Great Timing. I'm just about to start building the Airfix 1:24 Typhoon with that amazing Napier Sabre sleeve valve engine. I was wondering how the sleeve valves worked and this paper showed me the answer. I also didn't realize that it wasn't the only sleeve valve engine and saw why those Bristol radials looked as they did. Read it top to bottom, and while much was pretty arcane, the summaries were easy for me to grasp. Sleeve valves had some performance advantages, but they were inherently heavier, harder to cool and were more difficult to maintain. That may account for why they didn't penetrate the market. I still think the Napier Sabre is one cool engine, even though it may run hot.