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Mounting A Thermometer

Originally posted: 04/01/2001
Last updated: 02/16/2014

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Mounting a thermometer is one of the most common modifications made by Weber Smokey Mountain Cooker owners. This topic provides the detailed instructions you need for a professional installation.

As always...click on any of the pictures to view a larger image.

Trend thermometer through lid vent

Polder thermometer on cooking grate

Do I Want To Drill A Hole In My Beloved WSM?

This is the first question you should ask yourself. Measuring Temperature In The WSM provides a thorough discussion of all the options you have for measuring temperature in your cooker, including approaches that don't require you to drill holes. For example, you can insert a thermometer through the lid vent (Picture 1) or use a probe thermometer to measure temperature at the cooking surface (Picture 2).

You should not undertake this modification unless you're comfortable working with power tools and you're willing to accept the risk that you might screw things up. Also, mounting a thermometer may void the warranty on your WSM, so take this into consideration before proceeding.

Finally, when drilling a hole in your WSM, make sure to wear eye protection since bits of porcelain enamel and steel will fly in all directions.

Where Should I Mount A Thermometer?

You can mount a thermometer in the lid or in the middle cooking section. Some people mount them in both locations. In my opinion, the lid is the best place for a thermometer.

The most common lid location is on the other side of the handle opposite the vent. This is where I chose to mount my thermometer. Other popular locations are halfway between the handle and the edge of the lid, or very close to the edge of the lid, to get a reading just above the top cooking grate.

When mounting at the edge of the lid, make sure to install the thermometer far enough above the edge so the dial won't hit the ground when you set down the lid.

For the middle cooking section, the most common approach is to mount a thermometer just above the lower grate. This is done by drilling a hole in the body of the middle cooking section itself, and less frequently by drilling through the access door.

One creative approach that does not require drilling is to remove the top nut and bolt from one of the grate brackets and inserting a thermometer through the resulting hole. This allows you to measure the temperature just below the top cooking grate.

The two most common types of thermometers to mount in the WSM are candy thermometers and industrial-grade thermometers. Installation of each type is described below.

Candy thermometer

Candy thermometer secured with alligator clip

Candy thermometer secured with "e" clip

Mounting A Candy Thermometer

To mount a candy thermometer, drill a hole just large enough to accommodate the diameter of the stem. If you make the hole the right size, you will have a good fit and little smoke leakage.

It doesn't matter whether you drill from the outside or inside surface. If you're careful and use a sharp bit, you'll end up with a clean hole and very little chip-out of the finish around the hole.

You can put several layers of masking tape over the spot you want to drill, which will keep the drill bit from wandering. Some people feel this also minimizes chip-out during drilling, but I've not noticed any difference with or without tape.

The stem of a candy thermometer is not threaded and will not screw into the hole. To prevent it from falling out accidentally, hold it in place with an alligator clip (Picture 2), an "e" clip (Picture 3), or a set screw collar.

The Weber #9815 Replacement Thermometer used in Weber gas grills can also be mounted using this method and can be purchased at most stores selling Weber grills.

Trend thermometer - side view

Mounting An Industrial-Grade Thermometer

Industrial-grade thermometers have a male 1/2" NPT (National Pipe Thread) mounting base on the backside of the dial, as shown in Picture 1. There are two ways to mount this type of thermometer in the WSM. 

The first approach is to make a hole just big enough so you can screw the thermometer into the cooker—let's call this the "screw-in" approach. The second approach is to make a hole slightly larger than the thermometer's threaded base and fasten the thermometer to the cooker using a lock nut—let's call this the "lock nut" approach.

I prefer the lock nut approach because it results in a professional looking installation and I can easily remove the thermometer for testing, recalibration, or replacement without damaging the cooker. With the screw-in approach, I'm afraid that I'll chew up the hole after screwing the thermometer in and out a few times.

Below are the detailed steps to implement the lock nut approach. At the end of this page I'll explain how to do the screw-in approach. 

Regardless of the approach you choose, you'll need to make a big, perfectly round hole in your cooker. I've learned about two different tools that make the job easy—the step drill bit and the knockout punch. Either tool will do a good job for you.

Unibit step drill bit #4

Step Drill Bit

The Unibit #4 Step Drill Bit has a series of 1/16" graduated steps to drill holes ranging from 3/16" to 7/8". Its standard 3/8" shank fits most electric drills.

The Unibit makes a very clean hole with little or no chip-out of the finish, especially in the lid where the surface is curved both horizontally and vertically. It also makes the hole in a single shot, whereas the knockout punch requires a two-step process. The Unibit is readily available at better hardware stores.

The disadvantage of the Unibit is that it's pricey—about $40. However, it's a tool that you'll probably find other uses for on future projects.

1/2" knockout punch

Knockout Punch

Electricians use a knockout punch to cut holes in electrical boxes in order to connect conduit. It consists of a cup-shaped die and a cutting head (the punch) that run along a threaded shaft. The punch is removed from the shaft and the shaft is inserted through a pilot hole drilled in the metal surface. The punch is screwed back onto the shaft and when the shaft is tightened, the die and punch are pressed together through the metal, creating a perfect 7/8" hole.

Greenlee makes a 7/8" knockout punch for 1/2" piping that is sold through industrial product suppliers such as Grainger.

The advantages of the knockout punch are that it's a less expensive tool—about $25—and it is easier to use for the washer-enlarging process that's discussed further down this page.

The disadvantages are that it can be a difficult tool to find and it can only be used for one thing—making 7/8" holes. It makes a clean hole, but not as clean as the Unibit. It will produce a small amount of chip-out of the porcelain finish around the hole.

The knockout punch also requires a two-step process—you must drill a 3/8" pilot hole using a regular metal drill bit, then install the punch and proceed to cut the final hole.

Unibits and knockout punches are commonly used by electricians, so if you're lucky enough to know an electrician you might be able to borrow these tools or have them punch the hole for you.

Required parts & tools

Parts & Tools List For The Lock Nut Approach

Here's what you'll need to accomplish a thermometer installation using the lock nut approach:

  • Industrial-grade thermometer with male 1/2" NPT threaded mounting base; operating range 50-550°; stem between 2-1/2" and 6" long
  • 1/2" or 3/4" steel washer, about 1-3/8" in diameter
  • 1/2" lock nut
  • Flat black high-temp barbecue spray paint (optional)
  • Masking tape (optional)
  • Unibit #4 step drill bit
  • Clamps
  • 1/2" NPT conduit knockout punch
  • 3/8" metal drill bit
  • Adjustable wrench
  • Bench vise or plumbing pliers
  • Electric drill
  • Eye protection

You'll find industrial-grade thermometers in a variety of stem lengths, dial sizes, and temperature ranges. They cost $30-40 and can be purchased from suppliers like Ashcroft, Tel-Tru, and Grainger.

You can buy the washer, lock nut, spray paint, and masking tape at a hardware store. If you'll be using the Unibit, buy a washer with a 3/4" hole in it; if using the knockout punch, buy a washer with a 1/2" hole in it. You'll find steel lock nuts in the electrical department and brass versions at specialty fastener suppliers.

Drilling a washer using the Unibit

Washer knock-out

Enlarging The Hole In The Washer

The washer serves as a backing plate to help stand the thermometer off the surface of the cooker. The 1-3/8" diameter ensures that any chip-out of the porcelain finish will be covered.

If you're using the Unibit, clamp the washer to a piece of scrap wood and enlarge the hole to 13/16", then ream the hole a little so the washer slides over the threads on the thermometer mounting base. The result is shown in Picture 1

If you're using the knockout punch, center the punch on the 1/2" washer and hold the punch firmly with plumbing pliers or in a bench vise while you tighten it. The result is shown in Picture 2—the distorted piece on the right is the bit removed by the punch.

The washer shown in Picture 2 is 3/32" thick and was pretty hard to punch. You could use a thinner washer or several thinner washers stacked together, just make sure they're about 1-3/8" in diameter.

3/8" hole drilled in lid

Cutting the hole - outside lid view

Cutting the hole - inside lid view

Hole cut in lid

Punched hole and Unibit hole

Making The Hole In The Cooker

Now's the time to finalize your decision on where to mount the thermometer in your cooker. I chose a spot on the lid opposite the vent. The process described below is the same regardless of the location you choose.

If you're using the Unibit, just drill the hole to 7/8". Put several layers of masking tape over the spot you intend to drill to keep the bit from wandering, then start drilling carefully. If you prefer, drill a pilot hole first with another drill bit before making the final hole with the Unibit.

If you're using the knockout punch, drill a 3/8" pilot hole as shown in Picture 1. As with the Unibit, place several layers of masking tape over the spot you'll be drilling to keep the bit from wandering.

Next, install the knockout punch through the pilot hole. It doesn't matter whether you put the cutter on the inside or outside surface of the cooker—I've done it both ways and gotten the same result.

Picture 2 shows the punch cutter installed on the outside of the cooker (the work bench is at the bottom of the photo and the cooker lid is at the top, resting upside down on the work bench). Picture 3 shows the view inside the lid. Cutting the hole is as simple as tightening the punch slowly with the wrench.

Picture 4 shows the resulting hole viewed from the outside of the lid. You'll notice there's some chip-out of the finish all around the hole.

Picture 5 shows two test holes I made in a damaged charcoal bowl that I keep around for just this purpose. The punched hole is on the left and the Unibit hole is on the right. If you click the picture to enlarge the view, you'll notice that the Unibit hole is much cleaner. Please note that the lighting in this picture makes the punched hole look worse than it is in reality.

Touching up chips around hole

Touching Up The Finish

As an optional step, you can mask the area around any chip-out and spray on a few light coats of high-temp barbecue paint. This will protect any bare metal from potential rust. The paint won't be visible afterward because it will be completely covered by the washer.

Thermometer with washer & locknut

Stainless steel knockout plug

WSM with thermometer mounted

Side view of mounted thermometer

Inside lid view of mounted thermometer

Installing The Thermometer

Picture 1 shows how the washer slides fits over the threads on the thermometer mounting base. I put the lock nut on part way to illustrate how the pieces fit together. The lid will be between the washer and the lock nut.

To install the thermometer, slide the washer over the threads and insert the thermometer through the hole in the cooker, fastening securely with the lock nut. Do not over tighten the lock nut.

To remove the thermometer for testing, calibration, or replacement, simply remove the lock nut. If you decide to remove the thermometer permanently, you can plug the hole using a stainless steel knockout plug (Picture 2) found at hardware stores.

Picture 3 shows how the thermometer looks on my WSM. Picture 4 shows the washer between the thermometer and the lid. Picture 5 shows the inside of the lid with the thermometer stem and lock nut.

Screw-in approach About The Screw-In Approach

To screw an industrial-grade thermometer directly into your cooker, start by drilling a 3/4" hole using the Unibit described above. With the hole drilled, try screwing in the thermometer. Since the hole in the cooker is not threaded, it will be a very tight fit. If the hole is too small, enlarge the hole ever so slightly. This takes a gentle touch—you've only got one chance to make this hole just the right size.

If you end up making the hole too big and the thermometer threads slide right through the hole, you can revert to the lock nut approach described above.

More Installation Ideas

To see examples of thermometer installations sent in by TVWB readers, visit More Thermometer Installations.

Closing Thoughts

This is a great looking modification if you have the courage to poke a hole in your cooker! Take things slowly and use your common sense. But remember, you can get equally good results by sticking a thermometer through the lid vent or placing a Polder probe on the cooking surface. What's important is that you measure the temperature of your cooker in some fashion.

Good luck and have fun!

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