Pinion Modulus explained

Matching the modulus of your pinion ( the gear on your motor ) to the helicopter main gear is vital to achieve efficient transfer of power and minimum wear on the gears.

But what is modulus and if you don't know what it is for a random pinion you found in the back of a drawer, how do you measure it?

Although it sounds mysterious all it really means is how many millimeters of gear are there per tooth. But it's millimeters of diameter not circumference that we're interested in here. So to calculate the modulus of a random pinion we need to carefully measure the diameter of the pinion across the middle, from half way up one tooth to half way up the tooth on the opposite side of the pinion. This distance is called the pitch diameter.

Then divide that number by the number of teeth:

Modulus = Pitch Diameter in millimeters / Number of teeth

For example: On my HK250GT I have a random pinion supplied with the motor which has a pitch diameter of approximately 4.4mm and 11 teeth:

4.4/11 = 0.4 Modulus

You may also come across gear pitch which is the same sort of thing but measured in inches and is teeth per inch rather than millimeters per tooth: 

Pitch = Number of teeth / pitch diameter in inches

So my 0.4 Modulus pinion above has a pitch of 11 / 0.1732 = 63.5

Some conversions:

32 pitch = about 0.8 modulus (actually 0.793 module)

48 pitch = about 0.5 modulus (actually 0.529 module)

64 pitch = about 0.4 modulus (actually 0.396 module)

85 pitch = about 0.3 modulus

Pinions are sometimes labeled as "metric 48 pitch which is actually 0.6 module.

To convert Pitch to Modulus:

Modulus = 25.4 / Pitch