More is usually better. More power, more torque, more, more, more. When it comes to transmissions, more gears is always better. The more gears you have, the faster you go while keeping your engine in the power band. That is the key characteristic that made the Muncie M21 “Rockcrusher” transmission so popular—it was a close-ratio gearbox, each shift was a small jump, so you could keep the RPMs up, no big drop in engine speed to slow you down as you blasted down Main Street. Compared to a 3-speed, the 4s were always better. The ultimate in manual transmissions these days is the Tremec T56 6-speed.
Like the Muncies of the late 60s, the T56 comes in a few flavors, so you have to make sure your version matches your actual needs. Originally designed for the Viper in 1992, the T56 has been a staple of high-performance American muscle for over 20 years. Essentially, the T56 is a 4-speed with two overdrive gears. Fourth gear is always 1:1, while fifth is .84 down to 74:1, and sixth could be as low as 50:1, meaning your driveshaft is spinning twice as fast as your engine.
Input shafts vary in both length and girth (spline count). This is the most important spec, as matching your engine to the transmission is tricky. The older LT (90s GM) input shaft is shorter and uses a pusher-type throwout bearing. Most late-model engine swaps, and in our case a Ford 302, require the LS version. What we happened to have in the shop for our project, was an LT1 T56 from a 1996 Camaro. In order to mate this bad boy to the 320hp 302 that is going into the 1965 Mustang, some modification is required. We needed some expert assistance.
American Powertrain has become the premier manual transmission swap supplier in the country. They have just about everything you could need, and moreover, they have the knowledge of how to make this thing work with that one. Our swap is a Frankenstein if there ever was one. The original plan was to use a 1994 T5, and we bought all the goodies for that swap, then we discovered that the T5 was in fact not what we thought it was (TKO and full of rust), and not suitable for our application. The LT version of the T56 is readily available in salvage yards for just a few hundred bucks, so we picked one up and gave American Powertrain a call.
Here is what we had already: Ford 302, a Quick Time T5 conversion bellhousing, and a PRW flywheel. In order to mate the T56 to the 302, American Powertrain sold us an LS input shaft, adapter plate, and a hydraulic clutch system. We sourced a new front bearing online. Swapping the input shaft is not complicated, but it is important to follow a few steps. Remove the cover and carefully pull out the input shaft. Press the new bearing onto the new shaft. When installing the shaft into the transmission, a slight rotation is needed to get the gears to line up. Then reinstall the cover.
The next part requires a dial indicator, a magnetic base, a piece of steel, and some clamps. Place the steel across the front cover, clamp it down, and lock the dial indicator to rest against the end of the input shaft. The factory spec is .0005” to .0035”. Push the input shaft in, zero the indicator, and then pull the shaft out. Note this number. The end play is adjustable through the shims behind the race in the front cover. Once the end play is set, apply some assembly lube to the front bearing, install the front cover with a carefully-placed thin bead of silicone and torque to 26 foot pounds. Too much silicone can gum up the internals of the trans, so use just enough to seal the case.
With the transmission settled, the work moves to the engine side. We installed the PRW flywheel with a set of ARP flywheel bolts, and bolted on the new American Powertrain clutch. In order to mount the T56 to the 302, we used an adapter plate from American Powertrain (APGM-40005) to go from the Quick Time T5-type bellhousing to the GM T56.
A critical process for a well-functioning clutch is the bearing clearance. This requires measuring a couple of areas. The first dimension is from the bellhousing mating surface on the engine block to the clutch diaphragm fingers, this is measurement A. The second dimension is from the bellhousing mating surface to the face of the release bearing. For the T56, this requires installing the bearing retainer to the adapter plate. This is measurement B.
The formula is:
(A-B-.150”)=X, X + shim thickness= number of shims needed.
Example: A= 2.45”, B= 2.125, shim thickness= .090”
2.450-2.125-.150/.090= 1.9 spacers.
The ideal clearance is .150” – .200” is optimum, but you can get away with as little as .100”.
The shims go under the release bearing on the retainer shaft. With all the components secured, the bellhousing was bolted to the transmission, and then mated to the 302 for installation in the vehicle. The T56 fits quite well in the stock chassis. We only had to do a couple of mods, namely a custom transmission mount, and opening up the shifter hole about a half inch. We built the transmission crossmember from 1×2 steel tubing and an American Powertrain ISO-X transmission mount.
Because the T56 uses a slightly angled pad, we built the crossmember flat and modified the ISO-X mount. The mount was cut on the bandsaw at an angle. We chose to modify the mount for height and angle, allowing us to tuck the crossmember as high as possible.
The last step for the T56 swap is the master cylinder. This Mustang was originally a 3-speed manual car that had been converted to an auto, but they never removed the clutch pedal. The pedal simplified the MC install, which was a simple drill-and-bolt affair using the American Powertrain kit. The bracket articulates, allowing you to adjust it at several points, making it very simple. The hard part was the vacuum brake booster. Our Mustang has power brakes, and even though the booster is small, it is right in the way of the clutch MC. Using the existing mounting plates for the booster, we pie-cut an angle into the bracket to raise the front of the brake master cylinder to clear the clutch MC. The bracket was bent to close the pie-cut, welded back together, and reinstalled in the car.
Swapping a T56 into an early Mustang takes some patience, especially if you are mixing and matching parts like we did. We spent several weeks off and on getting this swap dialed in, but if you have all of the specs and information up front, you should be able to complete the task within a few days.