checking port timings

Dumb question for all you wise men when checking port timings do you use the piston crown or top ring?

The piston crown edge for exhaust and transfer ports and the rear piston skirt for inlet ports.

Thank you Trevor I will double check my readings but it looks like I have some wild timings (inherited) compared with those on the forum and tuning book,I will put them up and you can tell me if I'm wasting my time.

Trevor
hope this is not a case of over experimentation by previous owner? the timings I have are as follows set with a dial gauge for TDC and a degree disc fixed to crank and a pointer set to zero to start. ex opens 97ATDC transfer opens at 117ATDC inlet opens 87.5ABDC clockwise rotation (.001 feeler gauge in port)?????

Mikewt,
Before we get too far into port timings, could you let me have dimensions for both stroke and con-rod centre lengths please. If the engine is a standard Bantam, the lengths will be 58mm and 125mm respectively. Variations of any combination of stroke and rod length for a 125 capacity produce different port heights for similar timings, and this can affect suitable choices.
Be assured about your current timings, they are very far from being in the category of "over experimentation" and seem quite modest!

Trevor