This is a typical diaphragm-style pressure plate with its Belleville spring (arrow). This
In The Clutch
Bill Snyder, Salt Lake City, UT: I have a '65 Impala with a 396 backed by a Muncie M21 four-speed. While I have the engine pulled, I am going to replace the clutch and pressure plate. This is a street car and will not be raced. I would assume there is a clutch package out there that is superior to an original setup-or not? What would you recommend?
Jeff Smith: There are many good clutch systems out there, Bill. Instead of giving you a specific clutch recommendation, let's talk about several I think would work for you, and you can choose the one that best fits. Roughly a half century ago, GM converted to what is now called a diaphragm-style clutch. The diaphragm uses a single Belleville-style spring that has an interesting advantage over three-finger (Borg & Beck or Long) clutches. These other designs use coil springs between the pressure plate and cover. Depressing the clutch pedal moves the levers, which compresses these springs. As with any coil spring, effort increases with pedal travel, which means your leg is pushing against maximum spring load with the pedal on the floor. That's why your left leg gets tired. An interesting by-product of the Belleville spring used in all diaphragm clutches is that as the fingers of the spring are compressed past a certain point, spring pressure is drastically reduced. This means that when the clutch pedal is fully depressed (at a stoplight for example) your leg is only working against a fraction of the spring's total clamp load. This is why diaphragm clutches are almost universally used in street applications.
Now that we've established the advantage to diaphragm clutches, all we're left with is choosing the model for your Impala. Your big-block requires an 11-inch clutch and pressure plate with a 10-spline input shaft with the Muncie. I looked up kits from McLeod (StreetPro PN 75124, Summit Racing, $256.69), Hays (PN 85-110, Summit Racing, $259.95), and Centerforce (Dual Friction, PN 73552, Summit Racing $269.95). All three of these kits include an 11-inch disc and diaphragm pressure plate. Most also include a plastic clutch alignment tool, and the Hays and McLeod kits also include a new throwout bearing, which should always be changed when installing a new clutch and pressure plate. That makes the Centerforce a little more money, but it incorporates a Dual Friction clutch disc, which offers a little more holding power compared with the other two organic discs. You really can't go wrong with any of these clutch kits.
We didn't talk about flywheels, since I'm assuming you will retain your stock one. But always have the flywheel surfaced whenever adding a new clutch and pressure plate. Another important step is a new pilot bushing to ensure that the transmission input shaft is aligned with the crankshaft. If the clutch kit you purchase does not include the hardware, then be sure to purchase new quality pressure plate bolts. We also use a drop of thread-locking compound on the flywheel bolts when torquing them in place. Inspect the clutch fork and especially the ball stud in the bellhousing for wear. The ball is often overlooked and can cause a side load on the clutch fork that can cause wear on the throwout bearing and the transmission input shaft collar. Look everything over very carefully, including all the clutch linkage pivot points. These are often badly worn. Remember, we're dealing with cars that are now almost 50 years old. Choose your clutch and enjoy how smoothly it will work.
Mr. Gasket (Hays)
Also at the Knott's Berry Farm Show was John Vermeersch's '61 Ford wood-side wagon pulling his boat that just happens to be powered by a Roots-blown SOHC 427 Ford motor. John is the longtime owner of Total Performance in Mount Clemens, Michigan, and a lifelong Ford fanatic. Car Craft did a cruisin' story with him and his SOHC-powered Starliner back in the '80s.