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.
One of the most common repair procedures for the 911 is the replacement of the clutch assembly. Unfortunately, it is a rather big process involving the removal of the engine. The good news is that it's really not a very difficult job if you have some information, and a few hints and tips.
The first step is to make sure that you gather all the required parts for the job before you begin. It is very frustrating to get half way through a replacement job, only to find out that you need a part or a tool that you don't have. Here is a list of what you will need to replace the clutch on your 1965-1986 911:
- Pressure plate
- Clutch disc (it is recommended to use the spring-center type instead of the rubber-centered type)
- Throw-out bearing
- Replacement throw-out fork (1972-1986 only)
- Pilot bearing (for flywheel)
- Resurfaced flywheel (good core usually required)
- New Flywheel bolts (six for the 1965-77 911s, and nine for the 1978-89 911s)
- New clutch cable
- New flywheel seal
- Clutch helper spring (1977-1986 only)
- Transmission fluid
- Throw-out bearing guide clips (1965-1969 911/912 only)
- Throw-out arm retainer clip (1965-1969 911/912 only)
- Throw-out fork arm bushings/seals (one bushing for the 1965-69 911s, and two seals for the 1972-86 911s)
- Clutch alignment tool
- Heavy duty torque wrench
- 911 Flywheel bolt removal tool
- Flywheel lock tool
As mentioned previously, the first step in replacing the clutch is to remove the engine from the car. Project 7 details the steps required for the removal process in preparation for performing the clutch job. The early 911s (1965-1969) were equipped with the 901 transmission, and had a 215mm push-type clutch system where the pressure plate was pushed inwards in order to disengage the clutch. While still using the same transmission design from the earlier years, in 1970 Porsche moved to a 225mm pull-type design, in which the pressure plate is pulled away from the flywheel in order to release pressure on the disc. In 1972, Porsche unveiled the 915 transmission, and still kept the push-type flywheel. In 1987 Porsche released the G50 transmission, complete with a hydraulically operated clutch mechanism. For the purpose of this project, we'll focus primarily on the early transmissions from 1965-86.
The first thing that you need to do after you remove the engine is to remove the pressure plate from the flywheel. The pressure plate is attached using hex cap key screws and should come off quite easily when they are removed. You may have to use the flywheel lock in order to keep the crankshaft of the engine from turning while removing the bolts. Remove the starter ring (1970-86) and place it on top of the engine. It is very important not to forget the reinstallation of the starter ring: it is a mistake that most professional shops have made at least once.
The clutch disc should fall out after you remove the pressure plate. On the early cars, there was only one type of clutch disc used. This clutch disc used a spring center system that cushioned the shock of clutch engagement. The springs help to make the ride a bit smoother when the clutch is re-engaged. Sport-type performance and racing clutch discs often have a stiffer center spring setup, or no springs at all to give a much quicker response when engaging the clutch.
Starting in 1978, Porsche used a rubber-center clutch disc design that didn't seem to hold up as well as the spring discs. Very often with age, the rubber eroded and disintegrated, rendering the clutch useless. With the use of the spring discs, the driver of the car can clearly feel a lack of performance from the clutch, prior to it failing. Unfortunately with the rubber-centered discs, clutch failure can be sudden and rather abrupt, leaving you stranded somewhere with a car that needs to be towed.
With this in mind, I recommend to install the spring-center clutch disc replacement instead of the rubber-centered type installed as original equipment. The performance is very similar, but the reliability and durability of the older, spring-type design has been proven superior. Porsche subsequently abandoned the rubber-centered clutch disc in later years, and returned to the spring-center type as original equipment.
Now, the flywheel needs to be removed. Install the flywheel lock, and use the flywheel bolt tool to remove the flywheel bolts. Be careful that you don't strip out the bolts. The heads of the bolts are very shallow, and the tool has a tendency to slip if you don't hold it perfectly vertical. If one of the bolts strips out, the only recourse is to grind the head of the bolt off with an angle grinder, which will most likely cause some damage to your flywheel.
After the bolts are removed, the flywheel should simply pop off. Take the flywheel to your local machine shop and have them clean, check, and resurface it for you. If the flywheel is too worn to resurface, then you will need to replace it with a new one.
The flywheel seal is a troublesome spot for leaks, and it is recommended that you replace it while you have access to it. To remove the old seal, carefully take a screwdriver and pry it out. Be careful not to touch or mar any of the metal sealing surfaces around the seal. It's ok to destroy the seal as you get it out: it's going right into the garbage after it comes out. The new seal installation process is simple: just tap it in straight with a dull hammer.
Installation of the new flywheel is almost always easier than the removal process. For 1965-79 911s, install the flywheel pilot bearing inside the center of the flywheel by carefully tapping it in with a dull hammer. If you pound on it too much, you will destroy the bearing and have problems later on. Tap it in straight until it is flush with the top surface. The 915 pilot bearing (for 911s from late 1979-86) bolts onto the flywheel itself. On both types of pilot bearings, make sure that you put a little bit of lithium grease on the bearing before you install it.
Tightening the flywheel bolts is a chore. Start with one bolt, and then move across the center to one on the opposite side. Set your torque wrench at about 20 Nm to start, and torque down all the bolts. Then repeat the process after increasing the wrench torque value by about 20 Nm. Don't tighten the bolts in a circle, but crisscross your pattern. Final torque values for the 1965-77 911s is 150 Nm and the 1978-86 911s is 90 Nm.
Once the flywheel is installed and torqued down, place the clutch disc against the surface of the flywheel, and support it there using the clutch alignment tool. The purpose of the tool is to keep the disc centered while you bolt down the pressure plate. If the disc is not centered, you will have great difficulty remounting the engine to the transmission.
If you have a 1972-1986 911, you will need to install and attach the throw-out bearing to the pressure plate. Assemble the bearing according to Diagram 1, and be careful to place all the rings and spacers in their correct orientation. If you have a 1965-69 911, you can just install the pressure plate on the flywheel: the throw-out bearing is installed on the transmission.
Install the pressure plate and bolt it down, pressing the clutch disc in place. Make sure you reinstall the starter ring on 1970-86 911s. When you are finished, you should be able to easily remove the alignment tool, and the clutch disc should not move at all.
Now that you have completed all the tasks related to the engine, it's time to move to the transmission. On the 1977-1986 911s, it is a wise idea to replace the shift throw-out fork (cost on the order of $65) because they sometimes bend and break over the years. Tap out the small pin that holds the fork to its shaft, and it should slide right off. There are two small seals that fit on each end of the arm as it rides on the shaft. Make sure that you replace these seals if yours are deteriorated or missing.
On the early 911s, the throw-out fork is prohibitively expensive to replace with a new one, so most people don't bother. Three important items to replace though are the throw-out bearing guide clips, the plastic bushing on the inside of the arm, and the throw-out arm retaining clip. Using a hex key, remove the bolt that holds the retaining clip to the throw-out arm. On the back side of the arm you will see the small ball cup bushing. Pick the old one out, and tap in the new one, adding some lithium grease as well. Reattach the arm using a new retainer clip. When you install the new throw-out bearing, make sure that you use new guide clips, and also grease the guide bearing shaft as well. If the shaft is significantly worn, replace it with a new one.
The final step is reinstalling the engine and/or transmission, adjusting the clutch (see Pelican Technical Article: Clutch Adjustment), and taking the car for a spin!
The 911 complete clutch package should include everything that you need. This particular package is for the 915 transmission from 1972-1986. Shown here are: 1-Pressure plate, 2-Flywheel (resurfaced and cleaned), 3-Spring centered clutch disc, 4-Clutch alignment tool, 5-Throw-out bearing & clips, 6-Clutch throw-out arm, 7-Helper spring, 8-Flywheel seal, 9-Flywheel bolts (quantity 9), 10-Pilot bearing
The procedure for removing the flywheel seal is a bit tricky. The goal is to remove the seal without damaging any of the important mating surfaces that surround the seal. Using two screwdrivers, place one across the end of the crankshaft to use as a brace to pry the seal out with the other one. Don't worry about damaging the seal: you will be replacing it anyway.
This photo shows the new flywheel seal and pilot bearing installed on the 915 flywheel. Make sure that the new seal is mated flush with the case by tapping it with the blunt end of a hammer or socket ratchet. Don't forget to place a little lithium grease on the pilot bearing before you attach the clutch disc and pressure plate to the flywheel.
Tightening the flywheel bolts requires a special tool, a flywheel lock, and a heavy duty torque-wrench. You can make your own lock using a thin strip of metal with two holes drilled through it. Bolt one end to the flywheel, and place the other around one of the studs in the engine case. When tightening down the flywheel bolts, start with one bolt and then move across the center, tightening them to about 50% of their final torque value. Repeat the procedure several times, increasing the torque value about 20% or so until you have reached the final torque. Check, and then double check the bolts to make sure that they are torqued to the correct specifications.
Place the new clutch disc against the flywheel, and set its center alignment using the clutch alignment tool. If you don't use the tool throughout this procedure, you will have great difficulty mating the engine back up with the transmission. The tool sets the location of the clutch disc so that the input shaft from the transmission can easily mate with the clutch disc when the transmission and engine are remated.
Install the pressure plate with the alignment tool in place. Make sure that you attach the throw-out bearing and check its operation before you mount the pressure plate to the flywheel. The alignment tool should easily slide in and out of the clutch disc in this configuration.
The 915 transmission throw-out arm is often a source of clutch problems. Over many years of use, it has a tendency to develop cracks and break. It is a wise idea to replace this arm while you have the engine out. New ones are relatively cheap insurance against having to remove the engine again. Make sure that you replace the thin upper and lower seals (shown by arrows) that flank both the top and bottom of the arm.
The 901 throw-out arm seldom breaks, but there is a small bushing contained within the arm that should be replaced with every clutch job. Remove the retaining clip from the rear of the arm to remove it from the transmission. Pick out the old bushing, and press in the new one. Make sure to spread some lithium grease in the new bushing prior to reinstalling it.
Diagram of 915 clutch assembly. This diagram shows the original rubber-centered disc, which is not recommended for use on your 911. Make sure that you replace your original rubber-centered discs with the spring-centered discs. 1-Socket Head Screw, 2-Lock Washer, 3-Pressure Plate, 4-Starter Gear Ring, 5-Clutch Disc, 6-Circlip, 7-Thrust Washer, 8-Release Bearing, 9-Washer, 10-Spring Washer