Improving Our Vintage Corvette With A Five-Speed Transmission

Improving Our Vintage Corvette With A Five-Speed Transmission

There was a time when any “respectable” hot street car had a four-speed and at least a set of 4.10 gears with a high-revving engine under the hood. Troy Hudson lived in that era. His previous streetable rides included a 9,000-plus-rpm, dual-four-barrel, 327 cubic-inch small-block in a ’57 Chevy with a four-speed and 6.50 rear gears. Next came a 1960 283 cubic-inch, dual-four-barrel, four-speed Corvette with 4.10s. Times and technology change, and where four-speeds were once dominant, five-speed transmissions featuring overdrive are now the norm.

When Troy retired, he searched for his dream car and found this beautiful 1958 Corvette which gathers awards and constant attention. But these days, Troy has a different vision of street performance, which includes the ability to drive anywhere with comfort. Still enjoying the scream of a stout small-block, his C1 has a 350-inch GM ZZ3 crate engine with GM’s “Hot Cam”. An M21 close-ratio four-speed Muncie and a set of 3.55 gears in the Positraction carrier deliver the power. All was good until he discovered the five-speed TREMEC TKX package offered by Silver Sport Transmissions (SST).

Adding a five-speed transmission is one of the best upgrades you can do to make driving more enjoyable. The TREMEC TKX transmission is small enough to fit into many spaces and strong enough to withstand some serious torque. As part of the Corvette kit, our transmission had the proper shifter installed so it fit in the original opening in the console.

Going From A Four To Five-Speed Overdrive

SST is at the forefront of modern driveline conversions by offering well-thought-out kits for a variety of cars and trucks. SST offers the most complete five-speed conversion kits available with the support to ensure the best experience. Troy did the math and selected the close-ratio TKX five-speed with a 2.87 first gear coupled with a 0.68 O/D ratio. Compared to the old Muncie, he would have needed 4.56s in the rear to get the launch capability of this new combination. Adding another gear reduces the 70 mph engine speed from 3,382 rpm to just under 2,300! Going from four gears to five offers the best of both worlds.

Silver Sport has done a fantastic job with its extremely detailed installation instructions. The process of fitting a five-speed in places never designed for them is made easy. Follow along as we navigate the quirks that arise when a 65-year-old ‘Vette has undergone a few changes over the years. We’ll also provide insight into some steps and equipment to make the installation easier.

Installing a five-speed in a C1 Corvette

The first step is to find a lift. While you can do this lying on your back, using a lift and a transmission jack makes life a lot less complicated.

Removing A Muncie Four-Speed

Removing the old four-speed transmission is straightforward on a C1 ‘Vette. Start in the interior by removing the console plate and shifter assembly. Remove the distributor cap to ensure it won’t be broken when you lower the rear of the engine. This car had a Hurst shifter, which after removing the shift handle, was easier to unbolt from beneath and drop out the bottom. From there, remove the driveshaft, disconnect the speedometer cable, unbolt the old Muncie, and slide it back enough to lower it out, nose down. It’s a bit snug, slipping it by the bellhousing, but it will clear. Then, disconnect the clutch linkage, remove the bellhousing, the clutch assembly, and in our case, the flywheel.

Out with the old! A C1 Corvette is a dream to work on with plenty of access (left photo). We did have to drop one of the exhaust pipes to provide a little extra room when installing the bellhousing back in place during the TKX install. It wasn't long before the old Muncie was out of the way of the new transmission (right photo).

The next steps will prepare the new transmission for installation; the instructions are clear and will guide you through the process.

First, ensure the transmission shifts properly out of the box. Ours felt perfect, but shipping can be tough sometimes. Then, remove the shifter assembly, fill the transmission with the proper amount of fluid (2.7 quarts), and assemble the speedometer gear adapter. Next, secure the transmission to the jack.

The TKX is adaptable to all speedometer types. This car retains its cable-driven speedometer, so we assembled the supplied drive gear on the adapter to match our 3.55 gears and slipped it into place. Don't forget the tiny O-ring inside the gear (left photo); if left out it will leak for sure. The shifter assembly is disassembled/removed (right photo) to provide clearance when installing the transmission... and then reinstalled once it's bolted in place.

Before rolling the transmission under the car, install the new McLeod flywheel. This high-quality piece is a 10-1/2-inch, 30-pound, neutral-balanced, steel unit. In our case, the late-model ZZ3 is partially externally balanced and uses a specific flywheel as it came from GM. The new SST-provided flywheel comes with an assortment of weights to maintain proper balance.

There are well-marked holes (left photo) marking the proper position where the plate is screwed to the new flywheel (center photo). We torqued the flywheel (right photo) in place with the supplied flywheel bolts using a little blue Loctite thread locker supplied in the kit. Our engine was still dead smooth when we started it.

Removing an old pilot bearing can be exciting sometimes. There are numerous methods. Some folks have success by stuffing pieces of bread or grease in the recess behind the bearing and using an old transmission input shaft as a driver to compress the goo and force the bearing out. A slide hammer tool with a specific attachment usually works best.

Even when using the correct tool (left photo), sometimes a little heat from the "blue wrench" torch is necessary. In a few seconds, the old bearing popped free (center photo). The kit includes a quality roller-style pilot bearing with the needle-bearing assembly pressed into a bronze pilot bushing. This design is superior to a simple steel-cage needle bearing, which can become deformed when installed in the crankshaft (right photo).

Checking Bellhousing Runout

Dialing in the bellhousing is vitally important and must be done. There will be a quiz afterward because SST requests measurements as a condition of the warranty. Why is it so important? Unlike an old Muncie, which used a ball bearing on the input shaft, the TREMEC TKX transmission uses a tapered roller bearing. This eliminates the radial movement of the input shaft of the old days for a smoother shifting transmission and clutch action.

As you can see in the video above, the measurement process is relatively simple but might take a few minutes to get the dial indicator positioned properly. Don’t worry, SST includes excellent instructions. Aim for .005-inch or less Total Indicated Runout (TIR). Divide the TIR in half for the actual measurement. Ours came in at .0025-inch.

Centering bellhousing for a five-speed installation

Take your time to ensure the bellhousing does not have excessive runout and take the necessary steps to eliminate excessive runout if necessary. You might consider ordering some offset dowels from SST ahead of time so you don’t get stuck halfway through the job just in case.

Installing A TKX Five-Speed Transmission

On the C1 Corvette, there’s a fiberglass “pocket” in the transmission tunnel where the console-mounted ashtray resides. This pocket must be removed and left with approximately half an inch of lip remaining. Otherwise, it will interfere with the transmission tailshaft. The instructions detail the trimming process and how to modify the ashtray itself.

We sectioned the steel ashtray with a cutoff wheel (left photo) and carefully trimmed and contoured the shape until it could be epoxied back in place on the console plate while allowing the metal lid to function properly. You can also weld it back together. Either way, it ends up being pretty shallow but still functional, as seen in the right photo.

It was time to slip the new TKX transmission under the car and into the tunnel. There’s a specific method to assemble the clutch, bellhousing, and transmission which is unusual, but works. Position the transmission at an extreme nose-down angle and as far rearward into the tunnel as possible. For safety, and to not break the new TKX, we attached a safety chain to the trans and wrapped it around the frame just in case it should try to slip off the jack.

Next, the bellhousing is loosely attached to the transmission with the clutch fork and throwout bearing in place. The pressure plate is placed into the bellhousing over the input shaft, and while holding it in place, the clutch disc is placed against the flywheel. This can be done alone with some interesting contortions, but an extra set of hands would be handy. With one hand holding the clutch disc, slowly raise the trans jack, ensuring the pressure plate, clutch fork, and throwout bearing remain in place. It’s a tight fit as everything moves into alignment.

McLeod clutch assembly

The C1 Corvette TREMEC TKX kit is shipped with a McLeod flywheel and matching Street Extreme clutch assembly that is capable of handling up to 700 horsepower.

Once the transmission and engine are approximately parallel, move the pressure plate forward and get a bolt installed to hold it in place while keeping the clutch disc in position. Begin adding the remaining pressure plate bolts as you rotate the engine to gain access.

The instructions direct you to shorten the supplied plastic clutch line-up tool so it can be used and removed with an attached wire. In our case, it was easier to loosely attach the pressure plate and use fingers around the edges of the disc to center it within the pressure plate, double-checking alignment by slipping the tool in place. We could also use the tool without it being shortened, but every situation could be different.

The final step at this point is to torque the supplied ARP pressure plate bolts. Then, slip the transmission forward and install the bellhousing-to-block and the remaining transmission-to-bellhousing bolts.

Clutching & Fitment Issues

Troy mentioned his old clutch didn't feel quite right. We noticed the clutch throwout arm had some significant grooves worn in it (left photo) so we dressed them smooth (right photo). 

Next, we focused on the clutch linkage operation. The pivot stud in the bellhousing was incorrect, throwing the geometry completely out of whack. When the clutch was depressed, the arm contacted the nose cone of the transmission before releasing the clutch properly. The previous Muncie was shaped a little differently which allowed more travel before contact and required pushing the pedal to the floor to release the clutch, explaining the previous troubles. If you’re changing the clutch design, or it’s not working correctly now, make sure to discuss it with SST. You must assemble the correct combination of clutch, pivot stud, and throwout bearing length for correct operation.

Fortunately, Troy had a proper length stud in his stash at home, which allowed everything to work as it should. There are adjustable versions available also. It only took a few minutes to slide the transmission back enough to unscrew the stud.  Now Troy just has to get accustomed to the clutch releasing without shoving his foot through the firewall!

During the installation, things seemed slightly tighter than the instruction photos indicated. A little investigation revealed that during restoration, the original front “saddle-style” engine mount was replaced with side engine mounts. As it turns out, the engine was placed approximately one inch to the rear. That may not seem like a big deal, but when everything is already tight, an inch can throw everything off. It made it much tighter to slip the clutch and bellhousing together, because the transmission can only slip so far back into the tunnel. The shifter still fits the console plate but toward the rear of the opening. The new SST-supplied transmission crossmember required a little additional grinding also. Be prepared for little things like this if your car may have had some changes over the years.

Bringing It All Together

Installing the shifter assembly requires some manual dexterity. You’ll be working from the top through the shifter hole to install the cover plate and then from below to attach the shifter. Take your time, and it all fits perfectly. We suggest doing this before installing the transmission crossmember so you can lower the tail of the transmission as much as possible. This will provide more working room. Finally, raise the transmission and install the crossmember mount.

Troy opted for the original style shifter handle combined with the cool five-speed shift pattern plate.

We rerouted the speedo cable to the driver’s side of the tunnel, which already had clips in place to hold it. The TKX also incorporates a neat internal neutral safety switch to prevent starting the engine in gear. Troy liked this feature, so we easily wired it in series with the start wire on the starter.

Do I Need A Custom-Length Driveshaft?

The last step of the process is to order the new driveshaft. SST has driveshafts available for some combinations, but others require specific measurements. The early Corvette is one of the latter, and with our engine setback issue, it’s a good thing! We filled out the form with length and U-joint measurements and sent it to SST on a Monday. The SST folks told us our custom-length driveshaft would be ready to ship in eight business days. Our new shaft arrived just days after that with a new yoke and sealed U-joints in place.

The driveshaft slipped into place easily and was the perfect length. But it was apparent the engine setback was going to snag us one last time. We could see the clearance between the front U-joint flange and the tunnel was almost non-existent. Spinning the shaft created slight rubbing on the steel tunnel insert. This is not a kit design problem. The tunnel begins to narrow right after the tailshaft, and if the engine was an inch further forward, there would be no contact.

We performed some contouring on the offending U-joint flange bulges. On our first test drive, there was still slight contact when hitting bumps. The only alternative to surgery on the transmission tunnel was to lower the rear of the transmission. Some 3/8-inch spacers saved the day. The good news is the driveline angles improved and we gained the necessary clearance to eliminate contact.

Driving The C1 Corvette With A TREMEC TKX Five-Speed

We took the car out for a nice, late-night top-down drive. The first thing we noticed was the nice, tight feel of that shifter. The gates are positive, and the shifts are clean. There was also the added advantage of the TREMEC TKX’s 2.87:1 first gear to get things moving. There’s a nice little gear whine in first and second gear. It’s not objectionable and will probably subside as miles accumulate, but we hope not. It’s cool. Running up through the gears, there was no vibration, clutch chatter, or strange noises up through the triple-digit mph range. Cruising at 2,600 rpm put us right at 80 mph whereas previously that engine RPM would have been 55 mph!

Adding a five-speed overdrive transmission is one of the best modifications you can make to any car. It gives you stronger acceleration as well as lower cruising RPM. Many folks have swapped with junkyard parts and a lot of adapting, cutting, modifying, time, and aggravation. The folks at SST have done their homework to provide an incredibly well-thought-out package to install the best-shifting, five-speed street transmission on the market. If you want to enjoy driving your car, this is the way to go!

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About the author

Jim Moore

Jim Moore built his first engine at the age of 14 and later became an ASE Master Technician. You can often find him cruising the street in his 8-second, twin-turbocharged, 1967 Corvette with a documented top speed of 200 mph in the standing mile.
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