Had to drill out an 823 rim a while back to fit a shraeder tube in an emergency. Unfortunately, I did a pretty poor job and the hole is not completely round. Tried to go tubeless with a Presta style valve and no joy, lots of air escaping, would a shraeder valve be any better? Or is there anything else I could try? I was wondering if yellow tape around the valve hole might help.

try a noflats schraeder valve – they have pretty wide bases (in fact I file mine donw on 2 “sides” to make them fit my flow rims a bit better

If that on its own fails, shim it with a circle or two cut out from an inner tube before you tighten it down (and / or mastic – I tend to put a bit of that under the valve seat these days but I wouldn’t buy it just for that use)

i had same issue and bought some plastic valve reducer . works perfectly .

Thanks, thought I might try the no flats valves, before the post, will definitely give them a go.

Slight thread resurrection but just to say I had almost given up. However, gave LSX a go (plumbing sealant) and it has worked a treat. Tyre is still up from overnight and not even any Stans fluid in yet!