Hammering sound at TDC

YZ426F. When at TDC, turning the crankshaft back and forth slightly (with a wrench) produces a dull knocking/hammering sound deep inside the bottom end, possibly at the other end of the crankshaft. What’s that? :thumbsup:

BTW the bike runs great.

Could be any number of things, normal or not. Primary gear backlash would be present in every case, and should not be different at TDC than anywhere else. Nothing to worry about.

Two possible abnormalities could be the primary drive gear nut being loose, or the big end bearing having excessive play. The latter speaks for itself. In the former case, the nut on the crank loosens slightly and allows the gear to rattle against the crank splines. If the bike is a 2000 model, this condition can also lead to the balancer drive gear key being battered away, and this would need to be repaired.

Either the big end bearing or the loose gear/key should make an abnormal noise compared with the usual gear lash you would hear in a healthy unit.

Could be any number of things, normal or not. Primary gear backlash would be present in every case, and should not be different at TDC than anywhere else. Nothing to worry about.

Two possible abnormalities could be the primary drive gear nut being loose, or the big end bearing having excessive play. The latter speaks for itself. In the former case, the nut on the crank loosens slightly and allows the gear to rattle against the crank splines. If the bike is a 2000 model, this condition can also lead to the balancer drive gear key being battered away, and this would need to be repaired.

Either the big end bearing or the loose gear/key should make an abnormal noise compared with the usual gear lash you would hear in a healthy unit.

Thanks a lot, Grayracer!

The type of sound it produces while rotating the crankshaft back and forth, makes the impression that the element that’s hammering deep inside the crankcase, is massive, top heavy.

The sound is only heard at TDC. While being in any different position, the crankshaft is silent.

When I just turn the crankshaft CCW several revolutions in a row, there is no knocking sound as well.

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