Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Transmission (M66; 2004-2010): Design: Synchronization
The synchronizing hub assembly consists of a coupling sleeve, flange and the hub. Each synchronizing hub is joined to its shaft by splines and rotates with it.
There are idler wheels on both sides of each synchronizing hub. The idler wheels rotate freely on the shaft (does not apply to reverse gear). Each idler wheel is constantly engaged in its pinion. The synchronizing set is positioned between the synchronizing hub and the idler wheel.
The synchronizing units are positioned on the shafts in the transmission as follows:
- The synchronizing unit for 1st - 2nd gear is on the intermediate shaft 1-2, 5-6
- The synchronizing unit for 3rd - 4th gear is on the intermediate shaft 3-4
- 5th-6th the synchronizing unit is on the input shaft
- the synchronizing unit for back-up (reverse) gear is on the back-up (reverse) shaft.
The synchronizing rings expand when heated by the same amount as the components they are in contact with. As a result no safety margin is required to counter expansion.
The idler wheels for single and double synchronization have no cones. An inner ring on the synchronizer unit performs this function instead.
The synchronizer unit is a modular system. This makes it easy to upgrade from a single synchronizer to double synchronization by replacing the synchronizer kit.
Single synchronization , which has one friction surface, is on:
- back-up (reverse) gear
- 4th gear
- 5th gear
- 6th gear.
The outer ring (to the left in the illustration) is manufactured in pressed steel. The inner ring is pressed steel and the friction surface is coated with sintered brass.
Double synchronization , two friction surfaces, is on:
- 3rd gear.
The outer ring (to the left in the illustration) is manufactured in pressed steel. The sealing ring is pressed steel and the friction surfaces are coated with sintered brass. The inner ring is machined steel.
Triple synchronization , three friction surfaces, is on:
- 1st gear
- 2nd gear.
The outer ring (left in the illustration) is made of pressed metal. The intermediate ring is made of pressed metal with a sintered brass coating on the friction surfaces. The inner ring is of pressed metal with a sintered brass coating on the inner friction surface. Triple synchronization also includes a ring integrated with the idler wheel.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.