Heli-coil into spark plug thread

[Rigid]

Driving home from work yesterday there was a sudden pop and a loud chugging from the engine.
No6 spark plug had fallen out.

When i first bought the car and checked/changed the plugs, putting No6 back in I made a mess and slightly cross-threaded it.
I bought a suitable tap and re-cut the hole and put the plug back in.
It did tighten but I didn't go mad with the torque.

Now obviously there isn't enough meat left to hold it in so question is - Can the thread be mended with a heli-coil without taking the head off?

I'll be asking a professional to do it, i know I can't get my drill in to cut the hole.
Or can the heli-coil thread be cut directly?

[new to this]

I think you would be better using a time sert for this problem

Dave

[MikeParker]

Driving home from work yesterday there was a sudden pop and a loud chugging from the engine.
No6 spark plug had fallen out.

When i first bought the car and checked/changed the plugs, putting No6 back in I made a mess and slightly cross-threaded it.
I bought a suitable tap and re-cut the hole and put the plug back in.
It did tighten but I didn't go mad with the torque.

Now obviously there isn't enough meat left to hold it in so question is - Can the thread be mended with a heli-coil without taking the head off?

I'll be asking a professional to do it, i know I can't get my drill in to cut the hole.
Or can the heli-coil thread be cut directly?

Yes you can helicoil the thread. The head material is very soft and it is quite easy to cut the thread for the helicoil. Just be really careful to get the helicoil tap nice and perpendicular to the hole. I made up some tube, with an ID just over the OD of the helicoil tap with a good square end (turned on a lathe if you access). Then make sure this tube is in contact with the land around the plug hole as you start to make the thread. Also cover the tap in grease to hold the cuttings. Remove every couple of turns or so to remove aluminium bits and re-grease. I have done three plugs so far and all good. No issues with the bits that go into the cylinder. I guess thay get blown out the exhaust!

Mike

[Rigid]

Thanks Mike.
Is all of this possible without dismantling the head or lifting the engine?
I've already bought a kit, just need some confidence before I actually try and cut the thread.

[Matt Cook]

I have a time sert kit, I have a couple of spark plug threads that have been repaired this way.

[Goldstar]

I wouldn't use a helicoil. Snapping the tang off in the bore is asking for trouble. Use a Time Sert

[trunt]

Another +1 for time sert type repair.

[MikeParker]

Thanks Mike.
Is all of this possible without dismantling the head or lifting the engine?
I've already bought a kit, just need some confidence before I actually try and cut the thread.

I did it with the engine in situ. The others are right about being careful about snapping off the tang. I used long nosed pliers and made sure I did not let go! If you do make sure you get it out with a magnet or grease.

It may well be safer to use what they suggest. I have no knowledge of using time serts. I think I did look at something similar but was worried that it moved the plug out from its normal position slightly. I might well be wrong about that as it does appear that time serts are the favourite. So go with the majority view would be my advice. I may have been lucky!

Mike

[jbuckl]

Timesert is a better choice for the spark plug thread repair, but a helicoil will work too.

The ‘stripping confession’ (!) means there’s been some lessons - learned…

Give it a careful, cautious go!

[richardthestag]

I cannot understand why you might risk an engine rebuild to do the job with the head attached.

fitting either part will require accurate drilling and thread tapping before fitting the insert.

even with rags stuffed into the cylinder and grease on your tool, you simply cannot guarantee that no swarf will be left in the cylinder.

if that does happen the chances are that it will get blown out of the exhaust valve but what damage might it cause first.

I would be surprised if any garage did that job at their risk.

BTW heli coil works for me everytime

[Rigid]

Thank you all.
I'll have a think and likely spend some money.
(and take a plunge)
Richard

[Hughesy]

I had this done the first time by a professional. He used grease on the tap to collect the the majority of the swarf. Then turned over the engine with the plug out (coil disconnected) to blow out any stray swarf that may have been still in the cylinder. Even if there was any remaining swarf in the cylinder being aluminium he said would not cause any damage. I have since done another two myself using a kit for spark plugs.

[dasadrew]

... putting No6 back in I made a mess and slightly cross-threaded it....

I think the decision whether to take the head off is hidden in your sentence. I too often have a struggle to get spark plugs in true and square. If doing that is a bit hit and miss I wouldn't risk trashing a head trying to cut a larger thread in-situ.

It's only the way I would go, but I would wonder if it's only one thread that is a bit "iffy", would take the head (probably both) off, use the opportunity to look at the core plugs too, fit good new gaskets, and get it set up for many miles of head-trouble free motoring

Only my opinion

Drew

[marshman]

I cannot understand why you might risk an engine rebuild to do the job with the head attached.

fitting either part will require accurate drilling and thread tapping before fitting the insert.

even with rags stuffed into the cylinder and grease on your tool, you simply cannot guarantee that no swarf will be left in the cylinder.

if that does happen the chances are that it will get blown out of the exhaust valve but what damage might it cause first.

I would be surprised if any garage did that job at their risk.

BTW heli coil works for me everytime

FWIW I would do it in situ. My thinking is as follows:

1) Removing & refitting a head on a Stag is a PITA with the whole getting the manifold to seal etc. also risk of the timing chain tensioner making a break for freedom resulting having to remove the timing cover to reset it, which means rad out, fan off etc. in my opinion no easier drilling and tapping it "square". That is assuming it wants to come off and there are no stuck studs or bolts.

2) As the plug holes are at an angle the job of drilling and tapping "square" is no easier with the head off than with it on. As has already been said easy to make up a tube as a guide keep it perpendicular

3) Grease on the drill and tap will minimise swarf entering the cylinder , after use of a bit of small bore rubber tube attached to a vacuum cleaner up to TDC and fed down the plug hole to suck out any swarf that may have entered.

If it were my car that is what I would do, potentially a 10 minute job. Just needs a bit of care. Of course if there are other issues that could be addressed by removing the heads then that is a different matter, and as Drew says could be used as an opportunity for a mini overhaul. Really depends on the overall condition of the engine.

[trunt]

When I did mine it I got a "kit" which included time-sert type inserts, a setting tool (to expand the top to lock it in) and a tap/drill tool that used the old threads to align the tap.
It is threaded to go into whatever is left of the the originals and clear them then then stepped to "drill" the hole and then tapped to the new size. Unless your threads are totally trashed it goes a long way to ensuring that everything stays aligned.

here's a picture of something similar..

s-l640.jpg