Okay I'm no professional like several of you guys. I'm just a garage hack and build my own motors. I was not aware that the guides needed sized different for marine use, and I'm not suggesting that they don't need special clearances. However, given the number of engines I have built over the years, without any guide failures, leaves me wondering why they need special sizing, or why I haven't had any guide issues? My latest build (http://www.performanceboats.com/dyno...o-results.html) used AFR heads but my head guy supplied the valves and springs. Since the new motor currently has less than an hour of run time, off the dyno, do I need to pull the heads to check the guides?