Anyone using '013' O-rings as mating surfaces for more 'grip' when attaching 2nd stage to LP hose ?

[Couv]

By design, "locking" SAE J1926 straight thread bosses should come from the metal to metal contact/friction caused by proper torquing of the threads instead of any torque limiting contributed by the O-ring. Could it be that you are not tightening the connection enough? The connection should be lightly torgued, but just enough that it doesn't unscrew or back off in use.

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I don't recall the proper torque for this connection, but @couv or @herman probably do.

SP gives a torque value of 44 inch pounds for hp (7/16") and lp (3/8") port fittings going into the first stage.

However, I thought the OP was asking about adding a washer to the outside of the second stage air barrel (9/16") to act as a friction device.

OP, "I've been installing '013' Viton O-rings when attaching 2nd stages to LP hoses..."

I don't see how this would be an improvement over metal to metal contact.

OP: "and is a better idea than just having raw chromed brass surfaces as the mating surface."

Again, the metal to metal interface provides the friction necessary to prevent loosening.

How much torque? I don't remember ever seeing a specified amount, so I've always used: Run the fitting down finger tight until the surfaces meet-then snug with a wrench. Snug? How snug you say. Unlike some hydraulic fitting where one uses a torque wrench or counts flats of the 'B' nut, you should not even see the wrench/fitting move.

Everyone reading this tread would be interested in Akimbo's excellent article:
O-rings for Divers