166 Manual Drive Trains and Axles Copyright by Goodheart-Willcox Co., Inc. fork to move the sleeve backward and forward, while allow- ing the sleeve to rotate. Some shift forks are made of steel, cast iron, or aluminum. Aluminum shift forks sometimes have nylon inserts at the point where they contact the sleeve. This reduces wear on the fork’s contact points. Manual Transmission Shift Linkage The driver, through a series of shafts, levers, links, and sometimes, cables, operates the shift forks. There are many ways to transfer the force of the gearshift lever to the gears. Some systems are simple, while others are more complex. Shift linkages connect the gearshift lever to the shift forks. They can be divided into two general types of systems— external and internal. External shift linkage External shift linkage is a system that is identified as a series of rods and levers that connect to the outside of the transmission. In addition, some later external systems use cables instead of rods. External shift linkage may be con- nected to the gearshift lever, which is either on the steering column or on the floor. At the transmission case, the external shift linkage con- nects to the inside through shafts that pass through the side of the case. Inside, each shaft connects to a shift lever. The shaft rotates to move the lever. The lever moves forward or backward to move the shift fork. In this way, shaft rotation is converted to straight-line movement at the shift fork. The internal shift lever usually has some type of spring- loaded detent. The detent is a holding mechanism that keeps the lever and, therefore, the fork and synchronizer sleeve in position when in Neutral or in gear. There is also a shift interlock, which ensures that each shift lever is in the Neutral position before the other shift lever can move the gears. This way, only one gear combination can be engaged at a time. External floor-shift linkage is used on modern trans- missions, Figure 8-16. The gearshift lever assembly mounts to the rear of the transmission, usually on the extension housing. The gearshift lever extends upward through a hole slide back and forth on splines cut into the output shaft. It can be moved to mesh with the low countershaft gear or the reverse idler gear. Shift Forks The shift fork, operated by the transmission shift link- age, moves in a straight line to push the synchronizer sleeve into engagement with the gear. Shift fork action is illustrated in Figure 8-14. Shift forks are made to slip into grooves cut into the outer sleeves of the synchronizer assemblies, Figure 8-15. The groove allows the synchronizer assembly to turn with the shaft. The design makes it possible for the Mating gear Matching straight-cut teeth Ridges assist engagement Slots for inserts Blocking ring Cone surface Inserts in place Fork assembly Figure 8-15. Shift forks such as these are used to move the outer sleeves of the synchronizer assemblies. (Ford) Figure 8-13. A typical blocking ring is a brass ring with exter- nal teeth, which match the straight-cut teeth of the mating gear, and a tapered surface, which mates with the tapered surface of the mating gear. Ridges in the tapered surface help the synchronizer to solidly engage the cone. (Chrysler) Shifter shaft Splined shaft Mainshaft gear Groove for fork Synchronizer sleeve (or sliding gear) Mainshaft gear Shift fork Figure 8-14. Shift forks move in a straight line to move the synchronizer sleeve (or sliding gear) in and out of engagement. The shift fork is operated by the shift linkage.