Over time, the plunger that operates the diverter got stripped threads: it could no longer screw into the mechanism itself and we were not able to use the shower.
I tried gluing it with epoxy but this didn't work.
Using this method we were able to repair it.
Step 1: Measure
The original threads were brass, I'm using stainless steel.
I measured the existing length of the plunger and tried a variety of threaded rods that I had, in order to get the correct size.
Step 2: Cut Off Damaged Threads
Step 3: Drill a Hole in the End of the Plunger
I used a small nail set to punch a divot into the end of the plunger so the drill bit would centre itself
Step 4: Tap New Threads
I then screwed the section of threaded rod in as far as it would go, and marked it to cut off based on the lengths measured from the first step.
Step 5: Glue in Threaded Rod
I waited for that to dry and then screwed it into the tub filler body.