Transmission Construction

Before understanding the operation and power flow through a manual transmission, you first must understand the construction of the transmission so you will be able to diagnose and repair damaged transmissions properly.

Transmission Case

The transmission case provides support for the bearings and shafts, as well as an enclosure for lubricating oil. A manual transmission case is cast from either iron or aluminum. Because they are lighter in weight, aluminum cases are preferred. A drain plug and fill plug are provided for servicing. The drain plug is located on the bottom of the case, whereas the fill plug is located on the side.

Extension Housing

Also known as the tail shaft, the extension housing bolts to the rear of the transmission case. It encloses and holds the transmission output shaft and rear oil seal. A gasket is used to seal the mating surfaces between the transmission case and the extension housing. On the bottom of the extension housing is a flange that provides a base for the transmission mount.

Front Bearing Hub

Sometimes called the front bearing cap, the bearing hub covers the front transmission bearing and acts as a sleeve for the clutch release bearing. It bolts to the transmission case, and a gasket fits between the front hub and the case to prevent oil leakage.

Transmission Shafts

A manual transmission has four steel shafts mounted inside the transmission case. These shafts are the input shaft, the countershaft, the reverse idler shaft, and the main shaft.

Input Shaft

The input shaft, also known as the clutch shaft, transfers rotation from the clutch disc to the countershaft gears (Figure 10-11). The outer end of the shaft is splined, except the hub of the clutch disc. The inner end has a machined gear that meshes with the countershaft. A bearing in the transmission case supports the input shaft in the case. Anytime the clutch disc turns, the input shaft gear and gears on the countershaft turn.

Countershaft

The countershaft, also known as the cluster gear shaft, holds the countershaft gear into mesh with the input shaft gear and other gears in the transmission (Figure 10-11). It is located slightly below and to one side of the clutch shaft. The countershaft does not turn in the case. It is locked in place by a steel pin, force fit, or locknuts.

Reverse Idler Shaft

The reverse idler shaft is a short shaft that supports the reverse idle gear (Figure 10- 11). It mounts stationary in the transmission case about halfway between the countershaft and output shaft, allowing the reverse idle gear to mesh with both shafts.

Main Shaft

The main shaft, also called the output shaft, holds the output gears and synchronizers (Figure 10-11). The rear of the shaft extends to the rear of the extension housing where it connects to the drive shaft to turn the wheel of the vehicle. Gears on the shaft are free to rotate, but the synchronizers are locked on the shaft by splines. The synchronizers will only turn when the shaft itself turns.