Look up "valve stem seal" on Ebay- - - - -you should find hundreds of them listed. I believe the stems on a 216 are 11/32" in diameter (0.343") You'll need a .343 inside diameter seal that's big enough to be a slip fit over the valve guide- - - -a dial caliper is definitely your friend, along with a little common sense. The seals that are usually used on exhaust valves have a much shorter "cup" section that the ones for intakes.
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!