The SST PerfectFit Kit For a Five- or Six-Speed Transmission Swap

During the muscle car era, it was a common practice to yank out an automatic transmission and swap in a four-speed. For those who already had a manual transmission, there weren’t many upgrades available, and overdrive wasn’t on anyone’s radar at the time.

A classic four-speed transmission swap is done primarily with all OE parts, but swapping to a modern overdrive transmission today creates all-new challenges. That extra gear — or two — can mean tighter fitment in the tunnel area and/or cutting a hole for the shifter. Now, you’re getting into sheet-metal modifications and encroaching on challenges that might be out of your wheelhouse.

transmission swap

This shift pattern on the console isn’t factory, of course, but it sure looks it.

A simple solution for these challenges is a PerfectFit Kit from Silver Sport Transmissions (SST). Everything you are faced with to accomplish a modern TREMEC upgrade is taken care of by the engineering team at SST. Also, every part needed to complete the install of the kit is designed and fabricated for you — right down to the pedal assembly and shifter location.

Creating A PerfectFit Kit

As you might guess, in order to create a PerfectFit Kit, SST goes through a lot of steps to make it ‘perfect’. We reached out to Jeff Kauffman and his team at SST to find out more about this process. We wanted to learn what challenges they have to overcome to create the ultimate transmission upgrade for a classic muscle car or truck.

SST tries to develop both a five- and six-speed kit, simultaneously. However, if they have a vehicle in the shop with an already developed five-speed kit, the company will see if it can design a six-speed fitment, as well. These PerfectFit Kits are the whole nine yards, and part of the process is proper shifter position and angle, and console fitment.

transmission swap

SST Engineering Department’s Dick McCord and Eric Foirster discuss design options under a car in the shop.

When it comes to developing a new kit, much of it is based on supply and demand. Jeff tells us, “If a kit isn’t worth pursuing, we just decline the opportunity. There has to be enough demand in the market to justify the development costs.” This is not new to anyone who has ever tried to buy parts for a classic car, especially anything rare.

Jeff adds, “It has to be a kit that has the potential to help us recoup the R&D costs of designing it and stocking our shelves with custom parts.” When it comes to development, he says, “We may post the need for a specific vehicle, or we might contact some people who’ve previously requested information from us for that vehicle.”

If you're interested in a new kit, Jeff suggests contacting SST directly or subscribing to the newsletter, where they occasionally reach out to the public regarding a particular vehicle, such as this El Camino.

While the PerfectFit kit means it includes everything, Jeff reminds us that some vehicles will require a RestoFit Kit. “We use the term RestoFit to indicate the kit is complete, but perhaps the vehicle was only available as an automatic or column shift car,” he tells us. This kit is more for a restomod, as it will look like a factory kit, but it isn’t. A Builder’s Kit is one that contains many needed components but is not complete, and the shifter position will not be in the vehicle’s factory location.

Engineered Performance

Many popular kits have already been developed, and they all started out in much the same manner: acquire a vehicle and turn the guys loose on it. It can take a few weeks to take all of the measurements, design a kit, and complete the installation. But, it can take months before a kit is actually ready for sale. Some kits require only a few new parts and are finished much earlier, while others create more of a challenge and require more engineering.

transmission swap

It’s not always an easy task, later model cars — specifically those never available with a manual transmission — create the most challenges. Here, a template is made so the customer can make the same tunnel changes at home.

“Modern cars have so many electronics and may have a computer that needs to be reprogrammed,” Jeff states. It also creates a challenge when it comes to the shifter position and sourcing pedals that look good in the car. But, classic cars can be a challenge, too. “While classic vehicles are easier, some models are a tight fit with very small transmission tunnels,” he says.

Sometimes, SST runs into design changes within a model generation which requires a different shifter position or a change to the crossmember. This can create additional time in research and development. Jeff affirmed, “The engineering department must make drawings, the purchasing department must acquire bids from manufacturers and place a work order, and finally, the parts must be delivered and pass inspection by our engineering staff.”

From design to actual components, it's all done in-house. Fred Reagan cuts out the template from Eric's computer design for a prototype crossmember.

Part of this process is something that might be an afterthought to some: shifter position. Where equipped, consoles are removed and when the transmission is hoisted into the tunnel, the shifter location is noted and measured. In many cases, SST uses its proprietary STX Performance Shifter, so the look and feel of the original manual transmission shifter is retained.

By creating the STX shifter, a factory console can be replaced in the vehicle without cutting or modifying the tunnel. All console parts and trim can be reinstalled. Once the kit is completed, it will take a keen eye (and a visual of the shift pattern on the shift ball) to detect the car has an extra gear or two in the transmission shift pattern.

transmission swap

What Is The STX Performance Shifter?

The STX brand is SST’s performance shifter for the TREMEC transmission. The STX (Street/Track/Autocross) provides a shorter throw and a tighter feel for the gearbox. It is also available as an upgrade to your existing TKO five-speed or Magnum six-speed transmission. However, sometimes the stock shifter is exactly where it needs to be. In this instance, SST will leave it as-is and offer the customer the option of upgrading to the STX shifter.

Transmission swap

SST chooses to do more than just place the shifter in the sweet spot, it goes a step further and designed the STX shifter. Here, a TKO is getting reassembled for a Corvette. Note the offset of the shifter to the driver’s side.

Regarding shifter placement, Jeff says, “Our shifter will reduce the shifter travel from 12- to 29-percent depending on the application. The STX shifter is standard in most of our PerfectFit kits.” To an enthusiast, this fits into the performance perspective of why we build our cars.

Because of specific location requirements, the STX isn’t a better shifter than the Tremec shifter. Rather, it’s a manufactured component, engineered for specific purposes, which includes placement, throw, and to provide an OE feel without the sloppiness many OE shifters possess.

While many OE shifters use soft rubber insulators to reduce vibrations, the STX shifter has a double trunnion design allowing for a shorter throw, firmer shift feel, and smoother shifts compared to a ball-and-socket design.

The STX also provides a stronger neutral bias for improved 2 to 3 upshifts. It will bolt onto the stock TREMEC in either the front or rear position with a simple flip of the shifter. For the PerfectFit Kit, the shifter is going to be right where you expect it to be when you reach down to grab some gears.

Jeff described the STX shifter: “It is very compact, which makes installation easier and improves the fitment of the transmission. This may be the difference between cutting or not cutting the tunnel. Unlike many aftermarket shifters, ours do not leak. You’ll also like the positive feel of the STX shifter. It’s a favorite of many people who race their vehicles because they can make quick, accurate shifts.” The Magnum’s STX shifter has dual leak protection to keep the fluid where it’s supposed to be — and out of your console.

transmission swap

For SST, the shifter doesn’t only have to fit the console, it also has to fit the driver. Placement is paramount, just as much as feel and firmness.

When developing the STX shifter, both the vehicle and transmission are factors in the design. Jeff told us, “In addition to location, we also want the proper angle for the shifter stub.” SST will add dual biasing springs to strengthen the return to center on the TKO’s STX shifter. This helps to guide your hand back to the proper gate. It also helps build the confidence you expect when you’re focusing on the road ahead.

SST will take the time to check for fitment and make necessary adjustments so you won’t have to do all of your own R&D.

Finalizing Your PerfectFit Purchase

We asked about things like driveshaft fabrication and clutch choices because those are also areas where a customer might have a preference. “Our clutch kits are going to be high quality, just like anything else. They are assembled for us by RAM clutches,” Jeff told us. As with other aftermarket mechanical components, many clutch components are made by a few companies and painted/branded accordingly. He assures us the customer will not be disappointed with the feel.

SST includes a driveshaft with your PerfectFit Kit. However, logic tells you that it doesn’t ship with the transmission. SST  only provides the driveshaft after the transmission is installed, and specific measurements can be provided to them. From that point, they will relay those measurements to Dynotech Engineering. Within a couple of days, a new driveshaft is on its way to you. Of course, if you’re like most of us (impatient) and don’t want the additional downtime, SST allows you to remove the driveshaft from the order and will credit you $100.

Once the transmission is in place, a few simple measurements are taken, and the driveshaft is ordered and shipped to you for installation.

Installation is up to the purchaser. You can install it at home or have a reputable shop do the installation. Either way, due to the nature of classic cars having gone through variations of tweaking and modifying, SST recommends to always align the bellhousing. This is because of the tight tolerances of the input shaft required by TREMEC transmissions. These procedures are necessary to maintain your warranty with TREMEC and SST.

Jeff reminded us of the age-old wisdom passed down over the decades. “Measure twice, cut once, you’ll be glad you took the extra steps if your angles are off. You always have to measure for the throwout bearing. Failure to do so can harm your clutch or hydraulic system.” Driveline angles should also be checked and adjusted where necessary. This is necessary because an improper driveline angle can have an impact on pinion bearings and performance.

While transmission swaps are nothing new, there’s something to be said about swapping a new transmission into an older car. A PerfectFit Kit not only provides extra gears, but it also instills extra confidence with two names you can trust: Silver Sport Transmissions and TREMEC.

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About the author

Michael Harding

Michael is a Power Automedia contributor and automotive enthusiast who doesn’t discriminate. Although Mopar is in his blood, he loves any car that looks great and drives even faster.
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