Glow plugs for nitro RC engines

The glow plug is the smallest part of a nitro engine and the one most likely to stop you running. A plug that's worn, too hot or too cold, or the wrong type is behind a huge share of "won't start", "won't idle" and "dies when I take the igniter off" problems. Here's how they work and how to pick and check one.

How a glow plug works

A nitro engine has no spark. The glow plug has a small coil of platinum wire. To start the engine you heat the coil with a glow igniter; once the engine fires, the heat of combustion and a catalytic reaction between the platinum and the methanol in the fuel keep the coil hot, so the igniter comes off and the engine keeps running (Wikipedia: glow engine).

That means the plug's temperature controls when the fuel ignites. Get the plug's heat range wrong and the engine is harder to tune, idles badly or loses power.

Heat range: hot, medium or cold

Glow plugs come in heat ranges, from very hot to cold. The difference is how hot the coil runs.

As a rule: smaller engines, cold weather and low revs suit a hotter plug; larger engines, high revs, high-nitro fuel and engines that run hot suit a colder one. A plug that's too cold can make the engine sag, especially when the igniter comes off; one that's too hot can cause pre-ignition, and a kick-back or backfire when you start it.

Start with the plug your engine's manual specifies, usually medium, and only change one step at a time. Traxxas, for example, specifies its heavy-duty long plugs for the TRX 2.5 and 3.3 and recommends part 3232 (Traxxas engine manual), which is a medium plug (product listing). The O.S. No.8 is a widely used medium standard plug (product listing).

Standard or turbo?

This is the mistake that catches people out, because the two look similar.

• Standard plugs have a threaded body, seal against the head with a copper washer, and the coil sits in the combustion chamber. Most ready-to-run car engines use these, including Traxxas's 2.5 and 3.3. They use a 1/4-32 thread (Wikipedia).
• Turbo plugs seal on a tapered seat with no washer, leaving the combustion chamber smooth, and use a different, metric M8x0.75 thread (Wikipedia). They're used in many 1/8 buggy and 1/10 and 1/8 on-road racing engines with turbo-type heads. O.S., for example, supplies its Speed R2101 on-road engine with a turbo head and RP7 turbo plug (O.S. manual).

They are not interchangeable. Different threads and a different seat mean one won't fit properly in the other's head. Never force a plug that doesn't screw in easily by hand: you can damage the threads in the head. Check the manual or the old plug before you buy. A turbo plug also has to be very clean to seal; a speck of dirt on the taper can cause an air leak (RCTalk).

Long or short body

Standard plugs come in long and short versions. Fit the length your engine uses; the wrong length changes how far the coil sits in the chamber, and in some engines a long plug in a short-plug head can sit too deep. Traxxas TRX 2.5 and 3.3 engines, for example, use long plugs. Never swap lengths.

Idle bar plugs

Some plugs have a small bar across the coil, meant to stop raw fuel quenching it. They're mostly used in model aircraft engines. For a car, fit what the manual says, which is normally a plug without an idle bar.

How to test a glow plug

1. Remove the plug with the plug wrench (let the engine cool first). Keep the washer if it's a standard plug.
2. Connect the igniter to the plug while holding it with pliers or in the igniter's clip. Do this out of direct sunlight, or the glow is hard to see.
3. Look at the coil:
   • Bright orange glow: good.
   • Dim, patchy or no glow: replace it (and check the igniter is charged).
   • Coil broken, stretched, sagging or touching the body: replace it.
   • Coil grey or frosted rather than shiny: worn; replace it soon.

Owners also warn that a plug can glow and still be weak (RCTalk). If the engine still misbehaves, try a brand-new plug: it's the cheapest test there is.

Signs your glow plug is failing

• The engine starts but dies the moment the igniter comes off.
• It won't idle or stalls when you lift off the throttle.
• It's harder to start than usual.
• It runs fine, then cuts out after hard running.

Before blaming the plug, remember these can also be the carburettor setting. A low-speed needle set too rich or too lean can cause stalling at idle too. Change the plug first, because it's quick and cheap, then look at tuning. Our RC car troubleshooting guide covers the rest.

What kills glow plugs

• Running lean. A lean mixture runs hotter and can burn out the coil (Wikipedia). If you're eating plugs, richen the high-speed needle slightly.
• Running the tank dry. The last of the fuel runs lean. Traxxas warns this can burn the plug and overheat the engine (Traxxas engine manual).
• High-nitro fuel. It shortens plug life, as O.S. notes in its manuals (O.S.). See nitro fuel.
• Running in. Deposits from running in can shorten a plug's life, which is why Traxxas recommends fitting a new plug once running in is finished (Traxxas). See running in a nitro engine.
• Overtightening. Snug is enough. Overtightening can damage the plug or the head threads, and on a standard plug, crushes the washer.

Buying glow plugs in the UK

• Buy the exact type your manual lists (standard or turbo, long or short, heat range) and keep two or three spares.
• Branded plugs from O.S., Traxxas, Novarossi and similar cost more than unbranded multipacks, but they're far more consistent. A cheap plug that fails at the track isn't a bargain.
• Plugs are small and post easily, unlike fuel, so model shops, Amazon and eBay all work.

New to nitro? Start with what you need to run a nitro car, or weigh it up in nitro vs electric. The igniter itself usually runs on a rechargeable NiMH cell, covered in NiMH vs LiPo. Back to the RC cars hub.