U Tips (spiral bone tips) 7/16 in (11mm)

Takes some practice...

...but on my third attempt I was able to fit these on my steel. This is purely on my skill and not on the product. Some advice: I found that it was easiest to use a pair of linesman pliers or something similar, which a thick flat pinchy end. Use that to gently pinch the cap flat, then keep a finger to press and hold the cap on the steel while you use the pliers to pinch the sides in. Keep switching between the two until the cap is snug--slow, incremental adjustments worked best. It might be worth buying a few extras.