Putting a drain valve in your air box



Today I’ll discuss the benefits of putting a drain valve in your air box and any negatives to putting a drain valve in your air box.

This drian valve install was part of my 4WD touring build, although it is massive overkill I still wanted to do it.

We will look at why you might be looking at putting a drain valve in your air box as well as covering the costs of putting a drain valve in your air box and any other things related to the job.

If you’re reading this then you probably know why you’re here, but for those who don’t, this post is about putting a drain valve in your air box after you have plugged the OEM drain valve as part of a snorkel install.


If you haven’t installed a snorkel, fear not, the manufacturer would have put a drain valve in your air box from factory.



Video – Putting a drain valve in your air box


For those who would rather watch the video, here you go!

This video covers what’s in this post but makes things easier to visualise as well.

Why you might consider putting a drain valve in your air box


If you have installed a snorkel recently, you’ve more than likely decided to completely water proof your air box, otherwise what’s the point, right?

An unfortunate downside of this is that you will need to block the manufacturer’s drain valve of your air box. The reason for this is obvious: to make sure the snorkel can do its job in not allowing water into the engine.

But that drain valve now won’t be able to perform its purpose, which is to drain water from the air box. That is why we’re going to be looking at putting a drain valve in your air box, to reintroduce the ability of your car to syphon out any water that enters the air box.

There are multiple options available because the options are only limited by your imagination, there aren’t any off-the-shelf solutions here, just whatever you can make.

How can water get into the air box anyway?


It is not a common thing for water to get into the air box in the first place, but there are some ways that it can happen such as:

  • Water over the bonnet and a bad rubber seal on the air box lid.
  • Coming down through the snorkel through heavy rain fall and a poorly designed snorkel.
  • Coming into the snorkel via splashing as you perform a river crossing.
  • Slow leaks in the silicone/Sikaflex of the snorkel system that allow small amounts of water to build inside the air box.

I have to reiterate here though, that these are very rare occurrences and most people won’t ever experience any water in their snorkels at all. However it is of course possible that over time there is some sort of water ingress into your air box.

Has this ever happened before?


The reason we know that this issue has occurred previously (outside of people having reported it in forums) is that the instructions in fitting a snorkel will clearly state that the choice to block the factory drain plug is entirely yours to make.

The snorkel manufacturers know that it is possible for this to happen, so they state that the decision to plug the factory drain valve is on you, therefore absolving themselves of fault in the case of something going wrong.

Purchasing a snorkel from a reputable brand will severely limit the chances of this ever happening, most claim that they expel water through various methods but these methods are not bullet proof and only really account for rain, not other ways that water can enter the air box.

So yes, it has happened. But it is not a common occurrence by any means. However the fix is cheap and easy to do.

Again to reiterate, this is 99.99% never going to be an issue, but there are ways in which it can still happen.

*Affiliates Disclosure

Affiliate links are present on this page. Through partnerships with, but not limited to: Amazon, eBay and Commission Factory, I will make a small commission through qualifying purchases. This comes at no extra cost to you and is just a way for me to try and support myself and the blog.  Thank you.

Why did I decide to put a drain valve in my air box?


I installed a Safari snorkel (eBay) on my car to help protect the engine in the event that I had to drive through water and accidentally went deeper than intended.

There are a few reasons that I decided that putting a drain valve in my air box was worth the time and hassle.

Firstly, I found it baffling that the snorkel manufacturers didn’t offer an alternate solution, because to me the drain served a clear purpose.

I wanted to seal it, but only when I needed it to be sealed. I wanted the benefits of a sealed system and a system that can drain water.

Secondly, it’s dirt cheap to do. My first instinct was that it could be done with PVC fittings and/or some cheap valves.

Thirdly, the curiosity got to me. I really wanted to see how easily it could be done, to help others potentially do the same thing and to test out how effective it would be.

Air Box Drain Valve – Considerations


Before you go putting a drain valve in your air box, think about whether you really need it. For most people (myself included) the answer is probably no. That is assuming a few things such as:

  • You installed the snorkel yourself and know the Sikaflex is applied properly.
  • You have purchased a reputable brand of snorkel such as ARB Safari snorkel (eBay).
  • You have looked at the seals of your air box and checked that they’re in good condition.

There are other things to consider:

  • How often you do water crossings.
  • Whether you’re forgetful and might forget to close the drain valve manually one day and risk damaging your engine.
  • Do you really want to drill a new hole in your air box and risk it being a point of failure?

If after all that you’re happy to proceed, then read on.

Drain Valve Objectives


The idea behind the drain valve for me was that it would be in the open position 90% of the time, when you’re doing your day to day driving or driving on basic dirt roads.

When there was a large 4WD trip with many water crossings, the valve would be closed for the duration of the trip. This would avoid confusion that could arise if you were constantly opening and closing it throughout the trip as you came across a water crossing or bog hole.

Step 1 of putting a drain valve in your air box is determining what you want to achieve from the job. For everyone this could be a little bit different, but for me these were the things I wanted to achieve.


I wanted to have a drain valve that was not over engineered, it needed to be simple to use, simple to install and simple to make waterproof.

This ruled out a few of the options that I was looking at that we will discuss further down this post.

It definitely ruled out the idea of having an electrically activated solenoid that had a button in the cab to open it and failed closed, as nice as it sounded it was just too much work.

Manual control

The whole point of putting a drain valve in your air box is so that you can drain water out in normal circumstances. However, you don’t want to put the same style of drain into your air box that the manufacturer had because we need to be able to open and close it as we see fit.

Manual control is what we’re after, in keeping with the above we are looking for simple mechanical control. To me when I was thinking of what style of drain valve I would be putting in my air box this meant something I could do without tools.

A quarter turn or half turn of some sort of valve was what I was looking for.

As small as possible

The reason for this is that we are unable to take the prime position in the air box, as the manufacturer has already taken the lowest point of the box and this has had to be sealed with Sikaflex to maintain the snorkels water tight design and protect the engine.

To get the new air box drain valve as low as possible, the smaller the item the lower it will go.

There is also limited space in the engine bay in which you can install the drain valve. Each car will be different but in the Mazda BT-50 and the Ford Ranger you can only use the side walls. The air box sits too close to the body to effectively install a drain valve on the floor of the air box.


You need to be able to trust that the drain valve will do its job, particularly in the closed position. You need a reliable valve or blockage of some sort.

When you’re crossing a river, the forces against the drain valve (if any) will not be too extreme, they will tend to be similar to something that has just been submerged in water and are unlikely to be the full force of the car entering the river, as a bow wave is created that pushes water away.

You do however want your drain valve to be reliable through many on/off cycles.


I wanted to have some ability to negotiate a few different scenarios and levels of protections in how I mounted the drain valve.

What I mean by this is that I could either put the drain valve in a closed position with one block, or I could double block it for extra protection and also redundancy if one of the block methods were to fail.

Where to put the drain valve?


The drain valve needs to be at the lowest point of the air box. This will usually be taken up by the factory drain valve.

In the Mazda BT-50 and Ford Ranger the air box sits low, there is no space beneath the air box to drill a hole and fit a fitting.

In other cars, if there is space beneath the air box than this is the best place to fit the drain valve, whatever design you have.

In the Mazda BT-50 I went for a location through the side wall, as low as possible, near the factory drain valve.

You need to have the car on level ground and have the air box installed in the car when you determine where you will be putting your drain valve so that you can see which way it slopes naturally to allow water to drain.

How to waterproof the drain valve?


This ultimately depends on what method you decide to use when putting a drain valve in your air box, but it will probably just be a case of applying Sikaflex 227 to the entire job.

The other way you could do this, depending on the materials used is with the use of IP washers, rubber gaskets and the like. However for me this didn’t quite work out due to the design I chose to go with.

I was also wary that the washers could fail at some point after years of heat in the engine bay and dust from the road making them less effective. Using silicone or Sikaflex gives a result that feels like it will last a lifetime.

Mazda BT-50 / Ford Ranger Air Box Drain Valve Installation


If you are going to install a drain valve I recommend doing it at the same time as you fit the snorkel, because the air box will be removed from the car and you will be using Sikaflex 227 to waterproof everything already.

I hadn’t yet decided what way to fit a drain valve when I did the snorkel and therefore had to remove the air box for a second time, which was a hassle due to the snorkel fitting being annoying to put back on. I had to remove the wheel and splashguards for a second time, just to get the air box back into the car.

Materials used


All of the components that I purchased were bought from a hardware store, with the exception of two lock nuts I needed to match a specific thread type.

  • 13mm barbed in-line tap $6.72
  • 13mm barbed end fitting $0.93 ea.
  • Clickable ratchet hose clamp $0.26 ea.
  • ¾“ BSPM lock nut $1.90 ea.
  • 1m of 13mm poly tubing $3.09

The total cost of this was just $15.06

*Affiliates Disclosure

Affiliate links are present on this page. Through partnerships with, but not limited to: Amazon, eBay and Commission Factory, I will make a small commission through qualifying purchases. This comes at no extra cost to you and is just a way for me to try and support myself and the blog.  Thank you.

Tools used


For this job I used a few basic hand tools that most people will have lying around and also a drill. Here is what you might need (links are affiliate links):

Nothing outrageous needed, no specialty tooling at all.

Drain valve methodology


In the end I decided to try fit a tap fitting, with a quarter turn or half turn to open and close. This meant no tools were required.

I would install the tap and this would be protection measure number one. As a backup and extra protection, I purchased a barbed end cap fitting and some tubing, this would be clamped with hose clamps in the event that I felt the need for a second layer of protection.

With this method I had gained manual control, as well as two layers of protection when required, without the need for using tools.

Installation guide


This section will be brief with a few photos of the key points that I raised earlier about how I wanted the drain valve to work and how I implemented that in practice.

Every car is different and everyone has different ideas on how to do the job, some probably better than mine.



Make sure you have clearance in the engine bay for your intended design, don’t do it all on the bench only to realise it won’t go back in the engine bay.

Putting a drain valve in your air box is easy, but you only want to drill the one hole, don’t rush it and end up ruining the air box with a hole in the wrong spot.

Below is an image of my air box drain valve post install, I put it there because it is the lowest point I could get it that had heaps of clearance around the tap.

You also want clearance to make it easier to operate the valve when you want to change it from open to closed or vice versa.

Close to the original drain valve

Go as close to the original drain valve as possible. The manufacturer has sloped the air box towards that drain valve and the closer you can get to it the better your drain valve will drain.

See below the view through the air inlet that shows the new drain valve sitting low, beneath the inner wall of the air box.

Double block

If you can build in some form of redundancy then go for it. The picture below shows the barbed fitting I bought that will allow the tap to be the second form of protection after that barbed fitting.

In most cases the tap in the closed position would be more than enough protection, but just in case, this barbed fitting attaches to the small bit of hose and can be clamped with those hose clamps.

Double lock nut benefits

Using a lock nut on both sides has a few advantages.

Firstly, it allows much more clamping force on the air box side wall which in itself will create a bit of a seal. This keeps everything tight and makes it sturdy.

Secondly, it prevents too much intrusion on the interior of the air box. If you are putting a drain valve in your air box and can do it in the floor of the air box, this will be even more important!

It means there’s less thread for the water to pool over before it can start draining.

Alternative drain valve designs


Here are some other drain valve ideas for putting a drain valve in your air box.

There were a number of different drain valve designs that I considered before purchase. All of these might be viable or preferable ideas for your particular goals so I will list them here in case they’re helpful.

Raising the air box


To get the most effective drain valve possible in your air box, you would ideally have a drain port in the bottom of the air box. The Mazda BT-50 and Ford Ranger don’t allow for this given how they sit in the engine bay.

I considered raising the air box on spacers, then drilling and fitting an elbow fitting into the floor of the air box that had its own tap design still with a quarter turn operation.

The constraints with this are the additional materials required, as well as the other affects such as the air inlet hoses and other periphery hoses being affected by the new air box height.

I decided the extra work and potential follow on effects weren’t worth the small gain in increased drain performance.



There were a number of different bung options I looked at but at the end of the day it would have been too much hassled putting them in and then removing them.

I wanted to avoid the use of tooling and the awkwardness of trying to undo a bung would have meant opening the air box and taking out the filter to get it in and out.

Vacuator valve (automatic one-way valve)


There are valves called vacuator valves, or duck bill valves, these expel things and then close when pressure is applied from the other direction.

The reason I avoided this was because of the reliance on an automated system that could get clogged with dirt and not close properly was too big a worry.

Other valve


Using the exact same method as I did here but with a different valve was another thing I looked into. Namely the Fumoto oil drain valve (Amazon).

Being an M12 thread meant I could have positioned it lower on the sidewall and it could have drained better. I also could have attached a short bit of hose with a cap to maintain the two levels of protection.

The main downside here was availability and cost. I couldn’t get my hands on one within 2 weeks so I went with the irrigation tap instead.



So, do you think it is worth putting a drain valve in your air box?

If you have better ideas for putting a drain valve in your air box comment below with a picture.

I think putting a drain valve in your air box is cheap insurance for those who are interested but most people really don’t need to do it. I was half doing it out of curiosity.

I feel like the snorkel manufacturers could come up with a low cost design for the different air boxes that fixes this potential issue and gives everyone full control over when their air box is fully sealed and when it can drain as per OEM design.

Hope this helped!

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