Hy 50long150GMC5w, did the shaft break in the support/stand, or did it break immediately outside of it? I can't think of anything that would cause that much stress on the shaft, that wouldn't show up as a bent pushrod or a broken rocker arm. If you put all the supports/stands in place, will the shaft pass through them easily? Is there any discolouration on the shaft once it breaks? Could you post some pictures of the broken shafts?
