I have technical books that are a great source of general technical information about cam design for slow running (1000 to 1500rpm) four cycle engines. I know the criteria used for valve lift, cam flank radius, nose radius, base radius, and almost everything else related to cam design. The one thing that I am vague about is cam duration. I make both intake and exhaust cams identical on the engines which I build. Most of my cams are made to more or less match the profile of cams designed and built by Malcolm Stride of the U.K., which were used on his Lynx and Bobcat engines. These cams had a duration of 115 degrees as per his drawings. Since there is a 1:2 ratio between the crankshaft and camshaft, this means that the cam affected the engine through 230 degrees of the crankshaft movement. This meant that the intake valve would begin to open 20 degrees before the piston reached top dead center, remain open thru 180 degrees of crankshaft rotation, and close 60 degrees after the piston had reached bottom dead center. But if you add those numbers up, that is a total of 260 degrees, so his cam should have a duration of 130 degrees, not 115 as per his cam drawing in one of his published plan sets. Maybe I'm reading his drawing wrong.---Or maybe his timing diagram is wrong--Or maybe he's taking valve tappet clearance into consideration. I have also read elsewhere that cam duration for a slow running engine should be 120 degrees, which translates to 240 degrees of affected crankshaft rotation, which lets the valve begin to open 20 degrees before top dead center, remain open for 180 degrees of crankshaft rotation, and close forty degrees after bottom dead center. George Britnel, if you read this, what duration are the cams you use or recommend for slow running four cycle engines?---Brian