I fabricated a stainless steal split pass thru for the main wiring between the cabin and engine compartment. I got the idea from an EAA article that combines a split design to cover the larger hole and then grommet covers to cover the wire grommets. You’re technically supposed to use silicone (red) grommets for the high heat resistance, but the synthetic rubber ones will do since I couldn’t get the silicone ones in the sizes I needed. The main plate is made from 1/8 stainless steel plate which is a pain to cut. The design allows you to pass large connectors, etc through the firewall and then cover the hole and reduce the exposure to fire and fumes from the engine compartment. The grommet covers are fro Aircraft Spruce and cost about $8 each. I still need to enlarge them a little since I’m using slightly large grommets then they were designed for (1/2″ ID, 1 1/16 OD and 9/16″ ID, 1 1/16 OD).
Original pass thru cut by the factory. I had to extend out the edges a bit to fit the grommets.
I made a silicone gasket to seal up the space between the firewall and the pass thru plates.
Half of the pass thru on.
And pretty much the final product (still needs some fine tuning). The top part of the plate was a bit of a pain to get one, but a little finagling got it on.