Forget helicoils and all that other stuff, the only real permanent fix is a time sert. Costs around $100 for the kit, agents were in Newcastle, but you could get them at other places as well.
Use a drinking straw in the cylinder and turn the motor over by hand until you feel TDC. This position will trap any grease and scwarf at the top . Tap and insert/install the time sert, cold roll it etc. Take out your household vacuum cleaner and attach a small clear hose into the nozzle end [use tape] then feed it into the cylinder through the sparkplug hole and suck out any metal shavings that could have dropped in there, twist the hose about and cover as much of the piston top as you can.
Job done, and if done right, you'll have no problems at all.
If someone has a down bore arthroscope, borrow it and check!