Basic description: When you step on the pedal you push in on a small piston in the clutch master cylinder, which pushes fluid out of the cylinder, and causes a similar piston to move in the slave cylinder. This in turn pushes a lever on the side of the transmission. The lever pushes on the throw-out bearing, which pushes on the springs on the pressure plate, and thus releases pressure sandwiching the clutch disk between the pressure plate and the flywheel. (whew! see diagram.) The flywheel and pressure plate are bolted to the engine, and the clutch disk rides on the transmission shaft. When you step on the pedal, the clutch disk (and transmission shaft) can freewheel, turning independently from the engine. Release the pedal, and everything "locks" together, and the transmission input shaft turns at the same speed as the engine.
=== Slave Cylinder and lever | | (some cars- return spring on lever) | | O --| | | \ | engine |======]=[======= transmission shaft --| | | / ^Throw out bearing | | | ^Pressure Plate Flywheel^ ^Clutch disk
Note: Many cars use a cable to act on the engagement lever, instead of hydraulics. A few, such as pre-war MG's, utilize a pedal connected directly to a shaft through the transmission. A sudden clutch failure is most often caused by hydraulic failure, (or broken cable). Slow deterioration could be anything. How to determine whether a problem is hydraulic or the clutch itself: Get somebody to activate the pedal. You should be able to see the lever on the slave cylinder move back and forth about 1 to 1 1/2". If not, you have a hydraulic problem. If you have sufficient lever movement, you probably have a clutch problem, and it's time to remove the engine or transmission. The total to replace all the parts (disk, pressure plate, throw-out bearing) may be about $75-150 for most cars. The flywheel might need to be turned, to eliminate warpage, scoring, or glazing, or even replaced if there is cracking. The labour time will be higher if you have to remove the engine to get at the clutch. On some cars, you can just remove the transmission, and for many front wheel drive cars, accessing the clutch is as easy as removing a cover plate or two.
|-----| |-- --| | | | | --------------------------------------| (optional- misc. valving or springs)NORMAL OPERATION:
The brake, clutch, and accelerator pedals on many British sports cars are located by a shouldered bolt or shaft passing through a simple bushing in the pedal. These bushings are often not lubricated, and can result in worn shafts/ bushings, and a sloppy pedal. An even worse problem, is the linkage to the brake and clutch master cylinder pushrods. This is often just a hole drilled through the pedal body, with a clevis pin joining the pushrod and pedal. Wear at this joint is often excessive, resulting in an ovaled hole in the pedal body, and a narrowed clevis. Due to the mechanical ratios, a small amount of wear at this joint will result in a large reduction in pedal travel. You can check this some by wiggling the pedal up/down, and side to side, but visual inspection is best. The fix is simple.
First, remove the pedal assemblies from the car. Fill in the ovaled hole by welding or brazing, and re-drill the hole. This should be a fairly close fit, probably no more than .005" of slop. Ream to fit if necessary. If the push-rod holes are worn, they may be repairable as well, or may be replaced. If the pivot bushings or shafts are worn as well, now's your chance to fix them too. When reassembling, coat the pivot bushings with some good wheel bearing grease. Fit new clevis pins, which only cost about 50 cents. Do not skimp and use a threaded bolt instead, as both it and the pedal will wear away quickly. A couple drops of ATF, or possibly motor oil here completes the job.
________ /___ _ \ Roger Garnett (Roger_Garnett@cornell.edu) /| || \ \ Agricultural Economics | South Lansing Centre For | |___|| _ | 3 Warren Hall | Wayward Sports Cars | | \ | | | Cornell University | (Lansing, NY) \| \ |__/ / Ithaca, N.Y. 14853 | (607) 533-7735 \________/ (607) 255-2522 | Safety Fast! Copyright © 1992-1995 Roger Garnett You may publish this in your club newsletter, provided full credits are given, and you send me a copy.