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Those are very plausible sets of circumstances. Particularly the one about a dropped valve getting trapped between the piston and head. I guess that there's no possible way the balancer could hit or interfere with the crank. My condolences to matheusoleiro. Thanks for the follow up.

50 or 50,000 RPM still doesn't answer the question, what was the weak link / what part failed first? That's the main question and it remains unanswered. Without ever seeing the inside of one of these engines, I'll guess that the balancer drive broke, allowing the balancer to hit the crank...

Could the balancer gears failed and allowed the balancer to hit the crank? FWIW; Engines in GP Bikes during the 1970's were only good for one race.

From day one, the engine in my Svartpilen has had a knocking sound. The sound is most evident below 4,500 RPM while under a load. A sound that has the characteristics of a low RPM rod knock. Since my dealer is over 300 miles away, I have no way of verifying if this sound is normal. There's...