HELP!! Spark plug thread crossed.

macaroni · Post by **macaroni** » 29 Apr 2005, 00:42

Brat and blast. I was just checking the condition of the spark plugs on my Xantia Activa and number 3 got it's threads crossed. It goes in about 2 coils and no more.
Does anyone have any tips to sort this out or is it head off time?
I though about trying a new plug.
I hate petrol cars, give me compression ignition anytime.

paranoid · Post by **paranoid** » 29 Apr 2005, 00:53

Old trick that worked for me was to hacksaw a diagonal line down the threaded part of the plug in about 3 places and very very carefully screw in plug a quarter of a turn and back, then in etc keep a vac handy to keep the swarf out the way.

Richard Gallagher · Post by **Richard Gallagher** » 29 Apr 2005, 00:58

You could try restoring the thread by using a 'Thread chaser'. Basically a specific tap type of tool, normally available from accessory shops.

macaroni · Post by **macaroni** » 29 Apr 2005, 01:11

Cheers guys I will try the "thread chaser" first. I am such a clumsy tw@t!

fastandfurryous · Post by **fastandfurryous** » 29 Apr 2005, 01:17

when you come to refitting the plug (or any plugs for that matter) if you jam a length of old fuel hose (about 6") over the ceramic, this grips enough to be able to do the plug up, but not enough to strip threads. Use the length of hose as a handle, and do up the spark plug until it needs nipping up with a socket. This method has saved me dozens of stripped threads in the past, especially on recessed plugs that are hard to reach.

macaroni · Post by **macaroni** » 29 Apr 2005, 01:32

Good advice and I was doing it up by hand whilst being distracted by 2 lib dem canvassers, I thought it was home and applied the ratchett. I shall not be voting lib dem!

ITMA · Post by **ITMA** » 29 Apr 2005, 03:27

Use plenty of grease on the thread chaser. Take it carefully, allowing plenty of time to do the job, and work the chaser in only about 1/4 turn at a time before backing out and cleaning off, re-greasing and inserting again. The swarf (metal particles) will stick to the grease like the proverbial to a blanket.
Work slowly, don't use excessive force and you will eventually be able to easily run the chaser right down the threaded hole.
Finally use a small screwdriver or similar in a piece of cloth to make sure that all the swarf has been cleared out of the thread and around the seating.
I have saved more than one "head off" job by this method.
Best of luck.

ActivaV6uk · Post by **ActivaV6uk** » 29 Apr 2005, 04:01

mate i have a thread chaser you can borrow ware do you live?
Andy

Post by **RichardW** » 29 Apr 2005, 12:54

If this fails, it is possible to put a helicoil in with the head in situ - once watched a mechanic do this on a GS - some feat! That's best left to a specialist though...

macaroni · Post by **macaroni** » 29 Apr 2005, 12:54

Tunbridge Wells in Kent. Thanks for the offer, but I suspect that is nowhere near you and I work near a good motor factors so will try and get one from there at lunchtime.
Thanks to everyone for their advice and support, I will report on how things go over the weekend.

Homer · Post by **Homer** » 29 Apr 2005, 13:52

I had one rethreaded in situ on a Xantia 2.0 petrol, the thread was so bad the plug would slot straight in and pull out again.

macaroni · Post by **macaroni** » 29 Apr 2005, 14:11

Homer, rethreaded with a "tap" or with a helicoil? How did you recover the swarf?

Kowalski · Post by **Kowalski** » 29 Apr 2005, 15:09

To recover the swarf, with the piston at the bottom of its stroke, you pump a load of grease down the spark plug hole, so it sits on top of the piston. After you've finished drilling, you turn the engine over by hand so it goes up to TDC and pumps the grease and swarf out. Then you've got to poke about and get as much of the grease out as you can to avoid hyrdaulic lock type problems when you start the engine, and it'll smoke pretty badly for a while too.

macaroni · Post by **macaroni** » 29 Apr 2005, 15:44

Ingenius! I doubt that is necessary for a bit of thread chasing though. I intended to just spin the engine with the plug for a few seconds to blow the cr@p out.

PeterMann · Post by **PeterMann** » 29 Apr 2005, 19:18

I've successfully repaired damaged sparkplug threads by helicoil method without removing head. Use plenty of grease on the thread cutter, backing out after every half turn advance, and clean off the swarf and reapply grease before cutting a bit further. After can run a plastic tube around inside the cylinder with vaccuum cleaner, pulling any debris that might have escaped off the cutter tip, although this is probably not necessary.