Detent springs for 1-2 and 3-4 gear selection
To replace or not replace?
Selection and sorting
Page list

The six detent springs fitted within each of the two synchronising sleeves have two functions, and the effectivness of both of these functions is directly proportional to the stiffness of the springs and their length.

First function: Synchronising effectiveness

When engaging 2nd, 3rd or 4th gear, the six springs determine the force with which the gear's synchronising cone and the operating sleeve, moving together, engage and thus how quickly and effectively the rotational speed of the two parts is matched. Then as the force on the gear lever is increased by the driver, eventually, the detent springs are compressed enough such that the operating sleeve moves towards its gear, now sliding over the synchro sleeve which cannot move as it is hard up against the gear's synchro cone.

In this phase of operation, the combined stiffness of the six springs directly affects the synchronising effectiveness. In the extreme, if the six springs were not present, or all were broken, there would be zero synchronising effect, and the gearbox would become a "crash box".

Second function: Maintaining the position of the operating sleeve

Once a gear is selected, in this case one of 1st, 2nd, 3rd and 4th, the six detent springs push the six balls against the inside of the operating sleeve and help keep the operating sleeve in its correct position engaging the selected gear. This is because there is static friction between the six balls and the operating sleeve, even though there is plenty of oil present. However, also keeping the operating sleeve in place is the relevant detent spring and ball acting on the recess in the associated striking rod. The latter is probably more important holding the car in gear than the detent springs inside the synchro sleeve.

For reverse gear, only the striking rod detent holds the car in gear. 

stiffness test

Measuring the stiffness using a 4kg load suspended below a countersunk screw passing through the spring sitting in a small 1/4" socket in a vice.

springs measured

All fifteen new springs ground flat on each end, and measured for selection.

 So, should the detent springs be replaced?


The short answer is yes. All (or virtually all) springs lose stiffness over time. But by how much is hard to quantify. However, the springs themselves are both low cost and readily available, so it is not a hard decision.

The new springs I purchased had not been ground flat on their ends. I rigged up a platform that enabled me to grind each end of each spring on the side of a conventional grinder (fine wheel), using a drill bit inserted inside each spring as a safe holder.

It is imperative that the springs are frequently cooled during the grinding process. If the metal turns blue, you are going at it too hard!

Having ground all fifteen springs (three spares!), I then measured the stiffness of one new spring and one old spring. The end result was that the tested new spring was 9% stiffer than the tested old spring (0.92mm/kg vs 1.00mm/kg). Good news.

The next step was to measure the length of all 12 old springs and all 15 new springs, and then select a set of six new springs for the 2nd gear synchro duty, and a second set of new springs for the 3rd-4th synchro duty.

The average length of the old springs was 16.60mm, and average length of the new springs was 16.65mm - pretty much the same.

But both the old springs and the new springs varied in length. So for the rebuild, I selected the six longest springs for the 2nd gear synchro duty (the toughest duty, and the next six longest for the 3rd-4th synchro duty. The end result was that the average length of the 2nd gear set was 16.90mm, and 16.4mm for the other set.

The overall result of the selection process was that the 2nd gear detent spring stiffness in operation was 10.9% higher than the old springs, and the 3rd-4th set 6.9%.

To what extent the effectiveness of 2nd gear synchro is improved will be determined only after the gearbox is back in the car.