My conures unpredictable reaction to having his head scratched

[emwee0400]

Around six months ago I rescued a conure, Chucky, and he seems to have settled in extremely well apart from one issue. He is obsessed with having his head scratched and often sidles up with his head tilted to ask. He seems to love it, but will suddenly turn round and bite very violently.
This has never been an issue with my other conure, who will only give a light warning bite if he isn’t enjoying his head being scratched.

I understand it takes a long time for a new parrot to truly settle in, but I don’t want to accidentally encourage this behaviour nor misread what it means.
Does anyone have any ideas about my parrot’s unpredictable reaction to having his head scratched? I don’t want anymore bleeding fingers lol

 

[Mizzely]

Does he have pin feathers? You could be accidentally bumping them and it doesn't feel good!

 

[Dartman]

Conures are known to be a bit nippy. Pippen does similar and I tell her gentle in a calm voice and she'll usually back off and try to be nice for a bit. You definitely could be hitting a pin feather wrong and he hasn't figured out how to tell you except for biting at this point. If he insists on biting maybe walk away and he'll get the idea bites do not get him the results he wanted and will try to be nicer to keep the fun going.
It's all about reaching a understanding with each other and figuring out what triggers him to bite.