For several years now, owners of '65-'70 Mustangs have been taking advantage of the overdrive capabilities of modern five-speed manual transmissions. But it's been more difficult for '71-'73 Mustang owners to make the swap due to the different clutch linkage. When Wayne Farnham took on the challenge of replacing the original four-speed in Brit White's '73 Mach 1 with a Tremec TKO five-speed, he figured out what was needed, then called us to come over to his Clearwater, Florida, Restomod Plus shop to document the procedure.
A modern Tremec TKO five-speed...
A modern Tremec TKO five-speed was used to replace the factory four-speed in Brit White's '73 Mach 1. A replacement for the earlier Tremec 3550, the 105-pound TKO transmission comes with a short-throw billet aluminum shifter and a .68 overdrive ratio for Ford applications.
For Brit's 351C-powered Mach, Wayne put together a combination of Spec clutch and pressure plate, Tremec TKO, Modern Driveline cable clutch linkage (made specifically for the '71-'73 conversion), and bellhousing, separator plate, clutch fork, and throw-out bearing from the '87-'93 5.0 Mustang. Wayne says the flywheel should go with the engine to maintain balance. Some minor modifications are required to fit the transmission. For example, a section of the transmission tunnel stiffener will likely need to be trimmed for clearance and in some cases the shifter opening in the tunnel must also be trimmed for the late-model shifter. With the Patriot headers on Brit's Mustang, one of the tubes had to be dented to clear the late-model bellhousing's cable ear.
If you're converting from an automatic to a five-speed, you will also need to add the clutch pedal.
As the company name implies, Restomod Plus specializes in vintage Mustang performance modifications. Wayne says that the five-speed conversion is one of the best mods anyone can make to an older Mustang. "It offers much better gear spacing," he says. "And because the engine rpm are lower at speed, it's quieter. Some owners start hearing noises that they couldn't hear before. The newer five-speed also offers better shifting with its internal shifter, unlike the old four-speed shifter with linkage hanging everywhere."
A key to the five-speed swap...
A key to the five-speed swap in '71-'73 Mustangs is Modern Driveline's cable clutch linkage, which retails for $259 from Modern Driveline's website. A bracket mounts the cable to the firewall, via the power brake booster studs, while the cable end attaches to the clutch pedal. Wayne Farnham recommends removing the factory pedal assist spring for a better clutch feel with the cable clutch. This requires removing the instrument cluster to access the pair of spring attaching bolts.
Underneath the car, the cable...
Underneath the car, the cable emerges through the original rubber grommet that's part of the steering column seal.
After installing the Spec...
After installing the Spec clutch/pressure plate and 5.0 clutch fork with 5.0 throw-out bearing, Wayne positions the 5.0 bellhousing into place.
On our '73 Mach 1 with headers,...
On our '73 Mach 1 with headers, the cable attachment ear on the 5.0 bellhousing interfered with a header tube, so it was "modified" for clearance. This may not be necessary with factory exhaust manifolds or a different brand of header.
The Modern Driveline cable...
The Modern Driveline cable comes with a new clutch fork dust cover. Slide the cable through the cover before attaching the cable to the clutch fork.
Secure the cable casing to...
Secure the cable casing to the clutch fork, then attach the end of the cable to the bellhousing ear. A special nut is supplied to secure the cable to the ear. Move the metal dust cover into position over the clutch fork and secure with two small screws.
When adjusting the cable,...
When adjusting the cable, remember that late-model throw-out bearings ride against the clutch at all times, unlike the free-play for older throw-out bearings.
Wayne uses small clamps, available...
Wayne uses small clamps, available from any auto parts store, to route the cable away from exhaust heat.
The Tremec TKO five-speed...
The Tremec TKO five-speed is installed next. It weighs over 100 pounds, so an extra pair of hands will come in handy when lifting into place.
In addition to some minor...
In addition to some minor trimming of the transmission tunnel stiffener, our Mach 1 also required a small of amount of trimming at the shifter opening to fit the late-model shifter. It's a good idea to trial-fit the transmission to check for interference issues and make needed modifications before final installation.
A great tool for adapting...
A great tool for adapting the Tremec TKO to the '71-'73 Mustang is the Universal Transmission Crossmember (UTC) from Dave Stribling Restorations. With numerous adjustment points, the crossmember is handy for many different types of transmission swaps. If you're planning big horsepower, you may want to weld the crossmember into place.
Installation of the Tremec...
Installation of the Tremec five-speed requires shortening the driveshaft by one-inch, which should be done by a reputable driveshaft shop.
The TKO comes with a backup...
The TKO comes with a backup light switch, which can be adapted to the '71-'73 backup light wiring harness. The neutral safety switch can also be wired into the car.
California Pony Car's vintage-style...
California Pony Car's vintage-style shifter, obtained from National Parts Depot, bolts in place of the shorter late-model shifter to provide both a period look and more comfortable shifting position. This particular shifter has a dummy '68-style reverse lock-out ring, not correct for a '71-'73 but vintage-looking nonetheless.
Here's the final Tremec TKO...
Here's the final Tremec TKO installation, with the exhaust system connected. A five-speed with overdrive is a great way to improve a vintage Mustang for better performance, improved fuel mileage, and a quieter ride at highway speeds.
Pedal Trick
Wayne Farnham says that one of his pet peeves is clutch pedals that ride too high. During our Tremec five-speed conversion, he showed us his trick for matching the clutch pedal height to the brake pedal. By using a simple hood bumper mounted to the clutch pedal assembly, you can adjust the clutch pedal's range of motion. Depending on the pedal assembly (this works for all vintage Mustangs, not just '71-'73s), you may be able to use an existing hole. Or you can drill a hole for the bumper stud. In some cases, Wayne adds a bracket to mount the bumper.