I've found some good buys on top end parts used in circle track racing. Does anyone have any EXPERIENCE with this stuff? I know they run at sustained RPM much longer than street/strip motors, but what can wear out on a set of heads? Guides, valve seats, what else. Some of these packages include shaft rockers and roller lifters. I would be leary of the lifers but have no experience with shaft rockers. What should I watch out for other than a spread port design, angle milling or expoxy porting?