Ford Mustang Spec Super Twin Clutch Install - Bite Marks

In 2003, we were blessed to finally get a T56 in a Mustang from the factory. This six-speed gearbox was considered bulletproof with its stout gearset and triple-cone synchronizers in First and Second gears. The 390-horse Cobra certainly needed this robust gearbox to handle all of its torque, but after a few track outings and dyno days, enthusiasts began to realize that even this ultimate Mustang still had some shortcomings in the driveline department. Over time, these issues have grown apparent and are now considered recurring problems on the '03 and '04 Cobras. Today, we will address the two most common-the clutch and the input shaft.

Built To Spec
The Cobra's factory 11-inch clutch has never been a model for long life, especially when slicks are part of the equation. It's also saddled with heavy pedal effort and vague action, making it almost unbearable for many to live with. Many Cobra owners relied on the aftermarket shortly after taking delivery of their cars, and the results were mixed, as finding the right combination of pedal effort, feel, and clamping power has been an elusive combination for a conventional single-disc design. However, with the advent of twin-disc technology being available to the Mustang masses (that would be you and I), we can finally get everything we want with an over-the-counter twin-disc clutch setup.

Although there are a few clutch companies out there that offer a twin-disc system for the 4.6L engine, few offer as many different versions as Spec. Its Super Twin clutch assemblies are constructed of billet aluminum and are a direct fit for any Cobra. They allow you to reuse the existing clutch release fork and require no machining or special fabrication. As a matter of fact, the assemblies are a direct replacement that replace your clutch and flywheel assembly in one fell swoop.

The first order of business is to pull out the transmission and drain it. Yes, it's a gruesome task, and you'll probably make the Missus upset when you smell like old ATF, but buy her a pizza, and then go under there and get crackin'! Heave it onto your workbench or tool cart for easy access to the front cover-you want that stock input shaft out of there.

Using a 15mm socket, undo all of the front cover bolts and lay them aside. Using a flat screwdriver, reach into the recesses between the cover and the case and slowly pull away the cover. The cover is indexed with two dowel pins, so keep that in mind. Once removed, simply remove the input shaft.

Spec's Super Twin clutch assembly uses two smaller discs to handle more power than a conventional clutch. The complete package comes with a billet-aluminum flywheel with a replaceable steel surface ring. The billet-steel floater disc rides within the billet-aluminum cover, providing surface area that's nearly twice as much as a single-disc clutch setup. Best of all, pedal effort is significantly less than stock. Owners of '03-'04 Cobras can certainly appreciate that.

For our test mule, we decided to go with Spec's most street-friendly setup, the SS-Trim (PN SF87SST), which uses one spring-hub disc and one solid-hub disc. Both use a combination of Kevlar and metallic facings for smooth engagement and, of course, massive torque-holding capacity. Speaking of torque capacity, Spec tells us that this SS-Trim can handle an incredible 900 lb-ft of torque without a whimper. For our Cobra, which currently belts out 453 rwtq (about 525 lb-ft at the crank), it won't be an issue.

Torque capacity is certainly great to have, but we also looked forward to taking advantage of the reduced pedal pressure. Because of the large amount of clutch surface area that a twin-disc clutch inherently has, there's less need for a radical spring rate, and this ultimately translates into a lighter pedal for a given torque load. This means we can finally cancel that knee replacement surgery!

Getting Shafted
The T56 is a darn-strong gearbox, but its weakest link is the 10-spline input shaft. When the folks at Tremec were tasked to fit the T56 into the Terminator, they equipped it with this 10-spline shaft to match up with the Ford clutch disc. Although it seemed like a good idea at the time, the problem was that in designing this piece, it was not as thick or robust as the 26-spline shaft that typically comes with a T56 when it is used in a Viper or Corvette. Of course, companies like G-Force Racing Transmissions can sell you the parts or build you a killer T56, but we went with this simple upgrade to keep down the cost.

Side by side, you can see how the stock 10-spline input shaft (left) often fails in the splined section. Its inner diameter is much smaller when compared to the 26-spline unit on the right. Where the disc sits is where these shafts often twist and break.

While you're in there, it's best to replace the input shaft seal. The old one simply pries out with a seal puller and the new one goes in the direction shown. We used the backside of a large socket and a hammer to install it.

When installing a new input shaft, you'll have to shim the mainshaft for the desired endplay of 0.000-0.002 inch. Simply assemble the front cover with the new input shaft, measure the endplay with a dial indicator, and add that number to the existing shim under the outer race of the input shaft bearing. We added 0.003 inch for zero endplay.