This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
Whether you have just installed a new clutch cable, or simply need to adjust it bit from regular driving, the procedure is simple and straightforward. There are a few different types of clutch cable assemblies used on the 911, and the adjustment processes are different for each of them.
The early 911s had a relatively simple clutch adjustment mechanism. Despite the fact that there were four different clutch adjustment mechanisms during this period, the procedure is very similar for all of them. To adjust the clutch, release the adjuster nut near the underside of the transmission until the clutch pedal free play at the pedal is about one inch. The free play is measured by pulling back on the clutch pedal from inside the car. The pedal should travel about one inch when you pull it towards you.
Now, tighten up the locknut underneath the transmission. Start the car and with it at idle, press in the clutch and wait about 10 seconds. Then select reverse with the shifter. If the transmission grinds going into reverse, then you will need readjust the cable at the release fork end, underneath the transmission. The grinding indicates that the clutch is not disengaging all the way when you press in the clutch pedal. You use reverse to check the gears grinding because the reverse mechanism doesn't have any synchros that will disguise poor adjustment in your clutch.
When the clutch pedal is pressed completely to the floor, the travel on the throw out release fork should be about 0.6 in (15mm). Of course, this distance is highly dependent upon the condition of your clutch components. Worn clutches may require more movement, new clutches may require less. If the release fork doesn't move enough to disengage the clutch, then you will need to adjust the cable again, or check the other end of the cable that is attached to the pedal cluster. If you need to tighten the cable more than the amount that you can at the transmission, then you will have to adjust the trunion and pin on the clutch cable that attaches to the pedal cluster. See Project 9 for more details on gaining access to the trunion end.
Depending upon the wear in your clutch disc and pressure plate, you may have to play with these adjustments quite a bit until you get the right feel. Unfortunately, there is no exact science for adjusting the clutch on these early cars. The rule of thumb to follow is that you tighten the cable underneath the car if the transmission grinds into gear, and you loosen the cable if you cannot engage the clutch to the drivetrain (the clutch slips). Adjust the nut only three to four turns at a time. The clutch is quite sensitive to changes in the cable length, and once you get into the ballpark, only minute changes in the cable are necessary to dial in the clutch to your preferences.
For 911s with the late-model 915 transmission, there is a specific factory adjustment procedure that seems to work quite well. With the cable completely disconnected, adjust the small stop bolt attached to the large release lever arm, until you have a clearance of 1.2mm between the stop bolt and the smaller actuating lever arm (see Photo). Use a feeler gauge to measure this gap. Once this distance is achieved, then tighten up the lock nut to keep the screw from turning. Now, attach the clutch cable end to the small hook on the lever arm. Using the nuts captured on the clutch cable, tighten up the cable until the previously measured gap decreases to 1.0mm. Tighten both nuts on the clutch cable, step on the clutch pedal, and recheck the measurement. Readjust if the clearance has changed from the 1.0mm baseline.
If you have trouble meeting this distance, within the range of the adjustment nuts, then you might need to readjust the trunion pin at the pedal cluster end of the cable. See Project 9 for more details on this procedure.
Once you have the primary adjustment set, you need to measure and check the amount of cable travel at the release lever (see Photo). You will need an assistant for this task. Measure the distance that the release lever travels when the clutch pedal is depressed. The total travel of the clutch cable should be 25mm (0.5mm as measured at the transmission). If the travel is not in this range, then adjust the rubber stop underneath the clutch pedal accordingly. Refer to the photos accompanying this project for the exact locations of the clutch adjustment points.
The clutch on the 911 from 1987 to present isn't adjustable. The clutch master/slave system that is implemented with the new G50 transmission is pretty much maintenance free, and self-adjusts for wear on the clutch components. You can however, set the total clutch pedal travel using the same adjustable stop on the wooden floorboard behind the pedal. The total horizontal travel for the pedal should be 150mm--10mm.
Before 1977, the 911s have a single arm that the clutch cable is attached to. Adjustment of the cable is performed by rotating the small nut (shown by the arrow) until the proper throw is obtained. Although there were four different variations on this design, the principles for adjustment are the same. Adjust the nut until you obtain about one inch of free play in the clutch pedal. Then check your results by starting and driving your car. Keep in mind that if your cable is too loose, the car will be impossible to shift into gear without grinding or stalling. If the cable is too tight, then the clutch will never engage, and the car won't move.
In 1977, Porsche installed a helper spring mechanism (See Pelican Technical Article: Replacing Your Clutch Helper Spring) to give a better feel to the clutch pedal. This mechanism is a bit more complicated to adjust, but involves less guess-work. With the clutch cable disconnected, adjust the small screw on the small actuating lever so that the clearance is 1.2mm (shown by arrow). After the clearance on the small actuating lever is set to 1.2mm, hook up the clutch cable and adjust the nuts on the bottom until the clearance gap decreases to 1.0 mm. At this point, tighten up all the nuts on the cable and drive the car to test the clutch setting.
The total throw of the clutch cable should be 25mm (.5mm as measured at the transmission). If this distance is not within the specified range, then adjust the rubber stop that is attached to the wooden floorboard behind the clutch pedal arm in the passenger compartment.