People mention about "leaky" front bearings. A metal shielded front bearing with 1/2-2 thou clearance between the shield and the inner race will never be air tight. The only air tight ones I have seen are the rubber sealed ones. In the R/C car forums, people blame a leaky front bearing for lean runs. Well I have Two OS CVR .15's, One with a metal shielded front bearing and One with a rubber shielded bearing. Both engines run the same with the same needle settings. I don't believe in blaming front bearings for lean runs.