Dan is "NOT" an old fart......... Yes the Thunder tiger Pro 40 is legal in 424, and so are a lot of other faster, and more expensive engines. In AMA 424, the only criteria, is the 16,500 RPM range. This number is a little bogus on the start line in my opinion. There are some high dollar engines that might peak out at over 19 K, that like to be backed off to the 16,500 RPM number. I have seen some guys throttle back, & even have a switch on the TX set, to attain the 16,500. The honest guy that has a lesser timed, smaller venturied engine, that runs best backed off 300 RPM to 16,500 suffers. But as it is, they check RPM on the line, ready for the flag.........I think peak RPM should be considered......