They normally do... have a look on the Kent or Piper website, I think even Catcams show it. Most radical cams have around 4-5mm lift at tdc, some show 6mm but only if the cam is radical with a lot of advance... but never much over the above. In a chamberd head it's not normally an issue. They found more power when slotting rather than just conventional cut outs... possibly due to disturbed combustion, but the difference was only a couple of hp... but a gain is a gain. In effect, you'll need to follow a Ford Crossflow (Kent engine) style piston... you'll need around 30cc in the piston with around 100 thou deep cut out's. You'll have to juggle the numbers subject to the cam, c.r required, valve diameter. A dummey build will be essential to check clearances regardless of how close you can get the machining.