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Stripped spark plug thread: can you fix it with the head on?
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#stripped spark plug thread#spark plug thread repair#spark plug thread repair kit#M14x1.25 thread insert#helicoil spark plug#repair spark plug hole without removing head

Stripped spark plug thread: can you fix it with the head on?

Uzair bin Haroon

Usually, yes. If the plug spins or pulls out but the hole is still round and the head is not cracked, you can drill and tap the hole and fit an M14x1.25 coil insert without taking the head off. The repair holds if three things are right: the valves are closed and the swarf stays out of the cylinder, the insert length matches the plug's reach, and the broken-off tang comes out before the engine turns over.

This follows Stripped thread: thread chaser, coil insert or bigger bolt?, which explains how a coil insert works. A plug hole is harder because it opens into the cylinder.

When should the head come off?

Take the head off when the thread came out with a ring of metal still on the plug, when the seat face the plug seals against is damaged, or when there is a crack near the hole. A coil insert restores a thread. It cannot rebuild a seat or close a crack. Take it off too if the workshop manual calls for it.

Head-off is also the cleaner job: no swarf reaches the cylinder and the drill is easier to keep square. Classic British Spares puts it plainly:

"I highly recommend taking your cylinder head off to perform this job." Classic British Spares

Head-on saves a head gasket, head bolts and hours of labour, so it is worth trying on a thread that is only stripped.

Which insert length do you need?

Match the insert to the plug's reach, which is the length of thread from the plug's seat to the end of the threads. Measure it on the old plug with calipers, or read it from the plug's specification. Summit Racing sums up the risk in one line: if the reach is too long, the plug "could hit the piston".

The same applies to the insert. Böllhoff, which makes HELICOIL, sells its M14x1.25 spark plug inserts in three lengths (8.4, 12.4 and 16.4 mm) for this reason. The TOPTUL JGEW0901 kit holds five 20 mm stainless inserts with its own drill, tap, installation tool and tang breaker. The 20 mm length suits long-reach (19 mm, 3/4-inch) plugs. In a head made for a 12.7 mm (1/2-inch) reach plug, a 20 mm insert would reach into the combustion chamber. Check the reach before you drill.

How do you do it with the head on?

  1. Close the valves. Turn the engine by hand to the compression stroke on that cylinder, so both valves are shut and no swarf can get into the ports. Leave the piston low enough that the drill and tap cannot touch it.
  2. Pack the flutes with grease. Coat the drill and tap flutes with thick grease so the chips stick to the tool. Pull the tool out often, wipe it clean and grease it again.
  3. Drill square. Drill out the old thread with the 14.5 mm drill from the kit, square to the seat face. The tap follows the drilled hole, so a hole that leans gives an insert that leans.
  4. Tap with the kit's tap. Cut the full depth of the hole by hand. Use the tap from the kit, not a standard tap of the next size up: the insert needs its own special thread, as the guide to a coil insert that came out with the bolt explains.
  5. Get the swarf out. Before the insert goes in, vacuum the cylinder through the plug hole with a thin tube on the nozzle, and look in with a light.
  6. Fit the insert below the seat. Wind the insert in with the installation tool until its top coil sits at least a quarter turn below the seat face. Böllhoff sets that depth for its inserts, so the plug's washer or taper still seals on the head itself.
  7. Remove the tang. The tang sits at the bottom of the insert, on the combustion side. When you break it off it falls into the cylinder, and it has to come out before the engine turns over. A stainless tang may not stick to a magnet, so be ready to vacuum it or pick it out.

Then thread a new plug in by hand. It should run all the way to its seat with no resistance before you torque it to the engine maker's figure. A plug that binds partway means the insert is sitting too high or at an angle.

What if the insert does not hold?

If the new insert spins, or comes out with the plug, the tapped thread is oversize or the hole is cracked, and a second coil insert will not fix it. That needs a larger solid insert or a head-off machine-shop repair. Find out before the car leaves: run the plug in, torque it and back it out once. The insert should stay put.

Order the TOPTUL JGEW0901 spark plug thread repair kit from Vesprr Tools, or ask Vesprr Tools for a different thread size.

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