I've got a Piko I use on my road bike and love it.

I don't love the fact that I keep losing the O-Rings used to mount to the helmet plate and the one on the bar mount.

Gretna sells the helmet plate replacements for $5 or so but I didn't want to wait for delivery or pay for the same.

Went to Home Depot w/ screws and O-Rings in hand...

Danco #36 (5/16"Outer Diameter x 3/16"Inner Diameter x 1/16" wall) fit the helmet mount (the blue O-Rings).

Danco #60 (1/4"OD x 1/8"ID x 1/16" wall) fit the handlebar clamp.

~$2.50 for a pack of 10 of each.

Anyway, those are the sizes you need. The kit should come with extra ones because they're super easy to lose when you're moving the thing from mount to mount or removing from the bike, etc.