Chapter 8 Manual Transmission Construction and Operation 159 Copyright by Goodheart-Willcox Co., Inc. • Synchronizers—bring certain transmission gears to the same rotational speed as the output shaft before sliding in mesh with them to lock them to the output shaft. • Shift forks—pronged units that move gears or syn- chronizers on their shaft for gear engagement. • Manual transmission shift linkage—allows the driver to operate the shift forks. • Manual transmission case—encloses transmission shafts, gears, synchronizers, shift forks, linkage, and lubricating oil. In some designs, it is combined with the clutch housing. • Extension housing—encloses the tail end of the trans- mission output shaft, which extends from the trans- mission case. It also encloses part of the shift linkage. In addition to these seven major parts, there are numerous accessory devices mounted on the transmission. Some of these devices, such as the speedometer assembly, receive their input from the transmission. These accessories will also be discussed in this section. Manual Transmission Shafts Transmission shafts are directly or indirectly involved in transmitting motion. A shaft may rotate to perform its function, or it can be a stationary support for a moving gear. Transmission shafts are made from high-strength steel. They are closely machined and polished. Often, they are heat- treated to increase their strength and resistance to wear. All rotating shafts are supported by bearings attached to the case. These shafts are machined with splines and grooves designed to mate with other transmission parts, such as gears, bearings, and snap rings. Nonrotating shafts are machined smooth so that the gears can rotate on them easily. They are held in place by pins or locknuts or are press fit. In some instances, the shaft is not attached to the case and can rotate in a random manner. There are many variations of shaft design and place- ment in manual transmissions. Figure 8-2 shows the four basic shafts found in all manual transmissions: the manual transmission input shaft, the countershaft, the manual transmission output shaft, and the reverse idler shaft. Manual transmission input shaft The manual transmission input shaft, splined to the clutch disc, delivers power to the transmission. Sometimes called the clutch shaft, this rotating shaft transfers rotation from the clutch disc to the countershaft gear. The input shaft, as described in Chapter 6, is supported on the front end by the pilot bearing. The transmission main drive gear is machined on the inner end of the shaft and contained within the transmission case. The transmission front bear- ing, which is held by the transmission case, supports the inner end of the input shaft. The input shaft and front bearing are held in place by the front bearing retainer. This retainer prevents forward shaft movement. The output shaft prevents rearward move- ment of the input shaft. The front bearing is kept from moving by a snap ring, which holds it solidly to the case. In some instances, a snap ring is used for holding the main drive gear to the bearing to prevent lateral shaft movement. Transmission input shaft Clutch Transmission case Shift forks Shift linkage Transmission output shaft Extension housing Transmission gears Synchronizers Bellhousing Figure 8-1. This cutaway shows parts of a typical rear-wheel drive manual transmission. Note the position of the transmission in rela- tion to the clutch. The output shaft turns the drive shaft. All rear-wheel drive transmissions are in this relative position—directly behind the engine and directly ahead of the drive shaft assembly. (Peugeot)