As mentioned in a previous post it is valve to piston clearance which is the critical issue more so than piston hitting head. Dont trust someones word that they used .0XX thou clearance so that should be good in your engine.
How deep the valve seats are in the head, how thick the valve heads are and what valve lash clearance you use will all affect what the minimum bump clearance will be. Probably individual cam lobe profiles have slight differences too. The valve opening overlap near TDC on the exhaust stroke has the valves well off their seats when the piston is at the top.
With new heads and valves I would be very cautious about going with less than recommended clearances. Receeded and worn valves will tolerate less bump clearance but you are in uncharted waters.
You could not have said it better on my 10/1 using the closer than recommended bump clearance did cause me to use a larger valve clearance than the owners manual called for. + the decompression leaver had to be modified to prevent the ex valve from hitting the piston.
Billswan
