Finally found some motivation to drive to that winter cold garage of mine for a short mechanical investigation on that second hand engine I bought. With the inlet and exhaust valves almost touching each other, I was curious about the exact measurements. The inlet is 41.3mm thus being exactly what I have in my engine now and the second biggest.
Available at Burton at the time being:
38.15mm
39.3mm
40.6mm
41.3mm
42.7mm
The exhaust valve is 34.9mm compared to 34.0mm actually in my engine.
Available at Burton at the time being:
31.6mm
34.5mm
34.9mm
36.0mm
36.2mm
So really nothing spectacular to report here. At first sight, the remaining flange between the valves looks quite narrow, but it is always best to unmount the parts when measuring. There is still some possible improvement when preparing the cylinder head of the replacement engine.
Cheers.