In my list mostly onroad engines: http://www.euronet.nl/users/tooms/timings/timings.jpg

Head clearance is determed by the combustion chamber. Older Novarossi's were set to 0.7 and later are set to 0.55. Picco is at 0.5 or even 0.4 so far I have seen (@ 25% nitro). Also the headclearance is set by the nitro percentage.