passthrough
Vans says the snap bushing and caulk with silicone method is OK. It is done that way on many certified planes as well, however, I find it a splendid place to upgrade. I used a cable clamp (Cable Safe) on my firewall passthroughs. Tight, and flame resistant, but also stabilizes the cable sheath to allow a predetermined amount of flex between your stationary firewall and moving engine. Very hard to achieve this without anchoring the cable sheath at the firewall.
YMMV,
Chris