Hi Neil,
According to your cam spec you have .309 lift at the lobe, with 1.65 rockers that gives .309 x 1.65 = .509" lift = 12.95mm

My cam spec(camtech 621) has .399 valve lift with 1.5 ratio rockers, this calculates back to .266 lobe lift. I'm using the std rockers at 1.44 ratio, so my lift should be .266 x 1.44 = .383" = 9.72mm..
So you have about 3mm more lift than me.. i would be hitting the pistons for sure... yet you have no problems...?i must have someting different somewhere, maybe its to do with the cam timing ?

I wonder if my head has been skimmed in the past... I cc'd the chambers and got about 30ml which according to tech wiki is correct...so i asssumed not.