In my opinion, before an engine is run, it's the best it will ever be, After it starts getting run, it's starts it's journey towards worn out. The only engines that might possibly benefit from a break is a ringless pinch fit engine. Oh sure, a new ring and cylinder may seal better after it run for a while, but it doesnt require any special break in, it will wear in at it own pace either way. Bearings don't require any breakin, there at their best when there new. I never break engines in, and I have never had a problem. I dont even run em fat when their new anymore.