# How to find exact same position on opposite side of a metal sheet before drilling through it when measuring on both sides is not possible?

I need a way to mark the same position at both sides of a metal sheet (in my case I need to make a hole in the passenger compartment of my car and end up in the right place both in and out, that is not hit any cables or other vital parts of the car) when it's not possible to measure. If it was just a flat sheet of metal, you could easily measure out the same location on both sides to see where the hole will end up, but on a curved sheet (like in my case) or on a large wall where it's not practical to find the exact point on both sides by measuring you have to use another method.

Another example is if you need to make a hole in a double layer insulated metal sheet wall, and it's not practical to measure to find the same point at the other side of the wall -- still you want to make sure that the hole will end up in an acceptable location on both sides of the wall (i e not hit a cable on the outside, and not hit a beam on the inside) -- you have to know where the hole will end up on both sides of the wall before you drill.

• If the sheet would have been transparent I could have seen where I was, but metal isn't transparent ...
• I could knock on the sheet and listen on the other side, but this is not accurate enough.
• Maybe use a magnet in some way?

Since this is a common problem I guess there must be people out there that has a great solution! Please help :)

• Is something preventing you from drilling from the other side, where you know the location? Or do you need to drill another parallel sheet? I don't quite get the problem...
– Zeus
Nov 22, 2021 at 23:31
• "Since this is a common problem…" Really? If it is, what is the way it is commonly done? What prevents you from doing that? Don't you mean that YOU don't have/know a way to measure the hole location? Please provide something (a picture) more precise for a workable answer.
– Stan
Nov 23, 2021 at 3:33
• I'm voting to close this question because it needs more detail, lacks clarity, and doesn't show any significant research. I don't mean that in a bad way. Jus' sayin'
– Stan
Nov 23, 2021 at 3:57
• Thanks for the feedback. I tried to clarify the question and added another example of the same situation. Of course this happens when there's a wood wall as well, but then there are some tools depending on magnets that can be used, so I excluded those situations from my question since I want to find something that works for a metal sheet. Don't know if these tools with magnets would work on metal. Nov 23, 2021 at 9:29
• How accurate are you wanting to be? If it's approximate you could apply heat and feel on the other side, at least with conductive material, but obviously that won't give the exact position. Dec 6, 2021 at 12:19

What I do in these cases is to use a centre punch to put a dent in the metal where you want the hole.
Then on the other side you can see a small raised lump - the opposite of the dent.

The dent will be only 1 or 2 mm across and the same deep so if you decide not to drill there is no significant damage.

Of course this does only work reliably on metal. Plastic may or may not show the bulge on the other side.

Using a magnet would not work if the metal is magnetic (any steel or nickel alloy) as the magnetic force is more-or-less lost in the middle. I can understand how a motor vehicle would be challenging to determine a matching location.

A method that comes to mind is a length of transparent hose/tubing. An ordinary garden hose with suitable adapters at each end will work. The adapters would be barbed fittings sized to take a short length of clear vinyl tubing, providing the necessary visibility for the non-measurement.

The one key factor is that one must be able to see inside the vehicle, in order to align the inside and outside hoses in the left/right direction. Alternatively, a horizontal measurement could substitute for the visibility portion.

Water will find its level. Hold the vinyl tubing filled with water at the point (outside) to be marked/drilled. The other end of the tubing can be snaked through a window or door to reach the inside location. The tubing should be long enough beyond the leveled location to be visible from inside and out. This provides the left/right reference. The inside tubing will then have the same water level as the outside, providing the up/down matching point.

In the image above, the blue represents the tubing with the water level ending at the white. The tubing runs outside the open door (not shown due to lack of artistic skill) and into the engine bay. This scenario considers how to create a hole in the firewall, and is thus wrought with great risk.

The other end of the hose is routed to the engine bay as close to the firewall as possible. For the left/right alignment, a stick is used, touching the hose at one end (inside the vehicle) and touching a wall at the other end.

Inside the engine bay, another stick of the same length is placed against the wall and the hose is located to the other end of the stick.

If the hood is removed, the engine bay end of the hose would be visible through the windscreen, eliminating the need for the sticks touching the wall.

Many variations of this method can be used to accomplish the result desired. For example, if it is acceptable to measure the distance to the wall, one stick could be removed from the project.

Freepik image modified for purposes of clarification, maybe.