I believe the stock o-ring is 2x54mm. I think I used a 3x54mm on mine, I went thru my o-ring collection and noticed (1) 2x54mm laying in the box, and 3.1x54mm and 3.2x54mm still in their packages but the 3x54mm zip lock bag is empty. There is but a tiny gap in mine between bezel and head YMMV?

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. https://theoringstore.com/

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. 3mm X 54mm (NBR) Buna-N 70 Duro Metric O-Ring [N3.00X054] : The O-Ring Store LLC, We make getting O-Rings easy!