The clutch cover (also known as the pressure plate) has 3 release levers that come into contact with the throw-out bearing during clutch operation. These 3 levers must be in the same plane (equal height) when the cover and disc are torqued to the flywheel. If different heights, the clutch cover is defective and should be replaced. Note that these lever heights are not at the same height unless the cover and disc are bolted to the flywheel. If you want to adjust them, get a FLAT piece of steel exactly 0.250" thick to be used in place of the clutch disc. Now bolt the pressure plate and this steel plate onto the flywheel. Once it's torqued down, measure and adjust the throw-out engagement bolts on the release levers so they're all the same height.
Clutch linings. The linings of the disc used to be made of asbestos. They're now made of fiberglass or other materials that do not contain asbestos due to the effects of asbestos to our health. However, asbestos provides superior cushioning and better wear than the newer materials at this time. The linings contain copper and/or bronze metal strands to aid heat dissipation and to help prolong disc life. The linings have grooves cut in their faces for air ventilation and to help prevent lining dust build up. Dust build up causes chatter and slipping due to the slip/grab action of the dust. The 6 to 9 springs around the disc hub may range from loose to tight. Not to worry, the looseness/tightness does not affect the operation of the disc. They're there to asborb shock and help eliminate engine pulsations (lumpy cams). By far the most critical element of the disc in eliminating chatter is the "marcel". This is the term given to the crimped plate, or wafer, between the 2 clutch linings. The purpose of the marcel is to prevent clutch chatter by giving the clutch disc some "give" during clutch engagement. The marcel also helps prevent the lining from sticking to the flywheel and/or pressure plate (due to the spring effect of the marcel) when the clutch is being disengaged. The marcel thickness (the distance the linings are held apart by the marcel when the disc is not under compression) will vary depending on the type of use the disc is designed and built for. Basically the thicker the marcel, the smoother the clutch will engage and the spongier it feels to your foot. The absence of marcel makes the clutch grab, but makes for a much more postive lockup (less slippage and ability to handle more horsepower). As the marcel thickness increases, it will require more clutch lever travel to engage/disengage the clutch. A pure drag/race car clutch marcel will be from 0.000" to 0.010" since engagement is quick and abrupt and chatter is not a problem. Truck clutches use marcels in this area also. A street/strip clutch will generally have marcels from 0.015" to 0.025". Pure street clutches will use marcels in excess of 0.025". A super soft clutch uses marcel in the neighborhood of 0.030" to 0.040". Mine is 0.025" and works well with no slippage and no chatter. When the marcel gets up to 0.030" and more, it may require as much as 60% of the clutch travel to engage. In 1949 or so, Ford advertised a "cushione clutch" in the Mercs and Lincolns. With the newer disc materials the marcel thickness has increased to 0.050" to 0.060" since there is little cushioning affect of the harder materials. When marcels are used, the linings are riveted. When they aren't used, the linings are bonded or riveted and bonded.
Rebuilding clutches. When a rebuilder is doing a clutch, they usually dismantel a bunch of clutches from all brands of vehicles and dump them into the cleaner. Then they grab a piece when assembling the rebuild. Consequently many of the parts are mismatched which will lead to clutch chatter that is unreal. That is the reason to always use a NEW clutch over a rebuild. If you HAVE to use a rebuild, then take yours to them and ask them to rebuild it and not mix the pieces. At least this way you know what chatter you had before and it shouldn't get any worse. Which is a lot more than I can say for a rebuild off the shelf! I won't use a rebuilt of the shelf. If I can't find new one, I take a core that I know didn't chatter to a cooperative rebuilder and explain my problem. They've always been very understanding and willing to do what I ask. Just a thought.
Truck clutches. Incidentally, you can make a truck clutch out of your 9" or 10" clutch. Just have the springs replaced with ones from an 11". Makes for a very stiff clutch pedal, but it sure doesn't slip. You just hope you never get in a traffic jam with one! The old clutch leg sure gets to jumping.... ask me how I know!
Pilot bushings. These are critical. The bronze bushing used in most flatheads has a maximum wear of only 0.006"! Bronze is fairly soft and wears quickly. Bearings are much better. Seems to me this is the same bearing as used in the front of a two brush generator, but my CRS kicked in on this for some reason. The pilot bearing supports the front of the main drive gear that goes into the transmission. When the pilot bearing/bushing gets worn it permits/encourages the main drive gear to jump up and down in the throw-out bearing and clutch disc.... thereby promoting horrendous clutch chatter. If it has 10,000 miles or more on it, I replace it with a new bearing just to be safe.
Throw-out bearings. Several years ago, these bearings began being manufactured with some cheap thin grease. They still are today. This gets like oil in a hurry because of the clutch heat. Consequently, the lubrication is thrown out of the bearing as it rotates in engagement. Maybe it was done this way to put some grease on our new clutches! Most throw-out bearings are merely out of grease. I drill a 1/8" hole on the outside edge of the bearing (not on the engagement surface). This is pretty hard and requires a quality drill bit. I use a needle type u-joint grease adapter on my grease gun and give it TWO (not 3) shots of chassis grease. Rotate the bearing halfway around after the first pump to disperse the grease. I use a very short sheet metal screw and some red lock tite to seal the hole. In one of my cars, I was able to get nearly 200,000 miles on the same throw-out bearing using this method.
Misc: If your flathead has engine stay rods be sure they're hooked up and tight. They really help in my opinion. One thing you might want to try if you don't have time to do a clutch job is to try some graphite. Remove the inspection cover in the transmission case so you can see the edges of the clutch. Now depress the clutch pedal to the floor and hold it with a 2X4 wedged against the seat. Now squirt some spray graphite, like for a door lock, in between the clutch disc and the flywheel and between the clutch disc and the pressure plate about every 60-90 degrees of flywheel rotation. Go around it only once. This seems to encourage the clutch to slip, but noy excessively. An old used car lot trick!! On 8BA cars, you can loosen the transmission bolts and pry the engine back enough to put a 0.030" shim between the lower right passenger side transmission bolt and the bell housing. This will cock the main drive gear in the disc and will often stop clutch chatter.....another used car lot trick...... rumble seat