ISSA - International Slalom Skateboarding Association

What is the size of axle nuts on new trucks?

Greg, they're usually a U.S. 5/16"-24Tpi. Unfortunately, the local Hardware stores tend to carry this type of stopnut in the wide-shouldered sizing, which can mean that you may run out of axle length. The Skateboard Industry uses the narrow-shouldered (low profile) stopnuts on most stock trucks. Automotive supply stores should have them. You can try www.oldschoolskates.com or go to www.McMasterCarr.com for online ordering of replacement nuts.
DO NOT use an all-metal 'Aircraft Type' locknut! These are crimped-end, made of hardened metal, and will wear the threads of your axles. You want 'nylon-insert' Stopnuts, as they'll allow you to change out wheels (clean bearings,etc) fairly often. The all-metal locknuts are for semi-permanant applications.
There are exceptions in sizing, usually being the 8mm X 1.5 (or in the case of one British truck I have, it was 8mm X 1.75, damn hard to find Stateside) on some European made trucks, or if the axles were custom upgraded and the fabricator decided not to reduce the axle size at the threads. Z-Rollers used both 3/16"-32Tpi and 1/4"-20Tpi on their center-roller trucks (which have an 8mm axle), but that's a pretty unique truck.

Thanks so much Eric.

I order all my hardware from www.southcoaststainless.com
They sell everything in polished stainless steel. Beautiful but expensive !!
That's why I wanted to be sure of the size.

Thanks again,
Greg