I have an SA XRD8 rear hub. The nuts use a 15mm wrench to loosen/tighten.

With my fenders on, the stays interfere enough that I can’t really pull the axle back far enough to tighten the chain adequately. When I remove a link, it tightens but the axle is only about half in the dropouts. It has big SA acorn nuts on it now.

I would like to put on regular nuts to tighten the axle with the fender stays unattached. Then, once the axle is tightened, put on the stays with another nut holding them in place.

So.... what size nuts are they? What measurement am I looking for to buy online or at LBS?