The Input Shaft

Input shafts are made of heat treated hardened steel. The shaft and the main drive gear are usually a one piece assembly. The main gear is located inside the case and remains in constant mesh with the countershaft at all times. The input shaft transfers torque from the clutch disk to the countershaft.

Manual transmission input shaft and bearings.

It's supported by bearings that are capable of rotating at very high speeds. The pilot bearing supports the tip of the shaft and allows it to spin at a different speed than the engine’s crankshaft. This difference in rotational speed occurs when the clutch is disengaged while the engine is running.

The pilot bearing's located inside of the engine's crankshaft flange. A faulty pilot bearing is loudest when the clutch pedal is depressed completely and the clutch is fully disengaged. This is because the input shaft has slowed while the crankshaft is still spinning at engine speed. This difference in rotational speed works the pilot bearing revealing its wear.

The transmission's front bearing or input shaft bearing's inner race is pressed onto the input shaft by the main gear and the outer race is fastened to the transmission case by a retaining ring. It supports the inner portion of the shaft. A faulty front bearing is loudest when the clutch is fully engaged and the input shaft is spinning at engine speeds.

Manual transmission main shaft.

The input shaft connects to the output shaft with a set of needle bearings. These bearings allow the input shaft and the output shaft to spin at different speeds. Faulty needle bearings make noise when the clutch is engaged, the vehicle is still, and the transmission is in neutral. This is because the input shaft is spinning at engine speed while the output shaft is held by the motionless vehicle.