Timing gears

[Mike V. Florida]

I have two timing gears, one aluminum, one brass. Where exactly does one measure the gear to determine if it is oversize.

[RHD]

I asked an old Model 'A' mechanic the same question many years ago. He said simply measure the diamater.

[MikeK]

The outer diameter has no effect on the true mesh diameter. That is measured at the tangent contact points of the gear teeth. Not really do-able accurately outside a machinist's tool room.

The easiest field method for comparing (but not actually 'measuring') two timing gears is to take a crank gear and, by hand, press it into the timing gear. Use a caliper to measure the distance between the closest edges of the two gear center holes. Repeat with the second timing gear. I've found "standard" reinforced (laminated) fiber gears that varied 0.006 from each other. So much for quality control.

You can also compare different crank gears the same way. They vary in mesh diameter a few thousandths, too.

[colin1928]

I was always told to measure with feeler gauge should be only a few thou
Colin

[duffeyrj]

You can take a span measurement over a "some" teeth, maybe 5, and compare the two gears.
You might not be want this much information on gears but take a look.
http://www1.gantep.edu.tr/~bozdana/ME472_10.pdf
Pages 4 and 7 will show you the span measurement.

I know that this does not seem like the measurement that you think that you need but it is a measure of the functional tooth thickness and assuming that both gears were manufactured to be functionally the same gear, (same pressure angel, same lead, same module) the tooth thickness is the "diameter" that is controlled in manufacturing.

[duffeyrj]

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Make sure that you are taking measurement perpendicular to the tooth and chose the number of teeth that makes it so that you are measuring right in the middle of the involute curve of the tooth.