How to Remove a Broken Glow Plug

Dealing with a broken glow plug in an off-road mechanical diesel engine is a real headache. What was meant to be a simple job replacement turned out to be a big, troublesome, and time-consuming problem – even shutting down the equipment. If the glow plug is stuck or broken, the situation is even worse— if it is not handled properly, it may also hurt the cylinder head, so it is not cheap to repair.

We write this guide just to help you avoid all this trouble. Next, we’ll take you through the common damage symptoms, the tools you need, and the detailed steps to safely remove the broken glow plug.

The Possible Broken Glow Plug Scenarios

When the glow plug is stuck, it usually breaks in one of two ways. Finding out which one you are dealing with is the first step to safe removal.

The Glow Plug Body Snaps Above the Threads: This is quite common. The hex head and the upper half of the glow plug are broken, but the thread and the heating element below are still stuck in the cylinder head. It is generally because the glow plug gets stuck too hard, and the torque is too high when screwing in hard. The culprit is usually rust or carbon deposits that are too thick, nearly “welding” the threads inside. Although quite depressing, the good news is that this judgment is relatively easy to handle.
The Glow Plug Tip Snaps Off and Stays in the Cylinder: This situation is more serious. Although you have screwed out the main body of the glow plug smoothly, the slender heating tip is broken in the preheating chamber or cylinder. This is probably the case because the engine is operated at high temperature for a long time, causing the tip to expand, deform, or become entrapped in thick carbon deposits. If this broken tip is not taken out, once the engine is started, the consequences can be very miserable, as little as damaging the cylinder, as heavy as scrapping it outright.

Both cases are about the harsh environment in which the glow plug works, because of exposure to high temperatures and combustion residues for a long time. Glow plugs have carbon deposition and metal fatigue, and can easily become stuck. If you want to remove it, first figure out what kind of damage it is, and then do it in a targeted manner.

Expert Tip: Always attempt to remove glow plugs from a warm, but not hot, engine. The heat causes the aluminum cylinder head to expand slightly more than the steel glow plug, which can help loosen the threads and reduce the risk of snapping.

Prepare These Removal Tools

To do this work, the right tools are essential. Making do on the fly only makes the problem worse. Before you start, get everything you need ready to avoid a work stoppage.

Here is a list of tools we recommend for the job:

Safety Equipment: Safety glasses are a must. Metal shavings are a certainty.
Penetrating Oil: A high-quality penetrating fluid is your best friend. It will help break down rust and carbon.
Socket Set and Ratchet: To remove any components blocking access to the glow plug.
Vice Grips or Locking Pliers: Essential for gripping and removing the central electrode.
Power Drill: A variable-speed drill will give you the control you need.
Assorted Drill Bits: You will need several sizes, including a center drill bit and bits correctly sized for your tap set. Left-hand drill bits can sometimes be helpful, as they may bite into the broken piece and unscrew it during drilling.
Tap and Die Set: Specifically, you will need a tap that matches the extractor bolt you plan to use (e.g., an M6 or M8 tap is common).
Extractor Bolt, Nut, and Washers: A high-tensile (hardened) bolt, a matching nut, and a stack of washers or a deep socket will be used to create a DIY puller.
Compressed Air Blower or a Shop Vacuum: Absolutely critical for cleaning metal shavings out of the cylinder.
Glow Plug Bore Reamer/Cleaning Brush: To clean the shaft and seat before installing the new plug.

How to Remove the Broken Glow Plug

With the tools and steps ready, you can start removing the glow plug. Be careful and patient in this work, and don’t rush. Next, just do it step by step.

Step 1: Preparation and Soaking

Clean the work area to make sure it has access to the broken glow plug. Then disconnect the battery. Spray enough high-quality penetrating oil on the broken glow plug to. It slowly penetrates the thread. It’s better to soak for a few hours. It’d be better to leave it all night. During this period, you can refill a little more every hour to help the lubricant go deeper.

Step 2: Remove the Central Electrode

If the top of the glow plug is broken, the central electrode inside is usually still stuck in the residual segment. It has to be taken out before drilling. Clamp the electrode with a vise and pull it out while shaking it gently. It may be tight and take a bit of effort to pull out.

Step 3: Carefully Drill a Pilot Hole

This is the most critical step— Drilling. The goal is to drill down the center of the broken glow plug housing. First, use a small positioning drill to make a guide hole, so that the main drill bit will not deviate. After setting the starting point, replace it with a drill bit suitable for the tap to be used later (the corresponding hole diameter is usually marked on the tap set).

Before drilling, you can use a drill bit limit ring or wrap a circle of tape around the drill bit to make a depth mark. The drill should be just deep through the body of the glow plug, but not too deep to hit the bottom of the plug or the cylinder. You can first measure the new glow plug to determine the safety depth. Slow down while drilling, stay steady, and remember to clean iron filings frequently.

Step 4: Tap Threads into the Glow Plug Shell

With the pilot hole ready, it’s time to cut the threads. Pick the tap that fits your extractor bolt. Put a bit of cutting oil on it. Start turning the tap into the hole by hand. Turn it about half a turn clockwise to cut, then back it off a quarter turn to break the chips. Keep doing this slowly until you’ve cut threads through the whole hole.

Warning: Taps are made of very hard steel, but they can snap if you twist too hard or let them jam. If that happens, it’s even worse than a broken glow plug. Take your time and don’t rush.

Step 5: Assemble the Extractor and Begin Pulling

Now it’s time to build the puller. First, screw a nut onto your strong extractor bolt. Then slide on a few washers or a deep socket that can rest flat on the cylinder head around the hole. The socket or washers will act like a base for the nut to push against.

Screw the bolt into the plug housing where you just tapped the wire and tighten it. Then one wrench holds the bolt head, and the other twists the nut clockwise. When the nut is pressed down, it will be pulled up with the bolt, and the glow plug housing stuck in the cylinder head will be pulled out bit by bit. It will feel quite a lot of resistance during the process.

Step 6: Clean, Inspect, and Install

After pulling out the broken part, the work is not over yet. Be sure to clean up the glow plug holes and the debris in the cylinder. First, use a vacuum cleaner to suck out large pieces of debris, and then use compressed air to blow off the fine powder.

Use a reamer or cleaning brush of suitable size to completely remove the remaining carbon deposits on the hole and contact surface. Before installing the “new glow plug”, apply a thin layer of high-temperature-resistant anti-bite agent on the thread, so that it will be much easier to disassemble in the future. Remember, don’t push too hard.

In Summary

The key is to be patient, to remove the broken glow plug, use the right tools, and follow the steps carefully. If you prep well, drill, tap, and pull it out step by step, you can fix it without making things worse.

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