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motor is at 3/8ths of an inch before TDC. This is the benchmark for stock timed Triumph 650 motors, 1950 to 1983. Set points juuuuust to break at that point. be sure you are coming up on the lobe in the Hunt mag going in the correct direction of rotation, otherwise you are setting it at 3/8ths AFTER tdc.
 

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wes is right,i go with 5/16 though,got a little ruler to find tdc,back up 5/16.i use a cig paper or celophane between the points to tell when there just about to open,works good,kinda farmer style but works everytime,,got one on my red bike tryin to find one for blue good luck
 

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accuracy and significant figures:

a measurement of 7/16 can only be said to be accurate between 6/16 (3/8) and 8/16 (1/2). an error of, say, 1/32" or less in this application will vary the rotation by a few degrees one way or the other, especially since we're looking at the piston as it nears tdc. remember that the rate of piston movement in relation to small changes in crank rotation varies from 0 at tdc to it's max near 3:00 and 9:00. at a few degrees before tdc, a very small error in measurement of piston position will result in a large error in crank rotation.

for anybody even remotely interested:

http://www.chem.tamu.edu/class/fyp/mathrev/mr-sigfg.html

(courtesy of Jdub)

i personally use a degree wheel
 
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