There seems to be more misinformation, confusion, and debate about glow plugs than any other aspect of the nitro RC hobby. Let’s dive into the topic of glow plugs so you know everything you need to know.
If you have information about glow plugs (supported by a credible source or industry experience) that contradicts or expands on anything below, please contact me.
The Problem with Glow Plugs
Actually, many nitro RC enthusiasts don’t have a problem with glow plugs. They run the plug that came with their engine and when it dies, they replace it with the same plug. If their engine is performing well and they can easily source a replacement plug, there is no problem.
If the engine is not performing optimally or a replacement of the original plug cannot be sourced, a different plug must be tried which creates a problem: what plug to use?
If you have access to a hobby shop near by, the easiest way to solve the problem is to tell them what engine you have and purchase the glow plug they recommend. The glow plug will undoubtedly work and you’ll be burning nitro again. Unfortunately, there’s a decent chance they will recommend a glow plug that does not match the original specs of the glow plug that came with your engine. It’s also possible that they don’t sell glow plugs at all or won’t be able to recommend one to you.
With all that said, let’s go deep into glow plugs so you are informed and can know exactly which plug is right for your engine, should you ever need or want to use a different plug than the one that came in your engine.
Glow Plug Basics
In a nitro engine, glow plugs are required to both start and keep the engine running. The glow plug supplies heat at the top of the compression chamber that ignites the fuel and air mixture. Ideally, ignition occurs at top-dead-center, or when the piston is at its highest point of its stroke.
If ignition happens too early or too late, the engine won’t perform optimally and added stress could be placed on internal components. Every engine has a specific glow plug length and heat range that will provide optimal ignition timing. Using a plug that is not the correct length or heat range for an engine will likely reduce performance and longevity.
Glow Plug Type
There are two types of glow plugs: standard and turbo.
The manual for your nitro engine should specify which type of plug is required. Most engines that come with basher-grade nitro RCs use a standard plug. Racing engines tend to use a turbo plug.
Standard plugs have a straight threaded body with a copper washer that seals the compression chamber.
Turbo plugs have a chamfer at the end of the threaded body that seals the compression chamber.
For simplicity, and because this site is about nitro RC bashing (as opposed to racing), the rest of this page will focus on standard plugs.
Glow Plug Heat Range
Each glow plug has a heat range. Typically you will see a plug rated as cold, medium-cold, medium, medium-hot, or hot. The heat range tells you how hot the plug gets, which affects the ignition point. A hotter plug will ignite sooner in the piston stroke and a colder plug will igniter later. Other factors like the environmental conditions and the percentage of nitromethane in the fuel may alter the ignition point.
As far as I know, there is no objective way to determine which glow plug heat range is optimal for a given engine. Additionally, there does not seem to be any standard when it comes to heat ranges. A “hot” plug made by one manufacturer may have a different heat range specification than another “hot” plug made by another manufacturer. As far as I know, manufacturers do not publish the heat range specs of their glow plugs.
The best you can do is start with the brand and heat range recommended by the manufacturer of your nitro RC model, then experiment with different glow plugs to see which performs best.
Changing glow plug heat range when the weather changes
The old school adage on glow plugs is to run a hot plug when it’s cold outside and run a cold plug when it’s hot outside. The idea is that in hotter weather, the engine runs a little hotter and therefore the glow plug doesn’t need as much heat to ignite on time. This seems to make sense, but I’m convinced it isn’t necessary to change plugs when the weather changes.
I was first convinced by Adam Drake in this video about glow plugs that it is not necessary to change heat ranges when the temperature changes. Adam Drake is a professional RC racer and if he isn’t changing glow plugs with the weather, I don’t see why I should bother when I’m just bashing.
Though not as credentialed as Adam Drake, I’ve come to greatly respect the knowledge of thebug 404 on YouTube when it comes to nitro RC engines. He swears and almost burns his kitchen down in each video, but he knows his stuff. In this video about nitro myth busting, he makes the same recommendation as Adam Drake, which is to not change heat range of your glow plug with the weather.
If you do want to experiment with heat ranges, the general consensus is that a hotter plug will provide a more consistent idle, a larger tuning window, more low end performance, and shorter engine life. A colder plug will provide the opposite: less consistent idle, smaller tuning window, more top end performance, and longer engine life.
Changing glow plug heat range when changing fuel
The standard recommendation is to change to a colder plug when increasing the nitro percentage of the fuel. So if you were using a hot plug with 20% nitro fuel, you could try a medium plug when switching to 30% nitro fuel.
Glow Plug Size and Length
All glow plugs made for nitro RC engines that I know of have the same diameter and thread pitch, but the length varies from plug to plug. Like the heat range of a plug, the length also affects the ignition point of the engine. The opinions about which length plug to use for which engine are all over the place, which leads me to believe that plug length is not be super critical, especially when bashing an RTR engine (as opposed to trying to get peak performance from a top quality racing engine). That said, let’s dig into all the details of glow plug length.
Is there a standard glow plug length?
The term “standard” is used by manufacturers and the nitro RC community to describe the length of certain glow plugs, but as far as I can tell there is no universal definition of a standard length plug.
First of all, let’s get clear on what actually matters when it comes to glow plug length. Below is a technical drawing of a Force B4 glow plug. As you can see, the threaded portion of the plug is 4.9mm and the washer is 0.8mm thick. The washer sits on the threaded portion of the plug and outside of the engines internal combustion chamber, so it’s only the portion below the washer that determines how far into the combustion chamber the plug will sit. In this case, that length is 4.1mm (4.9mm – 0.8mm). In theory, any plug that has 4.1mm of threaded length below the washer could replace the Force B4 glow plug.
The table below is a list of popular glow plugs and their critical dimensions which should help you determine which glow plugs are 100% compatible with each other. Unfortunately, I’m still working on gathering info on various plugs. Contact me if you have any info that could help complete this table.
| Glow Plug Brand/Model | Threaded Length (mm) | Washer Thickness (mm) | Length below washer (mm) |
| OS #6-10 | ?? | ?? | ?? |
| OS LC3, LC4 | 5.4 | 0.7 | 4.7 |
| Traxxas Super Duty Long | 5.7 | 1 | 4.7 |
| HPI, Force | 4.9 | 0.8 | 4.1 |
| Kyosho | 4.6 | 0.5 | 4.1 |
| Dynamite | ?? | ?? | ?? |
| LRP | ?? | ?? | ?? |
| SH | ?? | ?? | ?? |
Glow Plug Length for Traxxas Engines
Traxxas recommends you use their Traxxas Super Duty glow plugs for all of their engines. Traxxas calls their plugs “long” and are often referred to as “long reach plugs” by the nitro RC community. A very commonly used alternative to the Traxxas glow plugs is the OS LC3 glow plug, which is carried by many hobby shops in the US.
In general, the OS LC3 glow plug is considered higher quality than the Traxxas plug, but of course Traxxas only officially recommends the Traxxas Super Duty plug.
Glow Plug Length for HPI, Force, and Kyosho Engines
HPI, Force, and Kyosho Glow plugs use the same size plug. Well, not exactly, but the threaded portion of the plug below the washer is the same length. Here’s what I know…
HPI support told me that their plugs are the same dimensions as Force plugs, which we already calculated as having 4.1mm of threaded plug below the washer.
HPI Glow Plug for the F4.6 or F5.9
Kyosho support told me the threaded portion of their plug is 4.6mm and the washer is 0.5mm thick, which leaves the same 4.1mm of threaded plug below the washer.
Kyosho Glow Plug for KE engines (e.g. FW-06)
When I asked HPI support and Kyosho support which plug they would recommend as an alternative to their plugs, they both said the OS #8 glow plug, which is probably the most popular glow plug and is carried by many hobby shops in the US.
Glow Plug Length for LRP, Dynamite, and SH Engines
I’ve had no success determining the proper length of the glow plug for these engines.
There is a Dynamite branded glow plug, which is probably the best option for any Dynamite engine (.18 to .46).
LRP also has a glow plug which I would assume is the perfect size for their engines.
If you have any of these engines and can’t source an exact replacement, your best bet is to pull the original plug from the engine and measure the length below the washer with calipers. You could also take it to your LHS and compare it to the glow plugs they have in stock to see which matches.
Glow Plug Troubleshooting
PROBLEM: Engine won’t start OR engine starts and dies when glow plug igniter is removed
Make sure the battery on your glow plug igniter is charged. It’s helpful to have a fresh glow plug on hand so you can test the igniter on it and ensure it is lighting up the coil.
Once you are sure your igniter is charged and working, test the suspected faulty plug with your igniter. It should glow bright and stay bright. If it doesn’t, you know it is dead and needs to be replaced.
If it glows bright, that doesn’t necessarily mean it’s good. I don’t know the exact science, but glow plug coils are coated in fancy metals like platinum and iridium that create a catalytic reaction that helps them continuously glow while the engine is running. If the fancy metals are no longer present on the coil, it may glow when you apply the igniter, but will not continue to glow in the engine after the igniter is removed. This is likely the issue if the engine starts, but then dies when the igniter is removed.
It’s best to always have a fresh glow plug on hand. If you are having problems, you can always try the fresh glow plug in the engine and if the issue is solved, you know the plug was the problem.
PROBLEM: Glow plugs aren’t lasting very long
The general consensus on glow plug longevity is that a quality plug should last between 1-3 gallons on a properly tuned engine. The exception is when breaking in an engine, which is hard on plugs and therefore can shorten their lie.
If your engine has already been broken in and new plugs aren’t lasting at least a gallon, there’s a number of factors that could be shortening the life of your plugs:
- Using a cheap, knock-off glow plug.
- Using either the wrong plug length or heat range specified by the manufacturer.
- Running your engine lean.
- Worn out internal engine components like the connecting rod, wrist pin, or engine bearings.
- Foreign material such as dirt is inside the engine.
If you have confirmed that you are using the right plug and your engine is tuned properly, the next step is to open up the engine, inspect it, clean it, and replace any worn parts.
It’s also possible that your glow plug is fine, but your engine has poor compression. When an engine has poor compression, it will often start when it is cold and run for a little bit, but once it gets up to a certain temperature it dies and won’t start again until it cools down. If you’re running into this issue, take the engine apart and inspect the pinch on the piston and sleeve. If the piston goes all the way to the top of the sleeve with little effort, it’s time to rebuild the engine or get a new one. See Rebuilding a nitro RC engine.
Glow Plug FAQs
My local hobby shop carries OS #8 and OS LC3 glow plugs. Which one should I buy?
If you have a Traxxas engine, buy the OS LC3 plug.
If you’re not running a Traxxas engine and you want to a plug that is the same size as the original plug that came with your engine, look for a replacement of the original plug online. If you don’t want to buy online or can’t find the original, bring the button of your engine (the part where the glow plug is installed) and the glow plug that came with your engine into your hobby shop and see if they’ll let you check which plug most closely matches the original plug when installed.
There are many reports of using the OS #8 and the OS LC3 plugs successfully in all kinds of engines. Your engine will very likely work with either so if you aren’t concerned with perfection, just go with what whatever plug your hobby shop recommends.
Additional Info
- All About Glow Plugs (Roger’s Hobby Center)
- Glow Plugs for Off-Road Racing (Adam Drake on YouTube)
- Nitro Myth busing! GLOW PLUG and what you should know (the bug404 on YouTube)