A manual transmission is designed with two purposes in mind. One purpose of the transmission is providing the operator with the option of maneuvering the vehicle in either the forward or reverse direction. This is a basic requirement of all automotive vehicles. Almost all vehicles have multiple forward gear ratios, but, in most cases, only one ratio is provided for reverse.

Another purpose of the transmission is to provide the operator with a selection of gear ratios between engine and wheel so that the vehicle can operate at the best efficiency under a variety of operating conditions and loads. If in proper operating condition, a manual transmission should do the following:

Be able to increase torque going to the drive wheel for quick acceleration.

Supply different gear ratios to match different engine load conditions.

Have a reverse gear for moving the vehicle backwards.

Provide the operator with an easy means of shifting transmission gears.

Operate quietly with minimum power loss.

TRANSMISSION CONSTRUCTION
Before understanding the operation and power flow through a manual transmission, you first must understand the construction of the transmission. This is necessary for you to 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 (fig. 4-13). 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 (fig. 4-14). 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 either a steel pin, force fit, or locknuts.

REVERSE IDLER SHAFT.- The reverse idler shaft is a short shaft that supports the reverse idle gear (fig. 4-15). 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 (fig. 4-16). 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.