Being as replacing my avigo motor has been a pain to research I'm going to post the info I find to make life easier for other motor noobs.
Perhaps it's not that common knowlege what the "pitch" of a gear is, or how it's worked out. So here's a nice quick tutorial on how to work out what pitch your gears are
Firstly: the pitch of a gear can (in a very round about sort of way) be thought of as how small the the teeth on the gear are.
A better way to explain it would be as the number of teeth on the gear divided by the diameter of the gear.
So, an example...
Say I have a gear that I found in a parts bag, down the side of a sofa or some other unlikely place, and I didn't know what pitch it was. Here's what I'd have to do:
Firstly, I'd need to know how many teeth it had on it. If it wasn't stamped on the side, I'd have to count them; using a fine marker pen to put a dot on the side of each tooth as I counted it, as to not loose track of which teeth I had counted, and which teeth I had not. Lets say it has 33 teeth.
The next thing I'd need would be the diameter of the gear. Now, this next bit is important! The diameter of the gear is not measured from the ends of the teeth, it's measured from the gaps between them. Pitch of a gear is strictly measured half-way up the tooth profile.
Because the majority of RC cars use imperial pitch, I'll want to measure the diameter in inches.
Now, I'd used a digital caliper to measure my imaginary pinion gear. But if you don't have one: a good steel rule will do the job well enough, as long as you have a sharp eye. Or perhaps a freind with a sharp eye.
So let's say my imaginary pinion gear turned out to have a diameter of 0.515 inches. To work out the pitch is a nice easy calculation, and it would go a little something like this:
number of teeth / diameter of gear = pitch
33/0.515 = 64.077669902912621359223300970874dp
So, I can tell that my gear is near as dammit 64dp. The 0.7766 etc difference between my answer and 64dp can be attributed to engineering tollerances and error in my measurements.
If you're using a steel rule rather than a digital caliper to measure your diameter, you should expect your answer to be further out from mine.
Measuring my pinion gear with a steel rule would have probably given me diameter of 0.5 inches, and therefore a pitch of 66. But because the following are the most common pitches used in RC cars: 64dp, 48dp and 32dp I can pretty much assume that my pinion must have a pitch of 64dp.
Hopefully all of that's helpful to someone.