OK, you've graduated from oil and filter changes to tune-ups and even brake jobs. You've got a tool box fairly well-stocked with metric and SAE combination wrenches and sockets, and all the screwdrivers and pliers you'll ever need. You also have the obligatory locking pliers and a BFH or two.
But now you find it tougher and tougher to light up the rear tires on launch, and may even feel a bit of slippage in the clutch. You have a little free play at the top of the pedal, so you believe the clutch is properly adjusted (more on this later), and it's been 40,000, 50,000 or more miles since the clutch was last replaced. Or, more likely, your prized Corvette has seen a number of owners before you, and you have no idea of when, if ever, the clutch was replaced. Suddenly that clammy feeling drifts over you as you have visions of a giant bill to keep your Vette on the road, and you think to yourself, "Geez, is there any way I can do this myself and save some money?"
Good news! You can, and you can, particularly if yours is a C1, C2, or C3. Clutch replacement in these cars is surprisingly straightforward, requiring about as much muscle as it does know-how. To get the low-down, we contacted the folks at S.P.E.C., Inc. who offer a full line of performance clutches and related products. They supplied a clutch kit for our '58 C1 factory 283/270 dual quad car, and also supplied some useful technical information. Here's the deal:
How It Works
A clutch is surprisingly simple in operation. A clutch disc with friction material on both sides is sandwiched between the flywheel and the pressure plate, also referred to as a clutch cover. The clutch disc has a splined center and "floats" fore and aft on the splines of the transmission's input shaft. The pressure plate is bolted rigidly to the flywheel, and has a friction surface that moves fore and aft to clamp the clutch disc firmly to the flywheel when the clutch pedal is released.
A clutch release bearing, also called a throwout bearing, presses against levers (fingers) in the pressure plate which are hinged so that when the pedal is depressed, the bearing pushes against the levers, pulling back the friction surface of the pressure plate. This allows the clutch disc to "float" between the flywheel and the friction surface of the pressure plate so that no power is transmitted to the input shaft of the transmission. The bearing is activated by a pivoted fork and mechanical linkage from the pedal on early cars, and by hydraulics on C4 and newer Corvettes. Simple, right?
A clutch is nearly maintenance-free, save for occasional adjustment and, ultimately, replacement. Since the throwout bearing absorbs rotational energy, it's important that it is separated from the levers of the pressure plate as much as possible. Otherwise it will be spinning constantly and will quickly wear out. A faulty throwout bearing can be readily identified by a whining sound when the clutch pedal is depressed slightly, bringing it in contact with the levers of the pressure plate. This whine tells you that clutch replacement is in your future, since it is labor-intensive to access and replace the bearing and, if the bearing is shot, it makes sense to replace the clutch disc and pressure plate at the same time.
Clutch adjustment is surprisingly simple. A threaded rod with lock nuts is part of the clutch linkage, and the idea is to adjust the linkage to assure that the bearing is withdrawn slightly from the levers of the pressure plate when the pedal is disengaged. Easy rule of thumb—adjust the linkage so you have about an inch of free play at the top of the pedal travel, and so that the clutch starts to engage when the pedal is about a third of the way up from the floor.
S.P.E.C. Inc. supplies clutch kits for street, racing and off-road use. Their clutch kits include all the parts needed for a clutch job—all-new disc, pressure plate, release bearing, pilot bearing/bushing, and clutch alignment tool and, unlike rebuilt clutches, do not require return of the old clutch core. And, happily, all their clutches are machined and assembled in the good ol' U.S. of A.
Diagnosing Clutch Problems
Typically what goes wrong is that the friction material on one or both sides of the clutch disc simply wears out from use, just like brake pads or shoes wear out. The most common symptom of a worn-out clutch is slippage, especially when launching or hard up-shifting. The easiest way to verify this is to set the parking brake, carefully(!) give the engine some gas while holding the brake pedal firmly, and letting the clutch pedal out slowly with the transmission in fourth gear, where it transmits the least torque. If you can't stall the engine with the car stationary, and/or you can release the pedal entirely with the engine still running, you've got a clutch job in your future. Likewise, if you have the throwout bearing noise described above, you're looking at a two-night job with a hefty friend or two.
Less common is fatigue in the spring action within the pressure plate that leads to slippage or total clutch failure, but the result is the same. And, of course, if you install a brand new quality clutch like one from S.P.E.C., such a failure is highly unlikely.
Time To R&R
Clutch replacement in early cars is not too difficult, even if you've never tried something this involved before. Basically, the job involves removing the transmission and the clutch housing, also called the bell housing since it's kinda bell-shaped, unbolting the clutch assembly, installing a new clutch and reinstalling the tranny. Of course you'll need to remove a couple of other items first, including the drive shaft, the shifter, the speedometer cable, and disconnect the clutch linkage. You'll also have to support the engine in order to remove the transmission cross member, which also serves as the rear engine mount. But that's not as tough as is sounds.
Just like you have brake drums or rotors turned to restore a like-new friction surface, the folks at S.P.E.C. strongly recommend that you have your flywheel resurfaced during clutch replacement in order to provide a fresh, proper friction surface for smooth, reliable clutch action. And you'll also want to replace the pilot bearing which, on early Corvettes, is actually a bronze-like bushing that's pressed into the back of the engine's crankshaft to provide a support for the front of the transmission input shaft.
There are really just a couple of cautions when doing this job. The most notable is assuring that the new clutch disc is perfectly centered with the pilot bearing/bushing, since you'll be lifting a heavy transmission up and sliding the input shaft through the splines of the new clutch disc and into the pilot bushing. The tricky part is not in getting the splines to line up, but rather getting that last inch or so of travel as the nose of the transmission input shaft enters the pilot bearing/bushing. It is an absolute no-no to allow the transmission to hang on the splines of the clutch disc without the input shaft firmly planted in the pilot bearing/bushing. Otherwise you will almost certainly bend and warp your nice new clutch disc and suffer poor clutch performance and premature wear. S.P.E.C. includes a clutch alignment tool in their kits to ease installation and prevent damage to your new hooker-upper. Remember, there's never enough time to do it right, but always enough time to do it over again! So do it right the first time.
The following photos illustrate the procedure for changing a clutch in a C1. The process is similar for newer Corvettes, although you'll be working in closer quarters on the '63-up models and, therefore, there's less working room and you'll need a bit more time and patience. But the individual steps are pretty much the same. Starting with the C5 Vette, these cars used a "torque tube" to connect the bell housing to the rear-mounted transaxle (transmission/differential), and so clutch replacement on these cars becomes more involved and may be best left to a specialty shop or experienced friend. And, finally, we strongly recommend that you put the squeeze on your favorite friend with a lift, since clutch replacement is possible, but challenging, if done on the floor.
Specing Your New Clutch
The engineers at S.P.E.C. have helped put together a very user-friendly buyer's guide on their web site, along with detailed descriptions of their various stages of clutch design. In simple terms, if your engine is stock or near-stock, you can easily select the right clutch based on the data in the web site. If your engine is modified, or if you have questions on selecting the best unit for your usage, S.P.E.C. has a team of engineers available to walk you through the selection process in choosing the right unit for your needs, and they welcome the opportunity to discuss individual customers' needs.
For the record, they recommended their Stage 2 clutch kit for our C1 installation, and we couldn't be happier with the results. Engagement is smooth and secure, and pedal pressure actually seems lighter than stock, which they confirm with their test bench results. Plus, the unit they supplied us featured an optional aluminum pressure plate housing (with steel friction surface, of course), resulting in a saving of about 6 pounds of rotating mass. The result is quicker throttle response and faster engine spool-up.
Finally, as they say, reinstallation of the trans, shifter, and drive shaft is the reverse of disassembly. Two cautions: 1: When reinstalling the transmission be certain the input shaft engages the disc splines and also the pilot bushing; do not try to draw the transmission the last inch to the bell housing by tightening the trans bolts, and do not let the trans hang on the splines of the clutch disc. And 2: adjust the length of the threaded clutch fork push rod to achieve 1-2 inches of free play at the top of the pedal and clutch engagement at about one third of the way up. This will assure that the clutch fully disengages for shifting, and will also assure that the throwout bearing is not contacting the pressure plate fingers when the pedal is at the top of its travel.