Cylinder head work, 1918

Unknown

I'm at the stage with this engine of looking at the cylinder head. Got the new valves from Egge but they could not supply the piston rings, still searching for these.

I am after your guidance on some issues that others may have already had to deal with:

The top face of the plug hole are very pitted and it is my intention to carefully machine these to give a better sealing surface for the plug washer, how much metal thickness is there here, anyone know?

Also one of the plugs has had a helicoil insert fitted at some time but this has screwed out easily, anyone had this problem and can offer advice?

As quite a few of the other threads are corroded/pitted and 'weak' I had considered tapping out the head for 22mm plugs rather than helicoiling again to 18mm. This would solve the weak threads and the helicoil insert problem but (knowing from previous posts) the heads are a weak point is there sufficiant metal there to drill and tap to 22mm? and if this has already been done by someone what make/model of plug was used?

I look forward to getting a reply.

Kind Regards

Peter

(England)