Taking a grinder to Britain's motorcycling heritage.
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Tuesday, 21 February 2012

Hunting Teeth

It's one of those mysteries that has bugged me and others I know, when the valve timing is set the marks are, as you'd expect , in line. Yet when the motor's turned over once the marks are not in line any more, and won't be until it's been turned over 42 times!  Knew it was called Hunting, but never quite grasped the ins and outs of it, time to Google her on up! It is, by all accounts, known as a Hunting Tooth Gear Train, and is done to even out the wear across all of the gear teeth on the circumference. 
Because the number of teeth on the Crankshaft Pinion and the Idler Gear cannot be divided by the same number, they are running out of synch with each other to an extent, so a tooth on the pinion must contact every tooth on the gear before it returns to contact the original tooth it started on. This has the benefit of distributing the hard work of lifting the valves off of their seats evenly around the idler, rather than continually hammering away at the same few teeth as it would if there was a common ratio, that is, if the idler was 50 teeth the same as the cam gears. Although all of the marks do not line up again for something like 90 odd revolutions, the crank and cam marks will be in the same relative position every two turns of the motor, due to their inherent 2:1 ratio, which is good. The idler and it's marks are spinning away "hunting" the start position and it doesn't matter where the marks are on that as it's position is not critical once the timing has been set once.