In addition to possible causes already offered here's one more: A cracked cam gear hub. At certain RPMs a flopping cam gear can make a mechanical tap or knock every 1/2 crank revolution, yet the engine will continue to run fine - at least until the gear hub breaks apart.
It's a common failure on Super M/400/450 gassers with the live hydraulic pump. I've had to replace cam gears on both my Super M and a friend's 400.
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Today's Featured Article - History of the Nuffield Tractor - by Anthony West. The Nuffield tractor story started in early 1945. The British government still reeling from the effects of the war on the economy, approached the Nuffield organization to see if they would design and build an "ALL NEW" British built wheeled tractor, suitable for both British and world farming.
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