I am thinking 5/32 ID.  I measured the NVA at 0.155 in.  5/32 is 0.156?

Where is this washer supposed to go?

Yes, 5/32 is 0.156 inch (4mm).  The ST spraybar is of larger diameter than the Fox factory unit.  I do not remember the diameter of the Fox spraybar.  I am thinking it is about 0.125 inch.  If you use the ST unit, it will result in a reduction of air getting into the engine.  Thus, you have to reduce fuel flow accordingly.  The Fox needs plenty of oil, which means reducing the fuel flow is not doing the engine any good.

When I used the ST needle, I had to neck down the spraybar to get the airflow back to what it should be.  The Fox venturi is cast as part of the case so I did not want to change that. The engine would overheat and "run away" into a constant 2-stroke part way thru the pattern with the 4mm ST spraybar. 

I never had that problem. They always seemed to needle better with the higher quality ST NVA. I will admit that I've had problems with some of the RSM clone NVA's. The problem has always been with the needle itself, and is cured with a true ST needle or one of the Enya ST needles from shtterman. FWIW, I have always run Fox 35's on 10% 11-11 fuel.

If you have an Ace hardware near you, they sometimes have small fasteners (loose) in bins/drawers.

     Hi Dave:

     I have been using a number 6 plastic washer found at the large building supply store’s hardware section.  You may want to alter the i.d. to suit and seal the joint, but they work!

     Stay well,

     Frank McCune