The limiting thing with any 4 stroke will be the valve clearance from the head and this is going to vary with different engines depending on design so some careful measuring will need to be done first. The critical part is the overlap period and in full size engines I believe around .060" is considered a minimum clearance. With model 4 strokes it would be much smaller than that and if I were to guess I'd say .010" would be my limit. Allowance has to be made for the fact that as the piston slows coming into the overlap period then the rod is doing all the work of slowing the piston to a stop at TDC so all the bearing clearances stack up plus the rod will stretch slightly (after all, the piston has to slow down at a rate of 2000G's or so). This decelleration can rip off the ends from a rod sometimes. Now where have I seen a photo of a rod like that?