Thanks FED. The sleeve valves were indeed the Achilles heel of the Sabre, and Bristol had similar issues of course with their ever more powerful radials. The way that their expertise and tooling was "appropriated" to ensure the reliable production of the Sabre, together with a rob Peter (in this case Pratt & Whitney) to pay Paul (Napiers) arrangement for centerless grinders is described here:-
Sabre II sleeve valve engine | Warbird Engines