Don't mean to sound like a broken record, but the biggest problem with popping the heads off the sodium filled exhaust valves was the seat pressures. Ford printed that the seat pressures should set up at 110# for the sodium exhaust valves. I raced a 427 with sodium exhaust valves and hollow stem intake valves for many years without a single failure. I set the seat pressure at 110# exhaust, and 130# for the hollow stemmed intake valves. Went through the lights at 72-7400 rpm nearly every run. I had a local racer that just had to have my 427, and he offered me enough money that I let it go. He took the engine to his local machine shop, did a fresh valve job. The engine broke a valve on start-up, and ruined that cylinder. He got all hostile with me, and claimed I sold him junk. I asked him what his seat pressure was on the exhaust valves, and he replied with "Huh"? They had set it up like a 460 Ford with killer seat pressures on the valve springs. I explained the problem to him, and he never spoke to me again for over 20 years. There are some nuances to the FE, and that is one every old timer should remember. 110# for the sodium exhaust valve seat pressure--max. I would not use them today, since the sodium eats away the interior of the stem over time. Joe-JDC