I was waiting for someone to mention the Sabre. The Sabre was the only 24 cyl. liquid-cooled aircraft engine ever to see mass-production. It had more than it's share of problems, but it worked well enough at the time. The Sabre's greatest weakness, perhaps, was that it put very heavy demands on it's cooling system. Any problem with it and The Sabre would quickly overheat. A high-altitude version of the Sabre would likely have been very impressive but was never developed. In test-stand runs where cooling limitations were absent, The Sabre achieved some truly staggering power outputs. The end of the war ended the Sabre's career due to the Brits' decision to concentrate their resources on the RR engines.