Im currently building a radical predator engine. I have pretty aggressively ported RTC-8 head. I was wondering if anyone made some stainless steel valves with the 5mm valve stems. If not how can I make some fit. I was thinking of drilling out the valve stems on a drill press but i don't know if I could get the tolerances right. Also thought of robbing some 5.5mm out of an older non-hemi head. But then I'm not sure the valve guides would fit.