies the space normally reserved for a Muncie. The only required transmission tunnel surgery involved cutting the shifter opening.
We chose to retain the 3.73 gears in the 12-bolt rearend that, when fronted by a Muncie close-ratio four-speed, would seem a great combination for cruising the local hangouts. Adding the overdrive capabilities to the TKX would clearly enhance our over-the-road, out-of-town travels.
Although top gear in the TKX is always overdriven, there are several iterations with various gear ratios, making it easy to select one that’s optimal for your application. Our choice was PN 17805, which channels output through a 2.87:1 first gear and a .68:1 overdrive. Consequently, our first-gear launch ratio is 10.7:1, which is nearly identical to a 2.20:1 first gear in the close-ratio Muncie four-speed when paired with a 4.88:1 rearend ratio. At highway speeds, a theoretical 3,000 rpm drops down to 2,040 rpm, allowing us to deploy our Bel Air as a true dual-purpose street machine, capable of covering long distances and the quarter-mile with equal aplomb.
With our game plan locked in, it was time to spin some wrenches. The process begins with removal of the TH400 automatic and the installation of the new clutch hardware, bellhousing, and transmission, then modifying the transmission crossmember, and having a new driveshaft fabricated. Even though it’s all pretty straightforward stuff, we’re plenty excited about the outcome, so follow along as we complete our Bel Air’s trans-formation by scrolling through the photos below.
Tremec TKX Installation
A proper clutch setup is needed to transfer the big-block’s power through the transmission and eventually to the pavement. We chose an 11-inch flywheel and clutch assembly from Hays, capable of harnessing the 500-hp output without undue drama.
Hays also got the nod for the bellhousing, throw-out bearing, and fork. The bellhousing is a substantially strengthened version of the factory “621” offering and accepts all the original-style hardware.
To establish proper mating of the flywheel to the crankshaft, we dressed down the crank flange with a flat file to eliminate any burrs or high spots. We also used emery cloth to clean up the end of the crank, ensuring an appropriate fit for a new pilot bearing.
To establish proper mating of the flywheel to the crankshaft, we dressed down the crank flange with a flat file to eliminate any burrs or high spots. We also used emery cloth to clean up the end of the crank, ensuring an appropriate fit for a new pilot bearing.
For additional longevity and reduced friction, we chose to install a Hays needle-type pilot bearing rather than a plain OEM-style bronze bushing. Tip: Place the bearing in the freezer for a few hours before installation to allow it to shrink slightly, giving it more clearance for an easier installation.
A clutch alignment tool is included in the Hays clutch kit. It locates the disc in the proper position while the pressure plate is aligned and torqued to the proper specs.
It’s always a good idea to use new pressure-plate bolts whenever the clutch assembly is replaced. The bolts have special shoulders that protrude into the flywheel to resist breakage in the highly stressed environment of power transfer.
Here are the clutch linkage components needed to convert our Bel Air from an automatic to a manual transmission. Although hydraulic clutch kits are available from American Powertrain, we chose the factory setup to maintain an original appearance.
Full-size Chevys of this era used a cross-shaft pivot ball that screws into the bellhousing. Many other Chevys use linkage with a ball that is threaded into the engine block, so consult the assembly manual or an online source to confirm which is correct for your application.
A frame-mounted bracket is used on the other end of the cross-shaft, which is welded in place. Lacking an original, we fabricated one from ¼-inch steel stock using dimensions gleaned from the factory assembly manual
Support for the TKX was accomplished by using a narrower Chevelle convertible crossmember and fabricating adapters that allowed it to be mounted 1½-inches rearward while using the original frame mounts.
With the Tremec supported by a transmission jack in the proper location, cardboard templates were used to establish the size and location of the adapters.
The template designs were transferred to ¼-inch steel plate and cut to size with a plasma cutter. Then the mounting holes were drilled in their proper locations and attached to the frame mounts using 3/8-inch, grade 8 hardware.
Although it may not look exactly like a factory installation, it’s solid and plenty strong. We could have possibly camouflaged our handiwork with a bit of black paint, but the aluminum color presents better in the pictures.
A magnetic angle finder shows that our output centerline is perpendicular to the floor surface. It’s close enough to get us rolling, but the crossmember may require some shimming to optimize the driveshaft and pinion angles.
Since we’re using an original speedometer cable, we purchased this adapter from American Powertrain. After providing the rearend ratio and tire diameter, they fitted it with the proper driven gear so our speedometer will read correctly.
Once the transmission was bolted up and properly positioned, we scribed the underside of the floorpan to help determine the shifter opening location. After removing the transmission, we were then able to drill a marking hole and make the required cut from inside the car.
Leaving the automatic steering column in place and simply removing the shifter handle would have been the easy way out, and less expensive, too. Instead, we purchased an original ’68 Impala floor-shift column to help keep our factory-stock efforts headed in the right direction.
Once the pedals and column were bolted in place, we began reconnecting the wiring and installing the dash trim and original carpet trim pieces at the floorboard.
After cutting a shifter hole in the appropriate location of our new floor hump (see “Hump Day” below), the original carpet was reinstalled, followed by the shifter boot and retainer plate. It’ll take a discerning eye to spot the Hurst shifter handle and black five-speed knob, but at first glance it all appears as factory original.
Regardless of the method for measuring driveshaft length (different shops may suggest different techniques), always follow the recommended procedure and make your measurements with the car supported by floor jacks at normal ride height.
A Gearbox For Gearheads
There are plenty good reasons to choose Tremec’s TKX gearbox, most notably that their compact size that easily slips into a space normally occupied by a Muncie four-speed, plus it’s strength and durability easily exceed that of even the fabled M22 Rock Crusher. On the inside, oversized gear widths and caged needle bearings at all gear positions ensure longevity while multi-cone-cone and hybrid synchronizers assure solid shifts at high rpm. It’s a replacement transmission engineered specifically for the aftermarket and is sure to live up to the expectations of most gearheads with 110 octane coursing through their veins.
Extra ribbing in a beefy aluminum case provides the strength needed to allow a 600 ft.-lb. torque capacity, while provisions for a traditional speedometer cable and reverse lights are engineered into the design.
A Gearbox for Gearheads
The output shaft is splined for a Ford C6 automatic driveshaft slip yoke, which is included with the transmission. The assortment of billet “links” are a part of American Powertrain’s unique White Lightning shifter setup that allows shift lever positioning in 64 different locations.
The White Lightning shifter plate replaces the original Tremec plate, seen here with the link combination that centers our shift lever in the shifter opening.
On the passenger side of the transmission are outputs for an electronic speedometer and a reverse lockout solenoid pigtail that interfaces with American Powertrain’s lockout module, allowing reverse engagement only when vehicle is not moving.
Hump Day
To help our Tremec installation appear factory correct, we needed a proper shifter opening and hump. Unfortunately, originals are extremely hard to find, and none are being reproduced. So, we decided to go with a reproduction 1968-’72 Chevelle shifter hump instead, which is close in both size and appearance. Reproduction shifter boots and bezels are easily found to complete the installation.
The Chevelle shifter hump fit the Bel Air’s transmission tunnel almost perfectly; even the shifter opening was in the proper location. We secured it to the tunnel with pop rivets.
Even though the shifter location was close, we centered the Hurst handle using the appropriate links from American Powertrain’s unique White Lightning shifter kit.
Be sure to seal the shifter hump to the floorpan to keep out any unwanted exhaust fumes or other odors. We used a rubberized spray-on undercoating to seal out the elements.
SOURCES
- American Powertrain, 931-646-4836, americanpowertrain.com
- Classic Industries, 800-854-1280, shop.classicindustries.com
- Hays, 866-464-6553, holley.com/brands/hays/
- Holley, 866-464-6553, holley.com
- Hurst, 866-464-6553, holley.com/brands/hurst
- Summit Racing Equipment, 800-230-3030, summitracing.com
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