How do I measure a small piston to head clearance

[Bluebell]

I just want to say a big public "thank you" to Brian (who I have never met)
My two C7RA heads arrived back today, all nicely machined to give 0.00" clearance (without a gasket) in each cylinder.
Brian wouldn't take any money for doing the job (not even the courier cost) He said these flatty's are his passion.
All the best, to you, Brian.