Interlock Arm Assembly

How it works
Removal

The 4-synch interlock arm assembly (arrowed) in the gearbox extension under the remote control housing:

The blue arrows indicate where the top plate is recessed into the top of the casing (under a paper gasket):

Removed:

How it works:

The Interlock Arm Assembly ensures that only one gear can be selected at any one time. The pivoting fork of the interlock arm (red) sits in cut-outs in the three selectors (blue), and has a slot for a finger on the selector lever (green). When the gearbox is in neutral the finger on the selector lever sits in the gap between the arms of the interlock fork. As the gear lever is moved from side to side the finger on the selector lever pivots the fork from side to side, through the cut-outs in the selectors, from being in line with the 1/2 selector to being in-line with the 3/4 selector or the reverse selector. Left is the 3/4 plane, centre is the 1/2 plane, right is the reverse plane:

When the selector lever is in line with a selector it is free to move that selector up or down to select a gear. As each gear is selected the selector moves up or down into the gap in the fork preventing the fork from pivoting away from that selector. The arms of the fork are in the cut-outs of the other two selectors so preventing their movement into gear.

Left: 1st gear selected. Right: 2nd gear selected.

Left: 3rd gear selected. Right: 4th gear selected.

Reverse gear selected.

For reverse the selector arm will be positioned lower left depressing the reverse plunger to operate the reversing light switch. For 3rd gear the selector arm will be upper right, 4th gear lower right. The interlock arm assembly has been removed here, the selector arm doesn't move this far across in practice:

Interlock arm assembly removed for clarity. Gearbox in neutral, all three selectors aligned horizontally, selector arm finger in the 1/2 selector:

1st gear selected (1/2 selector lifted up):

2nd gear selected (1/2 selector pushed down):

3rd gear selected (3/4 selector lifted up):

4th gear selected (3/4 selector pushed down):

Reverse gear selected (reverse selector pushed down):

The reverse gear selector is prevented from being pushed up because when disengaged the idler gear is hard up against the inside of the case.

Removal:

Select 4th gear to move the selector arm finger down out of the fork on the interlock. Lift the interlock out of the recesses:

Pull the lower, longer part towards you so the fork pivots and the top part comes out of the casing:

Angle the fork to the left to disengage it from the selectors and remove, a bit of wiggling will probably be needed:

Refitting is - inevitably - the reverse of removal, ensure 4th gear is selected and it should slot straight in. However! Fitting the remote housing becomes more of a fiddle than fitting the interlock because as well as having to slot the plastic bush on the remote arm into the socket on the selector arm the cover also has to be positioned so that the locating pin in the gearbox casing (arrowed above) slots into the hole in the remote housing. Until that happens the housing will not sit flat on the casing and it's all too easy for the plastic bush to get cocked in the socket, and because the selector arm is free to move the interlock can get dislodged. Really with the interlock in you need to select 1st gear by manually moving the selector arm, and that should keep both in position.