f1rocket
Well Known Member
I'm prompted to write this post after viewing some post on the Matronic's list about cutting holes in the firewall to pass an electrical connector. If you're a newbie and haven't yet carved your firewall into swiss cheese, I'd like to offer you a couple of suggestions.
First, maintaining the integrity of your firewall should be one of your primary safety concerns. I've visited many builder sites and I've seen some dubious things when it comes to this topic. In the rare event that you suffer an engine fire, seconds count and may make the difference between making a safe emergency landing or suffering other consequences. So think carefully before you drill any hole in that stainless.
The number one rule is to make sure that what ever you put in that hole, is at least as tolerant, or more tolerant, of heat and flames as the stainless steel you're replacing. That usually means putting stainless steel back in its place. The largest hole will likely be your cabin heat box. Make sure it is constructed of stainless steel and has a stainless steel flapper. There are aluminum ones floating around. They are unsafe. Don't use one. Van's used to sell both. I know they sell the stainless one.
When passing cables through your firewall, I am partial to the steel eyeball fittings. I know they are expensive at $30 a pop, but you only need three or four max. I like these units because they allow me to direct the cable in any direction, they don't come loose or unscrew, they look fine, and they are impervious to flame.
Finally, and the area where I've seen some real "creativity" is passing wires into the engine compartment. I try to drill as few holes as possible so I like to keep the engine wires into two bundles. One bundle contains all the noisy wires like alternator, starter, field, and magneto wires. The second bundle is for all the other wires like sensors, EFT, CHT, etc. I use the stainless firewall passthroughs sold by http://www.epm-avcorp.com/tubeseal.html. I put the noise bundle dowm low even with the crankcase split to facilitate running the starter and alternator wires forward. I put the the sensor bundle up high. These units are very good at sealing out fire and come with a tube of fire stop to apply in and around the wire bundle.
It's perfectly fine to use stainless steel grommet shells like Van sells as long as you cover the opening with some fire putty as a final step. I also choose to use steel bulkhead fittings for things like brakes, fuel, and hydraulic lines. They stand up better to the constant shaking of the hoses, although I suspect that there are many, many airplanes flying with aluminum fittings.
There are many ways to do the same thing. I'm not suggesting that my way is the only way or even the best way. I do think it's a safe way. If you have any doubt, consult with a mechanic friend or fellow builder.
First, maintaining the integrity of your firewall should be one of your primary safety concerns. I've visited many builder sites and I've seen some dubious things when it comes to this topic. In the rare event that you suffer an engine fire, seconds count and may make the difference between making a safe emergency landing or suffering other consequences. So think carefully before you drill any hole in that stainless.
The number one rule is to make sure that what ever you put in that hole, is at least as tolerant, or more tolerant, of heat and flames as the stainless steel you're replacing. That usually means putting stainless steel back in its place. The largest hole will likely be your cabin heat box. Make sure it is constructed of stainless steel and has a stainless steel flapper. There are aluminum ones floating around. They are unsafe. Don't use one. Van's used to sell both. I know they sell the stainless one.
When passing cables through your firewall, I am partial to the steel eyeball fittings. I know they are expensive at $30 a pop, but you only need three or four max. I like these units because they allow me to direct the cable in any direction, they don't come loose or unscrew, they look fine, and they are impervious to flame.
Finally, and the area where I've seen some real "creativity" is passing wires into the engine compartment. I try to drill as few holes as possible so I like to keep the engine wires into two bundles. One bundle contains all the noisy wires like alternator, starter, field, and magneto wires. The second bundle is for all the other wires like sensors, EFT, CHT, etc. I use the stainless firewall passthroughs sold by http://www.epm-avcorp.com/tubeseal.html. I put the noise bundle dowm low even with the crankcase split to facilitate running the starter and alternator wires forward. I put the the sensor bundle up high. These units are very good at sealing out fire and come with a tube of fire stop to apply in and around the wire bundle.
It's perfectly fine to use stainless steel grommet shells like Van sells as long as you cover the opening with some fire putty as a final step. I also choose to use steel bulkhead fittings for things like brakes, fuel, and hydraulic lines. They stand up better to the constant shaking of the hoses, although I suspect that there are many, many airplanes flying with aluminum fittings.
There are many ways to do the same thing. I'm not suggesting that my way is the only way or even the best way. I do think it's a safe way. If you have any doubt, consult with a mechanic friend or fellow builder.
Cheers G. 
