Over the years, clutches have presented a modest problem in rotary powered vehicles. In large part, the problem has been the result of poor low RPM torque; however, this problem has lessened as each successive generation of engines produced better low RPM torque characteristics.
Driver technique plays a large role in ensuring long life from a clutch disc. The clutch needs to be slipped somewhat more in rotary engined cars, because of poor low RPM torque, to start the vehicle rolling smoothly; however, if one slips the clutch excessively, the disc can overheat and result in premature failure. Technique is important: a clutch that is driven properly can reasonably last anywhere from 60,000 to 100,000 miles, or more.
Clutch Slippage Diagnosis
Clutch slippage eventually becomes a problem, independent of driver input: specifically, as a clutch disc wears it becomes thinner and the pressure applied to it by the pressure plate drops to the point where the clutch may slip under load. If the car is driven vigorously, it is not uncommon to first notice slippage when shifting into third gear. It is less likely that you will detect slippage in either first or second gear since you tend to move relatively fast through these gears under vigorous driving.
Another possible cause of clutch slippage is the slave cylinder release clearance. While there are too many factors to list that can jointly or separately contribute to this condition, checking for the problem is relatively easy. Reach into the bell housing area and pull the exposed, outermost end of the clutch fork toward the slave cylinder. If you can manually move the end of the clutch fork 4" or more, you have adequate fork-end-to-slave cylinder play at the moment. If you have less than 4" play you are almost out of clearance. If you are unable to obtain any play in the end of the clutch fork, you are completely out of clearance; the result is the same as if you were driving with your foot partially depressing the clutch pedal all the time. In this instance, if the clutch is not slipping already, it soon will.
At the first sign of slippage, replace the worn parts! A slipping clutch can be "nursed along" for only a short period of time. Do not allow the problem to persist; doing so will likely cause damage to other components that are more costly to replace.
An occasional problem which is also influenced by driver technique is clutch chatter when engaging the clutch in either first or reverse gear. The smoother the driving technique, the less likely the occurrence of chatter. We have developed an Engine Torque Brace for all 1979-92 RX-7s that will substantially reduce, or even completely eliminate in some cases, this chattering problem. This brace mounts to the rear of the engine which connects to the fender area inside the engine compartment. This brace will dampen the "shuttering" movement of the engine as the clutch disc attempts to mate to the pressure plate and flywheel.
In some instances, causes other than driver technique can cause clutch chatter. For example, oil or grease on the disc from: a leaking front transmission seal; lubrication from the input shaft splines at installation; or handling the disc with greasy hands prior to assembly. Once a disc becomes significantly contaminated it must be replaced. This problem is usually identified by greasy radial streaks on the hub of the clutch disc and black deposits on the lining.
Another cause of clutch chatter is excessive flywheel runout, which can be caused by a bent eccentric shaft, improper fit between the eccentric shaft and flywheel, or a mis-machined flywheel. While these problems occur less often, they must be considered when trouble-shooting a clutch chatter problem.
Unexpected Clutch Engagement
Unexpected clutch engagement is another problem that can occur. In this particular situation the transmission is in gear, you have fully depressed the clutch pedal and are keeping it depressed, but the clutch appears to be engaging without your release of the pedal. An additional unexpected clutch engagement symptom is the crashing of gears during shifting and/or difficulty engaging a gear from standstill.
The two most common causes of unexpected clutch engagement are: a leaking clutch master or slave cylinder, or a failing pilot bearing in the end of the eccentric shaft. To determine if the problem is related to clutch hydraulics perform the following:
Depress the clutch pedal fully for about a minute and then let it up slowly until the strong spring force of the pressure plate is released. If you find that there is a greater-than-normal amount of clutch pedal travel left before the pedal returns to its it "fully engaged" position, this excess travel strongly suggests a clutch hydraulics problem.
If a slave cylinder piston seal fails. fluid will leak into the slave cylinder boot, and eventually "overboard". You can remove the slave cylinder boot, and by visual inspection, easily ascertain if fluid is leaking. Also, the fluid level in the clutch reservoir will drop.
If the clutch master cylinder is the source of the unexpected clutch engagement problem the fluid level in the clutch reservoir may or may not drop. The master cylinder can fail and bypass fluid internally, leaving no trace of fluid leakage. If the master cylinder is indeed leaking fluid externally you can visually inspect the unit where it enters through the firewall by sliding the rubber dust boot off the end of the cylinder and look for fluid. Again, in this instance the fluid level in the clutch reservoir will drop.
Assuming the clutch pedal travel is normal and there is no sign of fluid leakage, it is possible that the pilot bearing in the end of the eccentric shaft has failed through inadequate lubrication. In this instance, the pilot bearing is binding against the input shaft and acting as a clutch of sorts between the eccentric shaft and the transmission. Whenever you remove a transmission always inspect the bearing and replace or lubricate with a heavy grease as necessary.
Pressure Plate Installation
When installing any 1988 and earlier non-turbo RX-7 pressure plate, either stock or high performance, it is critical that you note that two (2) of the bolts used to retain the pressure plate differ from the other bolts. These two bolts, commonly referred to as "reamer bolts", have an unthreaded section just below the bolt head. Be sure that one of these two bolts goes into the bolt hole on the pressure plate which has a small hole adjacent to it and that the remaining reamer bolt is fitted into the hole on the opposite side of the pressure plate. These two bolts must be mounted into the two threaded holes in the flywheel which have been counterbored to accommodate the unthreaded section of these bolts. Torque all bolts to 14 ft/lbs.
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