The Armstrong Siddeley booklet tells how to set up the toggle action using some marks which are allegedly to be found on the bus bar operating bush A33. None of the Riley units that I have seen have these marks so here are some British Army REME training notes for the Daimler Scout car. They describe a simple method for setting the toggle action if the original setting has been totally lost.
To be able to feel the "kick" described it is usually necessary to operate the gear pedal by hand. Make only small adjustments at a time and be prepared for the pedal to return with great force if you go far past the over centre position or you do not press the pedal sufficiently far to allow the operating strut D53 to engage with the bus bar D34.
The Scout car has 5 forward gears. For a Riley set 3rd gear as per the Scouts 4th gear, and set top as per the 5th gear. Reverse should be set similarly to 1st.
Each gear requires a different amount of toggle action. This is the reason that the gear change pedal returns further in the lower gears.
I have added links to figure 2 of the A/S booklet.
The procedure is as follows:-
This method is also described (for the Daimler P/S gearbox) in "Motor Engineering" by George Newnes.
The text says:
There is a limit to the possible increase in toggle action due to the brake pull rod and the pivotal point of the table eventually coming in line with the toggle link fulcrum. When this point is reached any further lengthening of the brake pull rod (due to unscrewing of the brake adjuster nut) will result in the gear coming "over centre". The exact moment when this occurs can very easily be obtserved, as the gear engaging pedal will kick back suddenly.
The "over-centre" position makes a very convenient datum from which to work when resetting gears, and the amount of toggle action given to a gear can be accurately described in terms of "the number of turns after which the adjsuter nut ceases moving when pumped up from the over-centre position". The fewer the number of turns from the over-centre position, the greater is the amount of toggle action.
The torque reactions exerted by the brake bands are greatest in the case of first and reverse, and in conswequence it is necessary to give these a greater amount of toggle action. Average settings for the various gears are as follows:-