RAW Rapid air leak in best fittings reg tester for non x tm1000 regulator

hi every one i got a leak from the best fittings reg tester the reg fits fine and perfect in the reg tester its just that the o ring supplyied by best fittings ( the one that comes with it) dosent seal the air i need a thicker o ring to be able to do for for reg tester with my raw tm1000 regulator..... the non x verson of the gun pre airforce gun ill incluse a pic and doens any one know what size of o ring ill need thats all im looking for is where to get or proper o ring for proper seal... thanks guys in advance

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For a Rapid that use 1/4"bspp threads....015
Here is the tester I use....made from Stainless by Dr Bob...
I/4"bspp on one end and 1/8bspp on the other.
It uses your tanks gauge and bleeder valve.....
regtester_1_.jpg
nice but im not buying any thing else to test my reg pressure all i need is a proper size o ring for my best fittings tester