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.
The transmission's front bearing or input shaft bearing's inner race is pressed onto the input shaft. The outer race is fastened to the transmission case by a retaining ring. It supports the inner portion of the shaft. A faulty front or input bearing is loudest when the clutch is fully engaged and the input shaft is spinning at engine speeds.
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. The input shaft is spinning at engine speed while the output shaft is held by the stationary vehicle.
Manual transmissions have an input, an output, and a countershaft. Manufactures incorporate endplay into the input and output shafts to provide heat expansion and lubrication.
Measure the input and output shaft endplay before and after removal. Check end play with a dial indicator fastened to the case and the tip placed against the input shaft. If the reading is out of specifications, look for worn or misassembled parts while disassembling the transmission. Typically a thicker or thinner selective washer is installed to adjust input shaft endplay.
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 completely depressed, and the clutch is fully disengaged. It's 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.