How would you remove an old helicoil from a sparkplug hole

[Johnny G Pipe]

This is an M14 spark plug hole in an aluminium Porsche 928 head. There appears to be a mixture of crossed head threads and an old helicoil in there. The hole will not take a plug without chewing it up. I want to try and fix this without removing the head (lots of hours and pounds even with the engine in).

I am planning to use a tap and insert (and have lots of contingency plans for keeping the aluminium chips out of the engine while tapping the hole) - but what is the best way to remove the helicoil first? There are special tools but they seem to only go up to M10. It is hard to show in the pic, but there is a loop of the helicoil clearly visible protruding out of the base of the hole, so there is definitely something to grab..

Any help appreciated, trying extremely hard to make the money pit that is a neglected 78 928 as shallow as possible :-)

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[Johnny G Pipe]

Thanks for the help folks. I wasn't sure if that top stepped thread was the beginning of the helicoil but I will have a pick at it to see.

If I can get it all out, the plan would be to use a solid insert kit, either timesert or one of the lookalikes on ebay. Basically a solid threaded brass insert with a top shoulder (so it can't get screwed down into the hole.

Hopefully I can tap enough material to hold the new insert's threads, but I will use some threadlock/adhesive on the insert-to-head part.

[Johnny G Pipe]

GreenV8S said:

They are wound in from the bottom and the comment you quoted was about unwinding from the top, so the effects are reversed. Winding the top counter clockwise will collapse it and pull it away from the wall so that it can be withdrawn.

I would expect to find pieces of damaged thread caught in there and I think this is very likely to end up with debris dropped into the chamber. Is there some cunning plan to block off the bottom of the hole?

Yep, when working on the coil I'll have a greased rag on a string sitting at the bottom of the hole. There are a few methods apparently of getting rid of the aluminium chips when tapping the new threads(like grease on the tap, repeatedly pulling back after each advance, and even attaching a blower to the exhaust!!), but the ally isn't as dangerous as steel fragments. I really need to watch the coil.

I'll be going around the chamber with a hoover attachment and a then a magnet, and I have bought a cheap boroscope camera off ebay!

http://www.ebay.co.uk/itm/Video-Snake-Tube-Camera-...

We will prevail..

[Johnny G Pipe]

Thanks guys.

Having compared with another hole on the other bank, it is definitely a helicoil in there from top to bottom, the top section of the helicoil looks good (but it isn't budging easily when I have at it with a pick) and I really don't know why it is chewing the plugs up..I presumed it had broken or jumped out of a thread somewhere.

Otherwise are you suggesting just grab the bit at the bottom and pull upwards? It is true I guess I am indeed committed as soon as I disturb it significantly. I think chasing it first is a very good idea to see if it can be left alone, will do that first.

With the inserts, which do look more foolproof, the tap is M16 for an M14 insert, so should be enough with some threadlock I hope.

Really appreciate the help.

[Johnny G Pipe]

stevieturbo said:

Well you would be wise to look at it and find out exactly why it is chewing up plugs, because if it is that bad, it should be really obvious.

You'd think! Even though this is a V8 and not a flat 6 thank goodness, getting a really good view of the plug hole is hard. There is nothing obvious with a light and magnifying glass, and my theory is that the helicoil is just sitting in wrong with a badly spaced thread somewhere, installed badly by the PO and the plug just screwed in hard and fingers crossed.

But I am awaiting delivery of my budget ebay boroscope camera before I do anything else, in theory it will give me a good view of the threads. In practice it might be a bit rubbish. :-)

If I can't see anything obvious, the least harmful next step is very careful use of the reverse tap thread chaser I think. You can vary how wide the tap is, perhaps that may just nudge the helicoil back into place if it has shifted.