Ignition is always set at the maximum advance.
It is a bit dangerous to use the cam lobe as the index point of ignition timing, partly because the exact moment of valve movement depends on the valve lash and partly because of the wear and play of the cam lobe and mechanism. That is if you haven't measured the exact time the valve closes/opens with a particular valve lash when the heads are off...and written a note in the engine logbook of it...
Now...it is also a bit dangerous to assume that the ignition cam opens the breaker at the exact time that it is supposed to, on both cylinders, because the breaker cam also wears and wears uneven...That should also be measured with the correct breaker gap setting, on both cylinders with the heads off...and noted in the logbook...the other cylinder can ignite way late if the cam is worn or if it is a cam for a engine with 45 degree cylinder vee split..
With iron heads, it is better to be on the safe side and ignite a bit later than early, there is a lot of different ignition timings in the service books floating around, everything between 7/32 for th 37ci, to 5/8 special racing.
Standard 101 37ci engine with iron heads is 5,5mm 7/32"
Standard 101 45ci engine with iron heads is 7,9mm 5/16"
Sport Scout, 3/8"-7/16" in one of the books. There is no specific notes and without knowing, I suppose aluminium heads 11,1mm 7/16" and Iron heads 9,5mm 3/8", but I would/might start with 5/16" on a fresh engine, not yet run in..
I tried 7/16" on my engine (by mistake) with iron heads and the pistons nipped.
So,what to do with the heads on? A rough setting can be done with a semi stiff bailing wire. If you can see down the spark plug hole and measure what distance below the cylinder top surface and the piston top at TDC, (maybe already noted in the logbook?) then take a bailing wire and bend an L on it, the L foot length should be 5/16" (or ign. time of your choice) + the distance to the piston. Put it through the spark plug hole, rest the wire on the cylinder top surface at the edge and when the piston touches the wire tip, then the breaker should just part, a paper between the points should loosen up..
But it is a bit crammed on the cylinder top surface, the exhaust valve can be in the way and prop up the bailing wire so watch out for that, position the bailing wire with care so you don't get a false reading.
