Parts of a Manual Transmission – 9 Parts Labeled, Cut Open

All 9 parts on the diagram

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Input shaft The shaft driven by the engine through the clutch. Its gear turns the countershaft.
Main shaft (output shaft) The output shaft, on the same line as the input shaft. Its gears turn freely until a synchronizer locks one of them to it.
Countershaft The shaft below, turned by the input shaft's gear. Its gears are always in mesh with the main shaft's gears.
Synchronizers Main-shaft collars that match a gear's speed to the shaft's. They then lock the gear to it, so the gears change without grinding.
Shift forks They slide a synchronizer's collar along the main shaft. They are fixed to the shift rails.
Shift rails Rods moved by the gearshift lever, each carrying a shift fork.
Gearshift lever The lever the driver moves to choose a gear. It pushes one shift rail at a time.
Reverse idler gear An extra gear that makes the main shaft turn backwards. It sits between the countershaft and the main shaft's reverse gear.
Transmission case Holds the shafts in their bearings. Also holds the oil the gears run in.

How this list was compiled: Read 2026-09-27. (1) US Army, TM 9-8000, Principles of Automotive Vehicles (1985), chapter 22 (paragraphs 22-1 to 22-8 and figures), https://archive.org/details/tm-9-8000-principles-of-automotive-vehicles-1985 : the input shaft, main shaft and countershaft of a constant-mesh transmission; the reverse idler gear; synchronizers; shift forks on shift rails (shifter shafts); the gearshift lever; the transmission case. (2) Wikipedia, Manual transmission, https://en.wikipedia.org/wiki/Manual_transmission : constant-mesh transmissions with an input shaft, a countershaft (layshaft) and an output shaft; output-shaft gears that turn freely on bearings until locked by a collar; synchronizer rings; the selector fork linked to the gear lever and the selector rods; an idler gear for reverse; the transmission casing. Licence: own work

The drawing: Drawn for this site by script, 2026-09-27. Own work, nothing traced. Licence: own work

How it works

These are the parts of a car's manual transmission, or gearbox, cut open from the side with the engine end at the left. The gears are drawn edge-on, as tall blocks.

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Power in, power out

The input shaft brings the engine's power in through the clutch, and its gear turns the countershaft. The countershaft's gears are always in mesh with the main shaft's gears, which turn freely until one is locked to the main shaft.

Choosing a gear

Moving the gearshift lever slides a shift rail, and its shift fork slides a synchronizer, which matches a gear's speed to the main shaft and locks it on. For reverse, the reverse idler gear makes the main shaft turn backwards.

Countershaft and layshaft are two names for the same shaft, the one along the bottom.

Questions and answers

What are the three shafts in a manual transmission?

The input shaft from the clutch, the countershaft (layshaft) below it, and the main or output shaft, which leaves the gearbox on the same line as the input shaft.

What does a synchronizer do?

It brings a gear and the main shaft to the same speed before locking them together, so the gears can change without grinding.

How does reverse work in a manual gearbox?

An extra gear, the reverse idler, sits between the countershaft and the main shaft's reverse gear. The extra gear turns the main shaft the other way.