Cross threaded helicoil spark plug

[chriz1]

My friends Fiesta 1.6 ST spark plug thread was cross threaded by another friend and then repaired with a helicoil which was overtightened while inserting the spark plug ! Dont ask ! Car starts fine and idles ok but any revs it leaks.
I've had a look for him and the plug screws in and out fine just at the last bit where you want it to tighten it doesn't..
What are the options here before I get the head off?

Can I simply run a new tap the same dimensions as the current thread to tap the last bit of damaged thread?

Or does the helicoil need removing and redoing ?

Obviously I will set the piston at tdc and fill hole with some rag / paper with grease that can be recovered after the work and hoover out cylinder wand check with borescope..

Is there any other options to look at ?

[OldGermanHeaps]

Do they do a suitably sized timesert?
In my experience those are better than helicoils

[chriz1]

Im sire they probably do, first thing though will be to try remove it

[E-bmw]

chriz1 said:

My friends Fiesta 1.6 ST spark plug thread was cross threaded by another friend and then repaired with a helicoil which was overtightened while inserting the spark plug ! Dont ask ! Car starts fine and idles ok but any revs it leaks.
I've had a look for him and the plug screws in and out fine just at the last bit where you want it to tighten it doesn't..

Can I simply run a new tap the same dimensions as the current thread to tap the last bit of damaged thread?

Or does the helicoil need removing and redoing ?

A couple of points here from my viewpoint.

How do you "overtighten a helicoil"?

Is it not more likely that the helicoil isn't as long a thread as the plug, so the little "hook" at the end is bottoming on the end of the plug stopping it from fully tightening?

First thing for me would be to remove the helicoil & try it on the plug whilst out of the head so that you can see what is happening to it.

[chriz1]

That's a good point actually as you say I think its best to get it out and check its length matches the plugs thread.

[donkmeister]

chriz1 said:

I've had a look for him and the plug screws in and out fine just at the last bit where you want it to tighten it doesn't..

What do you mean by this?

Sealing on spark plugs is provided by the crush washer, not the thread. Have you tried a new plug, just in case that resolves it? Fitting helicoils in situ, they're always going to be the tiniest bit odd axis, so reusing a plug with a pre-crushed washer is a potential leak point.

ETA re-read your post and it sounds like they didn't drill all the way through and fitted the wrong length of insert. That would need redoing. Sounds like your friend's friend is an expensive person to know!

[chriz1]

Yeah the spark plug just turns and turns, feels like its getting tight but it doesnt. Yes changed plug still the same so likely helicoil isn't correct size

[wildoliver]

I'll be honest I forsee a replacement head in this cars future.

I'm assuming you know, but when you helicoil you make the existing hole bigger, thread it then fit the helicoil which takes it back down to the original size. It's very unlikely that the helicoil has stripped, it's very likely that the actual head has stripped because it wasn't threaded properly.

In many years thankfully I've only ever had to do this to one car (a dolly sprint that some idiot had overtightened the plugs on) and I used the solid insert (timesert type) I think for many jobs and especially spark plugs they are superior to helicoils. That said I use helicoils regularly and never have an issue with them so it is mostly down to a degree of competence.

If that larger thread has been stripped, you could go up yet another size and then double insert it (definitely use timesert for this) but you are getting well in to the realm of emergency bodge territory, if it's been stripped again I'd look for another head I think.