Depending on the application and thickness of the spar, I would either use aluminum rivets or tap the spar and use bolts. If you use stainless be sure to coat it with silicon or something to prevent electrolysis. On my M 15 I used rivets to attach hardware to the boom. Bert Felton M 15, Zephyr ************** You're invited to Hollywood's biggest party: Get Oscars updates, red carpet pics and more at Moviefone. (http://movies.aol.com/oscars-academy-awards?ncid=emlcntusmovi00000001)