I am trying to buy some replacement o-rings for my front light, I think they need to be 50mm/40mm, but does any one know what thickness they should be? it is for a solar Storm x2

Not the answer to your question and no use if you want it for a helmet mount, but if you’re fixing to the bars this is better:

http://www.chainreactioncycles.com/hope-universal-handlebar-mount/rp-prod23533

2.5mm, as after looking at o-rings yesterday, they seemingly measure from the middle out.

Diameter is around 35mm ish.

thanks! 2.5mm sounds about right. I guess 35mm is the internal diameter of a 40mm ring?

just quickly measure my ‘large’ o-ring and it around 51mm x 41mm x 2.5mm, so 50/40mm is near enough right.
The smaller one is around 40mm x 35mm x 2.5mm (mine not round, so these are very aprox figures), which I think Bigyinn is alluding too(?)

brillant, thanks Z1ppy and bigyinn

You’re welcome!