QR Code Link to This Post Here's what NHRS (http://www.nrhsperformance.com/partsoemheads.shtml) has to say about these heads, better than I can say!The new XB and 2004 XL1200R cylinder heads represent a major step forward in port design from previous OEM cylinder heads. When comparing stock to stock, our testing has shown these heads to be capable of 5-6 more horsepower than typical Thunderstorm heads! Complementing the much improved ports is a set of special lightweight large-diameter valves, single beehive style springs with extra travel, ultra-lightweight retainers & keepers, and an improved valve stem seal design. As a result of these improvements, the new XB/04XL heads are quickly becoming the OEM performance head of choice for XL's and Buells.