Trying to figure out the air screw 79 KZ650BE ? Need an o-ring ?

I'm not so sure rtv will be a good idea for this application. As it's a paste when you screw the needle in the thread will clean so not really achieving your aim. Any that squeezes down down the needle taper could block it entirely. Once it's set it's set so further adjustment will not be possible without breaking the seal and getting you back to your start position.
What your trying to achieve is repairing worn threads likely on the carb body as its the softer material. 
PTFE tape on the needle threads will take up the space between male and female threads and will stay in one piece so no fear of blocking the port and will leave the needle totally adjustable for the future. 

My 1981 KZ650-CSR has the non-o-ring pilot adjusting screws & I re-installed the screws without sealing them. the aluminum plugs are long gone.  I would make sure the carburetor holders & airbox ducts are good w no air leaks >> the rubber should be soft. The airbox ducts should be soft & pliable, easily squeezed using your fingers, while the carb holder rubber is going to be firm. Replace any that are rock hard.
ps: the pic has me wondering if things are buttoned up tight.

Warren3200gt wrote:

I'm not so sure rtv will be a good idea for this application. As it's a paste when you screw the needle in the thread will clean so not really achieving your aim. Any that squeezes down down the needle taper could block it entirely. Once it's set it's set so further adjustment will not be possible without breaking the seal and getting you back to your start position.
What your trying to achieve is repairing worn threads likely on the carb body as its the softer material. 
PTFE tape on the needle threads will take up the space between male and female threads and will stay in one piece so no fear of blocking the port and will leave the needle totally adjustable for the future. 

When I said RTV I didn't mean around the thread, simply to replace the factory plug. Obviously common sense would need to prevail

Does the hole for the air screw get narrower after the threads? If so, an o-ring with ID to fit the smooth part of the screw and OD to fit the hole may work.

I found this screw. Part number 16014-1006 This was in the Fisch but not available. 
The other is a 16014-17 which was not on the Fisch. not sure if this would fit. But is on ebay for $34 each. Not going to happen.
None of these give any dimensions so who knows.

Keep digging.  They are out there.  The Suzuki GS VM's use the same air screw, with O-ring.