666: Installing the TKO Tremec & SPEC Clutch

Project 666 Mustang is moving along, with the hairy goal of putting down 666 horsepower and running 10s. With power comes great responsibility, and we knew the tired old T-5 stock transmission and wimpy clutch just wouldn’t be up for the job. So after 25 years of service, it was removed from our mighty four-eyed ’86 Mustang to make room for a fresh, durable TKO-600 transmission from Tremec and a fresh SPEC clutch.

To achieve that numerically demonic level of power without any forced induction or nitrous, we needed a big-inch, king of engines. We have a choice of a 408 or 427 Windsor, but rest assured, both make plenty of ponies, starting at 600, and going up to about 750 hp with a small nitrous shot. To handle (corral) that level of power, Tremec’s TKO-600 got the call, conservatively rated to handle 600 HP.

Tremec’s famed TKO-600 five-speed transmission

Tremec is an acronym, standing for “Transmissions and Mechanical Engineering”. Tremec first gained popularity in the early 90’s when Fox-body Mustang enthusiasts began breaking their T-5 transmissions in the pursuit of high power and a popular swap was the TR-3550

As Fox-bodies surged in popularity thanks to their low cost of purchase and modification, so did the line and popularity of the Tremec transmission upgrade. According to Nate Tovey of Tremec, “Tremec offers a line of five and six-speed manual transmissions for most every application, and since they are so easily adaptable, our transmissions have also become popular with the muscle car and resto-mod crowd, too.”

Tremec is a long-time player in the transmission game, having been in business since 1964, the very beginning of the muscle car movement. “We manufacturer transmissions for a wide range of products beyond just cars, even providing transmissions for John Deere tractors!” explained Tovey. “Tremec transmissions also found their way into many performance production cars today. The T-56 six-speed can be found in Vipers, ‘Vettes, Shelbys, and even the outgoing Pontiac G8 GXP.”

666 Gets TKO’d

Since Project 666 is getting 351W small-block stroker engine, we need a transmission that can handle gobs of power. Although an automatic is an easy choice, for our street/strip car, we knew we wanted to have three pedals. The goal if Project 666 is 666 flywheel horsepower and an equal amount of torque. Thus, we turned to Tremec and their world renowned line of TKO transmissions.

351W stroker small block meets Tremec’s TKO-600

Tovey filled us in a little more on the TKO-600: “The TKO is just one series of manual transmissions Tremec offers. The TKO comes in two flavors; the TKO-500, and the TKO-600. The TKO has an impressive lineage, dating all the way back to the legendary 4-speed “Top Loader” transmission found in Ford performance vehicles, including big blocks for the day.” Nate explained that the Top Loader evolved into the TR-3550, which eventually became the TKO.

For even more information on the TKO, we turned to Kevin Ryan, a Product Engineer at Tremec. Since we are going to be making in excess of 600 ft-lbs of torque, we opted for the TKO-600 and its 600 ft-lb torque capacity. The TKO-500 meanwhile, only has a 450 ft-lb torque capacity (which dwarfs the T-5’s 300 ft-lb capability). But what are the differences, beyond (metal) skin deep?

According to Kevin Ryan, “the main difference between the TKO-500 and 600 are the gear sets. The 500 utilizes a 3.27 gear set with a 0.68 overdrive 5th gear, while the 600 uses a 2.87 gear set with either a 0.64 or 0.82 overdrive gear set. “In general, the deeper the gear ratio set (higher numerical value) the lower the torque capacity.” The gears are made from SAE grade 4620, 8620, and 4615M carbon steel, and Tremec has several different tactics for improving strength of the gears to handle so much torque.

“A balance of base material and gear cutting design will dictate the torque capacity. We also use modern gear processes to optimize strength,” says Kevin. He goes on to explain that all of Tremec’s forward gears are helical (like a helix) in order to provide a balance of strength and gear noise. Straight-cut gears are stronger and easier to make, but result in a great deal more gear meshing noise. “It has always been an aftermarket transmission and, as such, a major design focus was to fill as many applications as practical,” Kevin explains. This is why you will find TKO transmissions in everything from classic Camaros to 5.0 Mustangs. It also explains why Tremec’s popular T-56 transmission can be found in high performance vehicles across all three domestic brands.

Other things were done to ensure the versatility of the TKO too. The 31 tooth spine input shaft was chosen because it is a very popular and proven strong design. Since the TKO was bound to wind up in different length vehicles, the shifter needed to be able to work from different positions on the transmission. Using the back of the engine block as a reference point, the shifter can be located at 19, 23, or 25 inches. A half-foot is enough variety for most applications, but Kevin says they are always working on expanding available shifter positions, including side-to-side

Tremec tests the strength of every area in their entire line of transmissions, because versatility would be worthless without durability. Gears and shafts are tested for long-term durability using strength-over-time measurements. The syncros and shift linkage are tested using shifts at different speeds over time. To test overall impact strength, the gear castings and linkages are shock loaded under extreme conditions to verify they represent a good value to the customer.

SPEC Clutch.. to the Rescue

All of this durability within the transmission would be of little use if the clutch isn’t up to snuff. Like the T-5 transmission, the stock clutch wouldn’t last long behind 666 horsepower. When it comes to manual transmissions, the clutch often gets more thought than the transmission itself, but you want to ensure both products are on par with each other. Thus, we went and asked our friends over at SPEC clutches to hook us up with a clutch package that would take all of the power that we dished out, and more so for the future.

SPEC’s single-disc Stage 2+ clutch kit with a steel flywheel

When we told them how much power we planned on making, they sent us their Stage 2+ clutch kit with a steel flywheel. The Stage 2+ clutch bridges the horsepower gap between the streetability of the Stage 2 and the higher torque capacity of the Stage 3. The Stage 2+ has a 15-20% higher torque capacity than the Stage 2 but maintains streetable manners.

We’ve used the Stage 2+, and there is no fighting this clutch to the floor to find a gear. The clutch is made from a hybrid Kevlar and Carbon-graphite friction material and comes with a high-clamp pressure plate, and has a flywheel torque capacity of 635 ft-lbs. It has a double sprung hub, too. It is a perfect match for the TKO-600.

Shacking up Tremec and SPEC with Project 666

As we have previously written about, the old engine and transmission have already been removed and discarded. But before we could install the 351W Windsor stroker crate engine, the SPEC flywheel, clutch disc, pressure plate, and throwout bearing had to be installed first. The benefit of a fully-equipped shop is there is no crawling under the car to do all this work, so we got right to work installing the parts.

Torque down the SPEC flywheel to 80 ft-lbs max

First goes the SPEC flywheel, which installs to the back of the crank shaft with six nuts. They must be torqued down to 80 max ft-lbs. Our SPEC steel flywheel is a stout piece, completely machined on CNC equipment, and with a removable facing so that we can re-use the flywheel for the life of the engine and vehicle.

The SPEC clutch disc comes next, and it has to be perfectly aligned to work properly. Thus we use a disembodied input shaft tool stuck into the crank shaft to ensure the right alignment. There is no room for mistakes here, and you can use the input shaft from your own transmission to make sure everything lines up; this is just easier… And we like easy.

The assembled SPEC 2+ clutch kit, lookin’ sharp. Too bad we don’t get to see it in action.

Finally, the SPEC pressure plate needs to be attached to the flywheel. When the pressure plate clamps down on the clutch, it engages and thus turns the wheels to the tune of 666 horsepower (minus drive train loss, natch). The SPEC pressure plate should only be torqued down to 28 max ft-lbs. Since this is a 351-based stroker and the engine was already put together, the throwout bearing was already installed — now it is merely a matter of mating engine with transmission.

With the SPEC clutch and pressure plate system installed, we could finally hook the TKO-600 transmission up to our engine. With both the engine and tranny out of the car, this is about as simple a procedure as it gets. Just torque down the bolts on the bellhousing, and get ready to hoist the engine up and in.

But wait! The 351W Windsor features a deck height that was 1.3 inches taller than the 302. This could create clearance issues with the stock hood. How do we remedy such a situation?

The easiest way is to drop the engine with motor mounts. Thus we used Holcomb Motorsports 351W Drop Mounts for better hood clearance. Zinc plated for appearance and protected from corrosion, these mounts made putting the engine in a pie. We just dropped the engine and attached transmission in and they slid into place with ease. The cross-member was cushioned with energy suspension polyurethane bushings, as was the transmission mount ensuring a smooth snug fit without too much bouncing for our engine.

Finally, we installed a Hays clutch quadrant and cable kit to connect the transmission and clutch together with our pedal. Hays Products adjustable clutch cable is designed to increase leverage and makes adjusting our upgraded clutch much easier. The Hays clutch cable also comes with a heavy-duty low friction cable, made from stranded steel. It has durable metal fittings with an adjustable fork end for exact adjustment of clutch release and pedal free play.. all in all, it helps us get that right feel in the pedal.

That was it – we were wrapped up and ready to go. We should be starting up 666 any day now, and we’ll bring you full test results before you know it.

Hay’s Clutch cable and quadrant, fully adjustable.

About the author

Chris Demorro

Christopher DeMorro is a freelance writer and journalist from Connecticut with two passions in life; writing and anything with an engine.
Read My Articles

Blue Oval Muscle in your inbox.

Build your own custom newsletter with the content you love from FordMuscle, directly to your inbox, absolutely FREE!

Free WordPress Themes

We will safeguard your e-mail and only send content you request.

Ford Muscle - The Ultimate Ford Mustang Magazine

We'll send you the most interesting FordMuscle articles, news, car features, and videos every week.

Ford Muscle - The Ultimate Ford Mustang Magazine


We will safeguard your e-mail and only send content you request.

Ford Muscle - The Ultimate Ford Mustang Magazine

Thank you for your subscription.

Subscribe to more FREE Online Magazines!

We think you might like...

Muscle Car & Hot Rods

Drag Racing

Ford Muscle - The Ultimate Ford Mustang Magazine

Thank you for your subscription.

Subscribe to more FREE Online Magazines!

We think you might like...

  • Next-Gen Ford Performance
  • Muscle Car & Hot Rods
  • Drag Racing

Ford Muscle - The Ultimate Ford Mustang Magazine

Thank you for your subscription.

Thank you for your subscription.

Ford Muscle - The Ultimate Ford Mustang Magazine

Thank you for your subscription.

Thank you for your subscription.