The cylinder head

The cylinder head
One of the first jobs on the bike involved removing the cylinder head for an inspection of the head, piston crown and cylinder bore, which is a fairly straightforward job.
On removal there was some expected carbon build up but generally everything looked ok.
One of Stan's specialities was vintage cylinder heads so it was expected that the head would have had some tuning work carried out on it.

Cylinder head at first inspection - looking OK, but what is that circle around the spark plug hole?

Carefully removing the carbon from the inside of the head revealed it to be very highly polished, as was the intake port. The circular line around the sparkplug hole was from where it has been plugged and then re-drilled for a 10mm plug at a slightly different angle, possibly to fit a racing plug.

The cleaned head with valves removed showing a high level of polish and the re-positioned plug.