When tuning ufo's should you drill out the disc to 5/16" and drill out the needle jet
or just do one or the other. I thought I heard somewhere that you should leave the disc
and just drill out the needle jet with a # 55 drill. What size is a #55 in inches?

:) I beleive you just drill out the UFO with a 5/16 drill bit . Did it on both my sleds no problems .

A #55 drill is .052 ". I drilled my out to the largest that they recommended.
I didn't really find that it made much difference. I am sure it did but mine was very rich at Idle. I reduced my pilot jets by about 7-8 sizes from what was stock. I installed the UFO in a 97 mxz 670. Good luck