1/2" Silicone O ring inner diameter in mm?

[k1100t]

I need to buy some of the orange 1/2" Silicone O rings, the last thing I need before starting the build. I was going to use RS, as you get 50 for ~£12, which seems better than ~70p each.

They have various diameters, and I don't have any callipers to measure my fittings, so do I get the 20.22mm or 21.82mm ID ones? I'm assuming 20.22mm, as I'm getting a 20mm Q.Max hole punch, but just wanted to check?

Cheers,

[Fil]

the 20.22 mm id option would be best imho, ebay china will be a lot less too.

ive used orings for a few seals and size isnt that crucial, you can always use 2x or 3x, and having to stretch them over fittings isnt a huge no-no either, other sealing options include dowtey composite washers, silicone baking sheet cut into diy washers, and healthy wraps of ptfe or a combination of the above

[Kev888]

The outer diameter of a 1/2" BSP thread is a shade under 21mm. If you want a tight/stretch fit without distorting the cross-section of the O-ring much, then the rule of thumb with O-rings is up to 4% smaller, so the 20.22mm ones sound about right. Though really thats for cylinders; threads may be different (and problematic) if the O-ring is small enough cross-section to fall into their minor diameter.

[Wonkydonkey]

This is what I got
http://www.ebay.co.uk/itm/10-Pcs-Red-Si ... 1c588c3168

Although it not the same seller,

They fit inside those big ss 1/2 inch backing nuts with the groove bit so it don't sqish out.

And I do use Ptfe tape on the threads

So that's 20mm I'd and 27mm OD

Cheers...