1977 Yamaha XS750
You're right, the stem doesn't stretch but have seen them break at the neck on car engines running huge spring pressures and solid lifters. What the valve will do is tulip, can post a pic later. Seats can recess.How are you going to stretch the valve
Valves don't stretch they expand with heat to grow in length, the valve springs are the only thing 'stretching' the valve length and the springs are not strong enough to deform the valve stems length. Ever.