It would also be possible to use a short 5/8-18 bolt, and drill and tap the bolt head for a 1/8" NPT thread. Use a copper washer from a brake caliper flex line fitting as a gasket between the bottom of the bolt and the sensing bulb seat in the cylinder head.
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!