igor, my vastly modified side valve Austin Big 7 (1937 manufacture) achieved 6,500 rpm with standard valve springs. The bouncing valves did make marks on the cylinder head, though. (The only valve train modification apart from bigger valves was lightened, reshaped tappets.)
In principle, a side valve engine can achieve higher rpm than a pushrod engine, because even after you put in ultra-strong valve springs, the main mechanical limitation is still valve bounce, and the side valve has less weight in the valve train (no pushrod or rocker). Of course they scavenge very poorly and have very long flame front travel distances, so they aren't likely to produce much power at high speed, and there is seldom any reason to try for it. I notice the ride-on mower racers take their side valve B&S engines to 6,500, but I don't suppose they expect an engine life of more than three or four minutes. Those aluminium rods seem to be able to achieve remarkable things, if you can only lubricate the big-end bearings, and the racing guys seem to make some modifications in that area.
It all reinforces my view that thrown rods in B&S engines are nearly always caused by lack of oil.
