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I need to run cables through my firewall (the wall between the engine bay and the seating area) but I can't seem to find any factory placed holes to pull cables through.

Now a lot of people told me its pretty easy to find a place to drill through safely, drill a hole through the firewall and then seal it off so water can't flow through. I agree with them but is there a good and smart way to do this?

  • Specific drill(bit) to use?
  • What kind of sealant to use?
  • Do I need to apply some anti-rust of any kind?
  • Any other suggestions?

TLDR; What do I need to pay attention to when drilling through the firewall?

PS. I don't know the proper name of the metal sheet that's between the engine bay and seating area, since its goal is to stop fire from spreading I called it a firewall (feel free to improve if you see fit).

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Your guess about what to call it is exactly right. It's called the fire wall. – cdunn Jan 27 at 15:34
3  
I don't know which kind of sealant to use so I'll leave this as a comment.. You're right, the things you want to be careful about are sealing the hole from moisture, preventing rust, and preventing chaffing of the wires against the edge of the hole. One way to go would be to cut the hole, put some kind of rust preventative on it, and then put a grommet into the hole, then pull the wires. Perhaps there's a style of grommet that allows you to seal against moisture despite the wires coming through. – cdunn Jan 27 at 15:38
    
Looks like there are a huge selection of grommets to choose from here: summitracing.com/search/part-type/firewall-grommets – cdunn Jan 27 at 15:51
1  
Agree with @cdunn -- this should be as simple as popping out a grommet, fitting the wires through it, and pushing it back into place. Also, I've never seen a firewall that didn't have an array of holes already cut and already filled with grommets (some already in use) that you can leverage. – Lynn Crumbling Jan 27 at 16:01
    
@LynnCrumbling My car's loom was (obviously) pulled through the firewall but its been sealed of with some kind of fibreglass of sorts. (the car in question is an 2001 Seat Leon.) Other than that spot there are no apparent holes to pull cables through. – Marco Geertsma Jan 29 at 9:19
up vote 14 down vote accepted

I think we have enough to call this an answer now. Here's how to do this:

  • If you can't use an existing hole, insure that there is clearance on both sides of the firewall before drilling. You can use whatever bit that fits. Although if you have limited space and can't use a prick punch make a dimple so the bit doesn't wander, a step drill might be more stable once you have it started.
  • Once you have the hole complete, deburr the hole, and hit the edges with a fine sandpaper to remove any remaining rough spots.
  • To be really thorough, degrease the hole on both sides, and hit both sides with a little primer just to keep the edges from rusting. Should only take two coats and each coat is one quick spritz.
  • Insert the grommet you selected.
  • Run the wires.
  • If the grommet is not the sealing kind and just has a hole, seal it with a silicone sealer. It will be a mess if you ever want to remove this, so if you need to get lines in and out easily, select a grommet that won't require any added sealer.

This accomplishes the big things on this task:

  1. Keeps water and gases out
  2. Prevents rust at the hole
  3. Prevents the wires from chaffing which can cause shorts

I hope that helps!

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