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Manual Transmission Box

Typically, the basic principle of manual transmission box is to change the rations of the gears to reach the different motive performance in different situations.
Manual transmissions are characterized by gear ratios that are selectable by engaging pairs of gears inside the transmission.
The dynamic power will flow through the input shaft ,the layshaft and then send to the terminal from the output shaft.

This is an exploded illus. of the transmission box T5 STD.

We can see that a transmission box is composed of over 100 differnet parts ,and the gears and the toothed parts are in the majority around over 70%. Except the case and some minor parts we can sort them up by the shaft into 3 sets.
Typically, transmission has three shafts: an input shaft, a countershaft and an output shaft. The countershaft is sometimes called a layshaft.

Synchronizer Rings

The synchronizer ring is the critical part in the manual transmission box.When the driver shifts the speed, the different gears of the input and output shaft will mesh up. This will cause a hard impact and may not make the meshing correct. The synchro is the absorber and delay the time of meshing up as a synchronizer between this motion. That is why it named Synchro. Some also called it baulk ring or block ring.

In this illustration , The Exploded of T5 STD output shaft.
Ignoring other 2 shafts, we can also imagine the relation between the matching gears on other shafts according to the previous illus.
The part #47 ,named sleeve, will be shifted by the part #24 ,the fork,to mesh up with the part #79,speed gear.
The #73, synchron ring ,is the important part to help this meshing fluent and correct.

Why replace them?

The synchro rings are friction rings that spin up the transmission to match the drivetrain prior to shifting. This allows the gears to mesh without grinding. As soon as the synchro rings begin to wear, then the gears start to mesh at different speeds, resulting in gear grinding. Synchro rings should be replaced as soon as gears begin to grind. Failure to do so will result in damage to the sliders and the synchro hubs which are much, much more expensive to replace.