I don't have the bench set up done yet. Maybe this week.
Here is the trigger wheel. The 3/16" OD $2 magnets are rare
earth
from Radio Shack. The trigger wheel has two because I want
to be able to test at over 7000 RPM and my drill motor
only
goes to 3700 RPM. I made this part with a hole saw and a
drill
press.

Not quite as small as a dime Tu Rong :)

Paul Lamar
Here is what the 555 crank angle sensor system could look
like.
The little black squares are the .1" by .1" Hall Effect
sensors.
The 3/16" round black cylinder is the $2 magnet from Radio
Shack.
The OD of this device is about the same as the
e-shaft pulley or about 4.5 inches. Not quite as small
as a dime but getting there. The magnet wheel is attached
to the e-shaft with spacers and long screws.

The Model T type ignition advance cable is shown. The
whole
circuit board rotates up to 30 degrees for starting (0
degrees TDC)
and high altitude advance (30 degrees BTDC).

The circuit board and the magnet wheel are enclosed to
prevent
a broken water pump belt from taking it out.

Paul Lamar

Another thought, High temp magnets, good to 300f,
whereas typical neodymium is safe up to 175f.
http://www.kjmagnetics.com/products.asp?cat=167
I dunno, could be an issue....
C. Smith