Clutch flywheel. Porsche 911 1984 - 1986

A flywheel in general (for a manual-transmission car) has several critical roles:

• Mass / inertia storage — It smooths out the pulses of the engine, aiding when cylinders fire and bridging the gaps between combustion events.
• Clutch friction surface — The clutch disc presses against the flywheel, so the flywheel must have a suitable, stable friction surface.
• Crank / sensor interface — In this case, because it is used with Motronic control, the flywheel provides trigger pulses (teeth or marks) used by the engine control (DME) to know crankshaft position or engine speed.
• Balancing — It must be precisely balanced (often as part of the entire rotating assembly including crank, flywheel, clutch) to avoid vibration and stresses.
• If the flywheel is worn, warped, cracked, or misaligned, all of these functions are jeopardized (vibrations, clutch slippage, sensor error, etc.).

Common failure modes / why / signs & symptoms

• Because the flywheel is a high-stress rotating component, over time or under strain it is subject to wear, damage and failure. For this particular part and era, here are common issues:
• Surface wear, scoring, glazing, heat spots on the friction surface: repeated clutch engagement or slipping can overheat zones, creating hard “spots” or cracks.
• Cracks / fractures: under severe stress or impact, cracks can develop, especially around bolt holes or near sensor teeth.
• Distortion (warpage): heat cycles may warp the flywheel face so it is no longer flat, causing uneven clutch engagement or chatter.
• Sensor trigger damage: damage or wear to the trigger teeth or notches may make the engine control module misread crank position or lost signals.
• Bolt failures / loosening: the 9 bolts attaching the flywheel to the crank are critical; if they stretch, loosen, or fail, misalignment or imbalance can result.
• Imbalance / degradation of balancing: over time, if the mating surfaces or mountings shift or fatigue, the initial balance may be compromised, introducing vibration.
• Oil contamination: if the rear main seal or other seals leak oil onto the friction surface, the clutch/flywheel interface becomes compromised (slippage, glazing).
• Fatigue / wear of dowel pins or alignment features: if alignment dowels (or roll pins) degrade or are missing, mounting may be off, leading to misalignment. In a forum thread, a user found small roll pins (alignment inserts) that had come off the flywheel.