Most helicopter manufacturers supply a range of pinions for their helicopters. You may require pinions with more teeth for some reason. The most common reason is for running outrunner motors which usually have a fairly low Kv and therefore require a pinion with a higher tooth count.
Some helicopters use metric pinions, and some use English pinions.
Here is a the conversion from English to metric.
module = 25.4 / pitch
Here is a table with approximate conversions from pitch to module.
There will be extra wear and some inefficiency due to not having an exact match, however this is good enough for most purposes.
32 pitch = about 0.8 module (actually 0.793 module)
48 pitch = about 0.5 module (actually 0.529 module)
64 pitch = about 0.4 module (actually 0.396 module)
There are about six different shaft sizes used in electric helicopters today: 1mm, 1.5mm, 2mm, 2.3mm, 3.17mm, and 5mm.
The 1mm shafts are used by the "N20" style tail motors used in micro helicopters. Wes-Technik stocks 0.5 module brass pinions with 6-15T for this shaft size.
The 1.5mm shafts are used by the GWS IPS-style "performance" tail motors used in micro helicopters. Wes-Technik stocks 0.5 module brass pinions with 7-12T for this shaft size.
The 2mm and 2.3mm (3/32 inch) shafts are used by main motors on the micro helicopters, such as the Piccolo, Dragonfly, Hummingbird, etc. The 2mm bore pinions are easy to find, but the 2.3mm bore pinions are much more difficult to find.
Here is a list of pinions usable in micro helis, sorted by bore size. w/ss means "with setscrew":
old 8T: 48005
old 9T: 48006
old 10T: 48007
old 11T: 48009
old 12T: 48010
old 13T: 48010
old 14T: 48011
new 9T: 48050
new 10T: 48051
new 11T: 48052
new 12T: 48053
new 13T: 48054
new 14T: 48055
new 15T: 48056
new 16T: 48057
9T (Tower Hobbies LXFNC5)
11T (Tower Hobbies LXFNC6)
13T (Tower Hobbies LXFNC7)
8T, 9T, 10T (T4027)
11T, 12T, 13T, 14T (T4023)
9T, 10T, 11T, 12T (34066)
13T, 14T, 15T, 16T (34067)
13T, 14T, 15T, 16T (#385701)
17T, 19T, 21T, 23T (#385700)
7T (Kyosho AZW002-07, Tower Hobbies LXGPF2)
8T (Kyosho AZW002-08, Tower Hobbies LXGPF3)
9T (Kyosho AZW002-09, Tower Hobbies LXGPF4)
12T: A 1Y 2MY05012
15T: A 1Y 2MY05015
16T: A 1Y 2MY05016
Note: -B pinions are blue, -S are silver
11T (MH011TT-B, -S)
12T (MH012TT-B, -S)
13T (MH013TT-B, -S)
The standard Hacker B20-series motors have a 2.3mm shaft, but Aircraft World carries some B20-S motors with a standard 2mm shaft, which makes pinion selection much easier.
Precision Model Products modifies the Team Losi Micro RS4 pinions bored out to 2.3mm for Hacker B20 series motors. These pinions work but are not the best because they are very noisy.
Sparrow Products now sells a pinion adapter (bore reducer) which allows you to use a 3.17mm pinion with a 2mm or 2.3mm motor shaft. This is available from their website.
If you know of another source for 0.5 module metal pinions for 2mm and 2.3mm shafts with setscrews, then please send me a message at RC Groups , because these pinions are difficult to find.
The 3.17mm (1/8 inch) and 5mm shafts are used for nonmicro helicopters. Tower Hobbies carries a wide range of 1/8 inch pinions in both 32 and 48 pitch sizes which are usable in electric helicopters.
Kawada makes 48 pitch 3.17mm pinions up to 40T. RCmart carries a full selection of Kawada pinions.
Please note the Duratrax Mini Quake pinions are NOT 0.5 module and will not work on most micro helicopters. The teeth are too big and the pitch may be 0.6 module, but haven't verified this.
The press-on pinions for GWS EPS gearboxes are 0.4 module and have a 2mm bore. These are usable for the Hornet, but they do not have a setscrew.
The smaller brass press-on pinions (1 and 1.5mm) can be very difficult to remove from motors. I do not recommend the GWS pinion puller for these - I have personally destroyed two GWS pinion pullers trying to remove this type of pinion. However, brass has a very high thermal expansion coefficient which can be used to easily pull these pinions off the motor.
The best way to remove these brass pinions is to clamp the motor in a vise, then press a hot blunt tip soldering iron on the end of the pinion (for maximum heat transfer), wait about four seconds to allow the pinion to become hot and expand, then pry off the pinion using needle nose pliers. Do not skip the soldering iron step, because otherwise you will pull the shaft out of the motor.
The best way to install a brass pinion on a motor is to use a vise. Put the motor and pinion between the jaws, and screw the vise shut to push the pinion onto the shaft. Be sure the back end of the shaft is pressed against the jaw otherwise you may damage the motor bearings.
When selecting pinions, be sure to check your helicopter manufacturer's pinions first. The pinions from the helicopter manufacturer usually work best for most models.