Hi,

I've just had a spark plug blow itself out. Luckily it didn't go flying as the coil pack was stopped by the plastic engine cover.

The threads on the plug look slightly burred but, more worryingly, they contained shards of metal that didn't look like they'd come from the plug, so my assumption is that they're bits of the cylinder head's thread. It's difficult to see down into the hole that the plug sits in but it looks as though there are shards of metal in there, too. I haven't tried to put the plug back in, nor do anything for fear of the metal bits disappearing into the cylinder and doing even more damage.

I phoned my local garage (not a VAG specialist but generally pretty trustworthy) who said that, assuming the thread in the head is damaged, we could probably use something called a helicoil. As I understand it, this is basically a steel thread within a thread - screw it into the head, then screw the plug into it. It's better than a new head but still sounds like a bit of a scary option.

EDIT: Just phoned a nearby Skoda dealership who suggested a tap and die set may be sufficient to clean out the threads. I'm a bit worried this might leave the head with insufficient thread, leading to the next plug also working lose.

It's a 4x4 estate - ARX engine, I think (or maybe AUX - I can check if it'll help).

I have several questions!

1. What's the easiest way to get a better look at the threads? On my old Mondeo, you could remove the "rocker cover" (top bit of the head). Can I do something similar on the Octavia and how easy / fiddly is it? In-depth dismantling is limited by my ability, time and not having a driveway.

2. Has anybody had any experience of these helicoils? The garage said that they can be a weak point and sometimes come out when plugs are removed.

3. Worst case scenario - how much for a new cylinder head? Would it be worth looking around a scrap yard? There seem to be loads of Octavias around - lots being used as taxis around here - so some must have been pranged and reached the scrap yard by now.

4. What do we think about using a tap and die? Will it last?

5. Any other ideas?!

Thanks in advance.

I just skimmed your thread but i'd be more concerned about finding out what caused enough pressure to cause the plug to leave!

however I've heard of helicoil before and I imagine it'd be fine, I wouldn't be happy with a tap and die set though for the same reasons you've mentioned, its also probably worth taking the head off to check the cylinder etc before fixing it as there could be something nasty going on, or bits of metal in the cylinder now.

if you need a head pm me;)

Ask your local garage about using an "endoscope" to inspect the threads and combustion chamber before proceeding.

Also, whether it's Helicoil or more involved engine work, I'd change the oil and filter to get rid of any metal particles left over from the failure.

Helicoils are great and any experienced garage will be familiar with their use. The plug must have been loose through insufficient tightening torque or someone forgetting the washer or shim on assembly.

Make sure you get all the metal out before running the engine. With the valves closed, use an airline with a thin extension to blow it out and check, as Ken said, with an endoscope.

I used to have a 3.9 V8 R/Rover, cross threaded one of the s/plug holes, removed the head and took it to a engineering w/shop, they fitted a Heli-coil, sorted !!! with no problems.

If you go the Helicoil route dont worry, its a recognised reliable repair for crossed / stripped threads in aluminium alloys.

NB: As above try to remove all fragments before starting.

Good Luck

Peter

I reckon you could have a case against whoever last serviced the car... sounds like the plug was either overtightened and partially stripped the thread, it was loose, or it was cross threaded and damaged the thread. I've never ever heard of a plug blowing out just because of cylinder pressure.

Thanks, everybody, for your advice. Much appreciated.

The car's going into a VAG specialist tomorrow for a proper inspection and probably a helicoil (still sounds like some bizarre contraceptive). The guy there said they use Time-Serts which, from what I've read, sound like the most durable option.

I think the plug was probably not tight enough and slowly worked lose over time as most of the threads seem okay, as far as I can tell. I've cleared away the lose fragments, put in a new-old plug (not the one that came out; a different one) and the engine seems to run okay (but I haven't dared do much more than crawl).

Cog, I think I'd only have a case against myself unless a garage has meddled with the plugs without my knowledge. I'm still surprised, though, that the problem took so long to present itself - those plugs have been in a long time (several months) since they were last disturbed.

I'd change the oil and filter to get rid of any metal particles left over from the failure.

How about using a pressure washer first?

Well, the car's back from the garage - "Martin's Volks Werks" in Tunbridge Wells.

They had a look at the remaining threads (presumably with an endoscope, as Ken suggested, because I couldn't see any other way of looking down the plug tube) and said there were only a couple of threads remaining. They've installed a Time-Sert and replaced all four plugs, along with the Number 2 coil pack as when the plug initially blew out, the coil pack took a bit of a battering and the hot exhaust gasses scorched it.

The garage said the only thing to be wary of is that any particles of aluminium that dropped into the cylinder usually burn off but could potentially have floated off elsewhere to do damage of their own. I think I'll follow Ken's other bit of advice and change the oil and filter soon, too.

The above cost £207 which I think sounds pretty reasonable. My heart sunk somewhat when the mechanic's response to my description of the problem was, "shiiiiit!" but, touch wood, I'm very happy with the garage - excellent communication and good attention to detail - would recommend them to others and will probably be going there for services in future (they're slightly closer than my usual place).

For your info, I have used helicoils etc in industry, they are a regonised engineering solution for a stripped thread, and sometime put into plastic during manufacture to give a better thread. The helicoil is wound in like a spring after drilling and tapping the hole larger with special taps, It could move but is generally tight after it expands. A Timesert is by a different manufacture and locates more positive on a collar, possibly a better solution.

We use helicoils and timeserts all the time in the bike trade mate, where everything is exposed things are always stripping threads.

I've never known one to fail

3 out of the 6 bolt holes that attatches my inlet manifold to the cylinder head have been helicoiled as of late.

Great little repair kit, not sure how one would stand up to a few thousand psi behind it though.

on (proper) BMX bike stems I have noticed that the holes for the stem bolts (which are typically done up very very tight) come helicoiled from the factory (Ally stem) so i have assumed if done right Helicoils are actually stronger than threads straight into ally, or at least least as strong and wont weaken with each time its undone and redone.

...not sure how one would stand up to a few thousand psi behind it though.

Actually, if inserted correctly they can be stronger!

The inner thread is made of much harder metal so it's less likely to strip.

The outer thread is of larger diameter and is in contact with more metal.

Have experienced the same with SWMBOs OCTY Mk1 and the Skoda garage did some kind of thread replacement, dont know if they used Helicoil, I've understood that there are even better methods today (Helicoil's a pretty old invention, 30+ yrs) but point is that there was no need for a new head.

Bill amounted to the same as yours, roughly - they had to remove the inlet manifold to get at it. 2 yrs since and no complaints.

Odd thing was that it didn't happen after a plug change (that had damaged the threads), the plug suddenly decided to leave home after 10K miles :confused:

I've only used helicoils on old motorcycles, but they were usually pretty solid. Some of the more highly tuned bikes sometimes needed a hotter plug fitted, as the helicoil didn't conduct heat so well, but the only one I ever saw break was on an old Triumph with a silly compression ratio (12:1 or something)

Phil

Just when I thought it was safe, the saga continues. Changed the plugs today as the (admittedly cheap) copper ones we put in a year ago were beginning to misfire at high revs when running on LPG. The plug in question was very stiff to remove and brought the timesert with it. The timesert's threads were covered in brown goo (presumably threadlock) and slightly burred at the end. I believe this burring was by design: a timesert is essentially a helicoil with a slight thickening at the end so that when a special tool is inserted, they splay out fractionally and lock themselves into the head - or so the theory goes. It doesn't seem to have worked in my case.

Was taken back to the garage that originally did the work and they refitted a new timesert. It seems to be holding up. However, I have been advised that for the timesert to have come out at all, it is likely that the head is cracked, and that the new timesert will probably go the same way.

Cost of a new head, anyone?

One other thing I noticed is that when the put the original timesert in, the plug (and coil pack) always stood slightly higher than the other three. Now it stands even higher still - probably nearly 10mm. Is this normal?

if you need head pm me;)

Modified for extra humour. How rude

If the insert is moving the plug out further than the others it may cause you some running issues as the spark will not be in the optimal position. Have you just got LPG or is car remapped as well? if you are getting missfires at higher revs is it only when under boost conditions or all the time? If it is only when boosting then you need to check the plug gaps, LPG is a bit more sensitve to poor spark than petrol and the copper plugs need to be one range colder so you need something like the NGK BKR7E's with the gap varied depending on state of tuning (.028 if mapped up to .032 if not, it can vary but will be somewhere in that range)

(trying to get image of clive writing on toilet wall out of my head now - not good :D LOL)

Sorry to hear about the insert coming out!!!!.

My Rangy (nick named Gilbert cos it was green ), if you recall had been fitted with a heli-coil,

and as has happened to you, when I cleaned and gapped the plugs the heli-coil came out with the plug, I just screwed the plug/coil back in

and torqued it up. Didnt have any probs with it (3 years).

Hope you can get this resolved as painless as possible.

Good Luck

Peter

I just screwed the plug/coil back in and torqued it up. Didnt have any probs with it (3 years).

Thanks, Peter. I might just try that myself if it happens next time I change the plugs.

With hindsight I think the things that concerned me most were the fact that it felt like the plug had been cross-threaded (but in fact this was just the threadlock) and the fact that the ends of the timesert threads looked burred (but this is to be expected as it attempts to lock itself into the head). Panic over, hopefully!

EDIT: If changing plugs, how do you get the helicoil/timesert off the old plug without mangling it?

Edited 19 August, 201015 yr by will_

Hi Will,

Good question, if I had to do that, i would spray the heli-coil / plug with WD 40, then clamp plug by its hex body in a vice,

and using a small screwdriver gently tap the end of the coil (unscrew direction), they are a bit springy and should give enough

to release the grip on the plug thread.

Peter

Ah - not sure that would work for a timesert. They're not springy.

( http://www.mphindy.com/tech/time.htm )

The FAQ here seems to suggest they can only be replaced, not re-used.