Waiting for the next chapter when you install the hydraulic clutch hardware. And we will see just how hard it is to put a juice clutch on an old Shelby Mustang.
Update by Tim Suddard to the
Shelby GT350 project car
Jul 25, 2024
|
Shelby,
Shelby GT350,
American Powertrain
Photography by Tim Suddard
It is a wonder that five-speed transmissions were not universally adopted earlier. Replacing a four-speed or, even worse, a three-speed transmission, with a five-speed is arguably the single best modification you can make to any old car that you want to drive faster than 55 mph.
While making this swap used to be a junkyard and internet-based exercise, today there are companies with well-developed kits using tried-and-true parts and methods to make these conversions much, much easier.
One such company is American Powertrain and its sister company, The Driven Man (aimed at British and German cars). They have developed reasonably priced kits based around the Tremec transmissions so prevalently sourced by the OEs for so many new cars.
Tremec has certainly developed a reputation for smooth shifting, durable five- and six-speed transmissions that can take the torque of a modified V8 engine. We just needed someone smarter than us to design and build an installation kit to make these swaps available to the average hobbyist.
The Problems We Were Trying to Solve
Original 1965 and 1966 Shelby Mustangs have an aluminum-case BorgWarner T10 transmission. By 1967, in an effort to simplify the cars and save some money, Shelby reverted back to the Ford top-loader four-speed. (It’s called a top loader because there is a big plate at the top of the transmission that accepts the gear assemblies.)
While we had our 1967 Shelby’s original transmission rebuilt and it was working fine, it was not the smoothest-shifting transmission. Think more Mack truck than Miata.
There are some tricks you can do to improve that and one of them is certainly a Hurst shifter–as we had on ours. Ultimately, though, these original transmissions use ’60s, if not ’50s, technology.
More importantly, despite replacing the original 3.89:1 final drive gear with a more highway-friendly 3.25:1 gear, out on tours we always found ourselves hunting for that fifth gear. North of 70 mph, we were running at about 4000 rpm. While certainly safe for the engine, this kind of constant rpm is tiresome, loud, hot and certainly not fuel-efficient.
On the other side of the equation, that tall gear ratio also hurt our acceleration. A car once known to be seriously quick no longer was so.
We had driven five-speed-equipped cars and marveled at how much better they behaved and felt.
Why Not a Six-Speed?
If five forward speeds are good, then six would be better, right? While this does seem to be what everyone is going toward, we feel a sixth gear (generally a second overdrive ratio) is not needed here, especially in a high-horsepower car with a wide powerband like a Shelby Mustang.
A six-speed (and Tremec does make some nice ones) is also larger physically, more expensive, and heavier, and it will not fit into an early Mustang without some floor pan modification.
Installing earlier Tremec TKO five-speeds is also a task reputed to need minor body modifications to fit into an early Shelby or Mustang chassis. With the rarity and value of our car, we were again loath to start hacking the transmission tunnel. The earlier transmissions are not all up to the nearly 400 horsepower our car makes at the rear wheels.
A few years ago, Tremec introduced the TKX, a wonderfully shifting, lightweight five-speed that fits into an early Mustang/Shelby with no chassis modification. This was the transmission we were looking for.
Enter American Powertrain
Tremec’s largest aftermarket customer is American Powertrain. While they can sell you any aftermarket Tremec available, their superpower is that they develop kits to fit various, mostly American, cars.
Its sister company, The Driven Man, specializes in Tremec-based kits for English and German cars. They currently have kits for most Jaguar models, Big Austin-Healeys and W113-chassis Pagoda Mercedes cars, as well as late-model Mustangs.
And by kits, we mean that the engineers at American Powertrain and The Driven Man have thought of everything. From transmission mounting brackets, to speedometer adapters and even the driveshaft, as well as step-by-step directions. These kits are way more than an assemblage of parts.
Which Transmission Do I Want?
There is a myriad of options here, including gearing. American Powertrain offers both close- and wide-ratio versions, and the close-ratio can be ordered with a 0.81 or super tall 0.68 fifth gear ratio. With a high-horsepower car, you can get away with the 0.68 fifth-gear ratio. Trust us, it is lovely to be cruising at 80 mph and only be turning 2600 rpm.
The close-ratio gears are a lot of fun, too, in a tour environment when you are working the corners and running through the gears.
Let’s Get Started
While the instructions give accurate, step-by-step detail, we can summarize a bit by saying the first step is to take out your old transmission.
You should always start almost any project on your car by taking the positive battery cable off. Also remember to take your speedometer cable off (an easy mistake to make). And don't forget that the shifter has to be removed before the transmission comes out of the car.
This whole job can be done on jack stands, but it is easier if you can beg, borrow or steal a lift. You will also need either a transmission jack (available at Harbor Freight for less than $200) or at least a good floor jack to do this job.
With the old transmission out (or even before), we would recommend you clean the chassis with a pressure washer or with at least rags and some solvent. This is a good time to get things cleaned up as no one likes working in a greasy mess. It is way easier to hurt yourself in a filthy, disorganized environment.
One other tip: The actual removal and installation part of this whole process is a two-person job. You might be able to do it with one person, but you would have to be way better at it than us, and we have been working on cars for about 50 years.
With your old transmission, driveshaft, bellhousing and transmission mount out, you are ready to begin the installation.
The next step is to inspect the new transmission and make sure it didn’t get damaged in shipping. Ours was stuck between gears and had a damaged tail shaft seal. American Powertrain covers in the directions how to get the shift rails realigned. This is an easy thing to do with the transmission on the bench, but is a real pain in the butt once the transmission is in the car.
The new five-speed Tremec TKX (top) compared to the Ford top-loader four-speed (bottom)
As for the tail shaft seal, American Powertrain admitted that while this has happened before, it is not common, and they sent us another one at no charge.
Speaking of customer service, we were very impressed with American Powertrain. Without them even knowing who we were, they answered their phones with knowledgeable staff who quickly helped us through the couple of minor issues we encountered.
Next, you will need to remove the pilot bushing or bearing from your engine. You will also need to check clearance for your new transmission’s input shaft. If it bottoms out against the crankshaft inside the pilot bushing, you will damage the transmission. While this is not a common problem, it needs to be checked.
Once you have the new pilot bushing/bearing installed, you can then install the plate between engine and transmission and the new flywheel that comes with the kit. While detailed instructions and torque specs are supplied with the kit, there is one footnote to be aware of.
Because of the tight tolerances associated with modern tapered roller pilot bearings, you will need to make sure the input shaft is almost perfectly aligned, or you will damage your transmission. This requires you to test fit your bellhousing. Separate instructions on how to do this critical step are included with the kit.
While it sounds a bit scary, with a dial indicator, again available at Harbor Freight, you can do this step in your home garage. American Powertrain has even thought to produce a video to help you learn how to do this.
With the flywheel installed and the bellhousing test fitted and checked for alignment accuracy, the next step is to take the bellhousing back off and then bolt the clutch assembly into place. Again, torque specs, ARP hardware and directions are included in the kit.
Throw-Out Bearing Installation
While you can use a standard hydraulic throw-out bearing with traditional master and slave cylinder configurations, a more modern approach is to go with American Powertrain’s Hydramax hydraulic throw-out bearing kit.
This system uses a hydraulic throw-out bearing that mounts right on the main shaft of the transmission, with a line that routes to an external master cylinder. There is no need for a slave cylinder and clutch fork.
While simpler, lighter and with fewer moving parts, the primary benefit is that this throw-out bearing provides a very light and direct clutch pedal. An original Shelby Mustang has a particularly onerous clutch pedal, so for long distance touring, this setup is a revelation.
As for installation, it is quite straightforward. The only catch is getting the throw-out bearing shimmed correctly with the correct air gap so that you get the correct amount of pressure and extension onto the clutch fingers.
To do this, American Powertrain once again has easy-to-follow directions.
You first need to get two measurements. “A” is the distance between the bellhousing face (the outer surface where the bellhousing touches the transmission) and the pressure plate (clutch) fingers.
Use a set of calipers and measure the fingers where they protrude the furthest. As these fingers are rounded, you can lay a flat piece of metal, like a machinist’s ruler, along them and then subtract the thickness of this flat piece of metal from your total distance.
The second measurement, “B,” is the distance between the throw-out bearing face and the mounting surface of the transmission (where the transmission touches the bellhousing).
Once you have these two numbers, subtract “B” from “A.” If your number is negative, you will require spacers. At least on a Ford, your ideal target is to get an air gap of about 0.150 inch. The minimum air gap is 0.100 and the maximum allowable air gap is 0.200.
With the shims correctly installed, you can install the floating stud carrier into position onto the transmission main shaft. You need to then attach the clutch and bleeder lines and run them through the hole in the bellhousing where the clutch fork was originally mounted.
Installing the Transmission
From here, you can now permanently bolt the bellhousing to the transmission. Then you can bolt the bellhousing to the engine block and install the transmission mount that is supplied in the kit.
We got the optional White Lightning shifter kit with our transmission and were thrilled to find that the shifter tower did not hit the floor pan of our Shelby. The shifter location was perfectly centered, too.
The shift lever bolted right up, and the shift boot still fit perfectly. To clarify, this installation took zero bending, cutting or pounding on our car’s unibody.
Final Steps
The next step in the process is to choose a speedometer gear. The kit comes with a selection of plastic gears that are easily clipped onto the end of your speedometer cable.
The kit has a chart to help you choose the correct gear based on your tire diameter and final drive ratio. Based on this info, we chose the 21-tooth red gear included in the kit and found the speedometer to be 100% accurate.
You also need to install a master cylinder for your clutch. The American Powertrain kit comes with an optional clutch master cylinder that is plumbed to go with the Hydramax hydraulic throw-out bearing. We mounted the remote reservoir next to our brake master cylinder with no issue.
You also need a new driveshaft. Thankfully this, too, is included in the kit (sort of). Once you have everything installed, you measure your critical driveshaft dimensions and then fill out the American Powertrain driveshaft worksheet and, in less than a week, a new driveshaft arrives that, in our case, fit perfectly.
How It All Turned Out
The whole job took about 20 to 30 hours. This included taking the old transmission out and installing the entire kit. While there is not a firm kit price, because there are so many options, you can expect to pay $4000 to $6000 plus labor to do this job.
Our three goals were to get rid of the horrible, very stiff clutch pedal while getting a light, easy-to-shift transmission and be able to cruise comfortably and quietly at highway speeds.
Our 1967 Shelby shifts as easily as a Miata now. While nicely positive, the gear change can be done with just a couple of fingers. The clutch pedal effort is essentially nonexistent. Again, it is reminiscent of a Miata, which is considered one of the best shifting manual transmission cars ever built.
As for gearing, with the 3.70:1 final gear ratio we chose (as opposed to the less steep 3.25:1 final drive ratio we started with), we can now cruise at 80 mph at a very comfortable and relaxed 2600 rpm.
Of all the modifications we have made to cars over the years, this is among the best. Instead of our Shelby screaming in fourth gear at over 4000 rpm at 80 mph, we now have a cool, quiet experience with super easy shifting and clutch modulation. Color us extremely happy with our choice to go with this American Powertrain kit.
Waiting for the next chapter when you install the hydraulic clutch hardware. And we will see just how hard it is to put a juice clutch on an old Shelby Mustang.
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