The extruder is the bit that pushes the plastic in to the hot end and this is a "Wade" type extruder which includes gears from stepper to the hobbed bolt that actually forces the plastic in to the hot end.
The gears are described as 11t17p and 39t17p. Well, this weekend, one of the teeth snapped off. It was just working well enough to print a new 11t17p and carry on, but naturally I felt I could make better gears.
The 11t and 39t is easy - "number of teeth", but the 17p fooled me. p is pitch, so I assumed it was circular pitch - i.e. tooth to tooth spacing along the pitch circle on the gears. I got close with 0.17" and even 17 pixels (where pixel is 1/90th inch). Not quite though. Turns out the unit is 17 teeth per inch diameter, so 17 per π" so no wonder I could not work it out.
The results are on thingiverse now. They work well. I made chevron style self centering gears, and I made the gears have 45 degree edges top and bottom for easy printing and nice appearance. They mesh well with no slack. Within minutes of posting I have several downloads.
Next replacement part - a Z axis wobble arrestor!!!