2nd and 4th/3rd Syncro hubs Diagram3 Diagram 4
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To eliminate the need to double de-clutch, the synchromesh gearbox was introduced. The basic gearbox is laid out in the same manner as the constant mesh, but with the addition of a cone clutch fitted between the dog and gear members, as shown in Diagram 3. The male clutch member is on the gear and the female on the dog clutch hub(it's the other way round in the Herald G/B)1, which has a splined outer sleeve located by a series of balls and springs. Movement of the selector moves the whole dog clutch until the cones make contact, when the friction brings them both to the same speed. Continued movement of the selector causes the spring-loaded balls to be overridden and the internal splines of the outer sleeve engage with the dog teeth on the gear. However, the change must not be rushed as it takes time for the friction at the cones to bring the rotating gears to the same speed. Change gear too quickly and there may still be some noise.

To overcome this disadvantage, the Baulk or Blocker Ring System was introduced(also known as Syncro Rings)2 - see Diagram 4, Radial movement of this additional ring is limited to half a spline width, which prevents the dog teeth from engaging until the speeds of the two members are equal.

If your car has a synchromesh gearbox it is recommended you do not double-de-clutch as this can cause undue wear on the synchro rings3.

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