I think I can get my head around a smaller valve having a higher percentage of its radius giving more percentage surface available for heat transfer, but still . . . doesn't the larger valve have more actual seat surface? The more seat surface, the more heat is transferred from the head to the valve. Heat, along with other (less important) paths, then is carried up the stem and dissipated. Excess fuel is used for heat dissipation too when we (wastefully) use it in that way. It's been ten years since I studied this stuff so obviously I could easily be mis-remembering things. I hesitate to question my understanding of valve size/function though since it seems to violate common sense. Or maybe there's just some confusion of ways of thinking about this. Quite apart from any geometry considerations, it seems if there is more seat surface (there's gotta be . . . it's a larger valve!), there will be more heat absorbed, thus a hotter valve. Heat transfer is one of the primary functions of the valves so it only makes sense that if the larger valve absorbs more heat, it's going to run hotter than a smaller valve. It's also going to do a better job of heat extraction due to its larger mass with which to absorb this heat. A smaller valve is of course going to run cooler in this comparison but since a primary function of the valve and seat is to transfer heat, what is the advantage of the smaller, cooler valve?
I've had no trouble at all from combustion chamber cracks between the valve seats. No trouble with over heating (because I prevent it through attention to oil temp and through extracting power using what I think of as torque as opposed to running it at a high RPM.) Other than the broken springs problem I've had no troubles at all with this GP 2180. My only problem was due to RIMCO re-using old valve springs when they did the TOH for Steve. Since I've no doubt they re-used the valves as well, given enough time I could have eventually had a valve failure. Putting things back together using the best components I could buy and operating the engine within its heat limitations has prevented any of the issues mentioned on this thread. I think if everyone carefully built their own engines from kits and, if not, re-built their engines to make sure everything is correctly lined up (replacing parts with better ones if necessary), there wouldn't be much discussion about problems with aviation use of the VW.
The idea of sodium-filled exhaust valves is a good one but when I went looking I didn't find any that would fit my engine. Maybe I just didn't look hard enough but don't think so . . . they just weren't made for the VW at that time. Since the D-3000 uses Chevy pistons and cylinders (I think), it's not unlikely there might be sodium-filled valves available for this engine.