The two most frequent complaints involving standard transmissions are noise and shift problems. Noise is a nightmare we'll discuss at another time; for now, we'll devote this article to the mysteries of synchronization.

To understand the shift process in manual transmissions thoroughly, we must first fully comprehend what's happening before, during and after the shift, and how the various components in the transmission are made to work in harmony.

To synchronize is to make two or more events happen at the same time. In war movies, the squad synchronizes their watches so that each member of the team can act together as one unit. Timing an engine is the process of synchronizing the stroke of the piston, the cam, crank, ignition, and fuel delivery to create a smooth, powerful throttle response.

Planetary automatic transmissions have one mainshaft, and shift with the throttle open. Manual transmissions have three or more shafts, and shift with the throttle closed. Once the driver takes his foot off the gas and depresses the clutch pedal, power is no longer transmitted to the input shaft; the front or rear wheels are driving the mainshaft of the transmission. To have clash free shifting, the shaft speeds must match the speeds that the gears are rotating.

Components of a Shift

The driver releases the throttle and presses the clutch. Now the driver moves the shift lever, and the shift fork moves the synchronizer sleeve to a neutral position. The detents and interlocks hold the shift mechanism in a neutral position while the driver selects another gear. As the shift fork moves the synchro sleeve toward the chosen gear, the synchronizer keys (sometimes called struts or dogs) move forward with the sleeve and push the synchronizer ring onto the cone of the speed gear (figure 1). Friction develops between the synchronizer ring and the cone of the speed gear, slowing the gear or speeding it up to match the speed of the shaft that the synchronizer is splined to.

Figure 1

At this time the teeth on the synchronizer ring aren't lined up with the coupling teeth on the speed gear, which blocks the sleeve from engaging the speed gear (figure 2). This is why synchro rings are also called blocking rings. When the speed gear reaches the same rotational speed as the shaft, the synchro ring relaxes and its teeth line up with the speed gear coupling teeth. This permits the sleeve to engage fully with the speed gear and couple it to the shaft, completing the shift.

Figure 2

Any of the parts involved can fail and cause a shift problem. More often a number of the parts will have various amounts of wear that together prevent smooth shifting. Here's list of areas you should always inspected for wear and damage. This list is the basis for an initial internal inspection of the gear box, before writing an estimate or quoting prices.

End play

Excess end play on the input and mainshaft, or countershaft on units that have speed gears mounted on the countershaft, will allow the shaft to move away from the synchronizer, causing grinding shifts, incomplete engagement, or gear jump-out. Excessive end play on the speed gear where the gears can move away from the synchro assembly prevent the ring from influencing the speed of the gear properly, and will cause grinding shifts.

Lubrication

Modern transmissions are designed with blocking rings that are lined with compound, paper or other friction material. All of these transmissions use a specific lube, formulated for proper shifting. The wrong lube fill may be incompatible with the lining on the ring, causing premature wear and poor shift quality. The wrong viscosity may prevent the ring from exhausting the oil between it and the speed gear. This causes the ring to skid on a film of fluid without being able to change the rotational speed of the gear, much like tires that aquaplane when driving over standing water.

Contamination

Inspect the inner threaded or lined area of the blocking ring for signs of steel or thrust washer material. Debris from failed bearings, gears or washers will work themselves into the blocking ring material. This creates steel-to-steel contact during a shift, and prevents the blocking ring from doing its job.

Synchronizer Wear

Before disassembly, match mark the synchro sleeve and hub with a die grinder. The synchro assembly will now be properly indexed for reassembly and you will automatically know which side of the assembly was facing you when you took it from the shaft.

Take the assembly apart and clean it thoroughly. Inspect the hub splines for wear. Look for broken or worn keys and springs, and don't forget to clean the inside of the keys for debris deposits.

Check the syncronizer sleeve-to-fork fit: Any slop here will prevent full application of the ring onto the cone.

Inspect the inner splines of the sleeve for wear. Look carefully at the points of the coupling teeth on the syncronizer sleeve for burrs or rounded, broken points. These usually come from end play problems, a worn clutch that isn't releasing fully, a dragging pilot bushing, or driver abuse (shifting without the clutch, or shifting too quickly in relation to the capacity of the synchronizer).

Look at the sides of the teeth on the synchronizer sleeve for wear or relief on the back taper; this will cause loss of torque lock and gear jump-out (figure 3).

Figure 3

Speed Gears

Inspect all journals on the gear for wear that will cause too much end play. Check the bores for galling or pitting that will cause it to bind on the shaft.

Check the points on the coupling teeth. Damage to the points on the synchronizer sleeve will have matching damage to the points on the speed gear. Reject any gear with rounded or fractured coupling teeth (figure 4). Inspect the side of the coupling teeth for wear or loss of back taper.

Figure 4

Check the cone of the speed gear for pitting, cracks, rust or other wear. Place a known good ring on the cone and measure the distance between the bottom of the ring and the front of the coupling teeth with a feeler gauge. Compare the clearance to the factory spec; if the clearance is too tight, there's too much wear in the speed gear.

But even if the clearance is okay, this measurement only tells half the story. Here's an easy way to check the contact area between the blocking ring and the speed gear: Use 'Prussian Blue' (machinists layout blue) to paint the cone of the speed gear. Place the known good ring on the cone, and twist it onto the cone as it would perform during a shift. Remove the ring and look at the pattern it leaves on the cone: It should provide a clear indication of the contact pattern.

Too often a ring will sit at the correct clearance, but will only be grabbing the cone at the very top or bottom. This reduced contact prevents the ring from putting sufficient friction on the cone. It's impossible for the average shop to measure the taper or micro finish on the cone: Get into the habit of checking the contact pattern, and you'll find that those mysterious shift problems with new rings will go away.

Syncro Rings

After checking for contamination, examine the ring for worn threads or burnt compound lining. Look for cracks through the ring and worn, chipped or broken points. Look carefully at the machined slots in which the keys ride. Wear here will limit the ring's forward travel onto the cone, causing another round trip for the unit.

Shift Mechanism

Worn, cracked, loose shift forks, bent or galled rails, burred or broken detents, or collapsed springs will all prevent adequate synchronizer travel (figure 5). Worn shifter bushing, a worn or loose shift finger on the stick, worn or out-of-round ball seats on the shift housing, and misadjusted or stretched shift cables will all contribute to shift problems. In short, any combination of excess play or slop in the shift mechanism will defeat your best intentions.

Figure 5

Like the old saying goes: "take care of the pennies and the dollars will take care of themselves." Rebuilding any transmission is an exercise in attention to detail. If you understand how it works and are meticulous in checking measurements and proper assembly specs, you'll build units that please your customer and give you the satisfaction of being a top professional.