I had two 50sx engines and currently have two 55AX engines. The first 50sx smoked it's rear bearing and started giving a death rattle. OS made good on it and replaced it. The second started going that way so I switched to the 55ax's and have not had a problem with either of them. No I did not run them lean and they never dead sticked. They just tossed their bearings. I have seen other posts here regarding OS 50sx bearing issues. This may be why the engine had a reletively short production time and was replaced with the, IMO, superior OS 55ax.