Rubi shower arm and toe spout have straight thread adapters

[Steve Robinson]

I have a new Rubi shower arm and toe spout (no diverter) to install. Each comes with 1/2" Male NPT (for the drop-ear adapter) and a male 1/2" straight thread to mount the arm/spout. Neither the arm nor the spout has a shoulder or washer and they are clearly meant to thread on until the arm/spout meets the trim ring. How do I seal those? I can see that the toe spout has little pressure on it (no screen, no aerator) but the shower arm might since the head has many fine sprays. Hope the picture works.
Thanks.