Spray Bar Inserting & Sealing

I assume this is a Cox engine and your making reference to a metal backplate? If so, the spraybar is knurled therefore it really doesn't require any additional sealer. The assemblies can come loose and yes, they can develop air leaks around not only the inside threads of the needle but the outside diameter of the spraybar. I would steer clear of any silicone unless it cures without creating acetic acid. Contractor grade silicone are no good for this purpose. On the other hand, I would use epoxy or JB weld. Both are removable with a heat gun and you don't want hardly any at all inside. First order is to clean outside of spraybar with solvent and thoroughly oil inside threads to avoid any adhesive getting inside. There would be no need to drill back through the nipple if this is what you meant. Insert the spraybar leaving about 1/8" out , apply JB weld of epoxy around perimeter and push it in. Clean residual epoxy using Q-tip and rubbing alcohol.