How do you find the correct PA of an existing gear

Amazon has 14.4° and 20° PA gauges in fractional and metric module. The lowest cost one is $70.00 for metric 20° and $88.00 for fractional 14.5°

A bit pricey for me, but for someone who makes a lot of replacement gears I guess its not too bad.

Roy
 
Handy little calculator here that gives all the measurements needed to figure out what you are dealing with with spur gears:

I got shown a neat trick which was to download and print out gear tooth profiles then roll a snake of Plasticine and lay it on a cutting board.
Roll the gear along the Plasticine then with a craft knife wet with some WD 40 or similar slice along the Plasticine. This will give you a clear profile to compare with your printed profiles.
Gauges were beyond my reach.
 
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