[Healeys] Core plug

[Per Schoerner]

Hi
Has happened to me too, on my newly restored Sprite engine. I thought I 
had hit the plug enough, but apparently not. Installed a new plug and 
gave it a couple of good smacks, and the others too, and they have held 
on ever since (7 years ago). Doomed plugs, you have to hit them in the 
centre so they expand in to hole. If you hit them correctly they will be 
impossible to remove without destroying I think.

Per