Unless the machinist who installs hard seats is VERY skilled, there's a danger of the seats coming adrift and wrecking the engine. The press fit of the seat in the head is very critical to the procedure. Too tight, and there's a danger of cracking a good head. Just a tiny bit too loose, and they fall out. The only time hard seats are a must are on LP gas or compressed natural gas fueled engines.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!