The two transmission shaft bearings shown in the figure are known as “eyeglass bearings,” due to their side-by-side arrangement together in a single mounting plate.
A defective twin-bearing mounting plate assembly shows no sign of damage on the outside. It usually makes a slight noise (e.g. whirring, rattling or grinding) while the transmission still operates perfectly.
The car manufacturer recommends use of the following procedure to check the twin-bearing mounting plate assembly:
- Operate the transmission until it is warmed up.
- With the clutch engaged and the transmission in neutral, increase the engine speed to about 4000 rpm.
- Step on the clutch pedal and shut the engine off at the same time.
- If you hear rattling noises which change as the speed of the transmission input shaft slows, the twin-bearing mounting plate assembly is defective.
The increased friction in a defective twin-bearing mounting plate assembly generates excess heat. This can destroy the needle bearing of the positioned fifth gear, and the gear can no longer be engaged. The same thing can happen if there is not enough oil available at the fifth gear. In both cases, the cage is damaged and the needles scatter around the shaft.