Xantia loose spark plug and slipped aux belt

[the_weaver]

When I had a stripped thread it was caused by over-tightening the spark plug. I used a torque wrench to tighten the spark plug to the correct torque but for some reason it was too tight. The thread stripped when I came to take the plug out a few months later. I always do plugs by hand now. I think the problem may have been due to using a big torque wrench at the lowest end of it's range. They become inaccurate at the extremes. I think tightening up by feel is much safer whenever you have threads in aluminium.

Paul

[merlin617]

Nope, they won't be related but damaged plug threads in the head are not unknown. It's a relatively straightforward job for a good, competent engineering firm to helicoil the plug thread and often they can do it without removing the head.

The baby Stig bit at the end was brilliant

Jim I have just changed from Bosch plugs to NGK (supposed to work better in Xantias ?)
On one of the plugs the thread only screws in 2 complete revolutions unlike the old Bosch that screws in about 4. ? I did have one come loose a couple of years ago but not been a problem since.

Anyone know if there is going to be a new Stig ?

[merlin617]

When I had a stripped thread it was caused by over-tightening the spark plug. I used a torque wrench to tighten the spark plug to the correct torque but for some reason it was too tight. The thread stripped when I came to take the plug out a few months later. I always do plugs by hand now. I think the problem may have been due to using a big torque wrench at the lowest end of it's range. They become inaccurate at the extremes. I think tightening up by feel is much safer whenever you have threads in aluminium.

Paul

I once over tightened a bell housing because I misread the numbers and put in Newton Metres instead of the intended foot pounds, curse!

[handyman]

Merlin, sounds like you have all the symptoms of a crossed thread in your spark plug hole! If it will only pull down two turns, the plug will not be seating correctly at the face and the plug will be recessed in the head, making for bad running and eventually the plug will pop out, taking the threads with it.

Suggest you investigate and resolve PDQ.

Handyman

[merlin617]

I agree that's what I thought but I put the new and old one in and out a few times and got the same result each time. Looking at the thread I wondered if it was damaged by the earlier pop a couple of years ago.

I was thinking of putting a helicoil in but to do so means tapping the existing thread and aluminium swarf will end up at the top of the cylinder. does it burn off anyone done this with head in place ?

[dnsey]

It's usual to apply a heavy coating of grease before tapping, which not only lubricates the tap, but catches the swarf too.

[handyman]

Merlin, read my first reply in this thread, as it has all the details. Alternatively look at 'AA Relay' post as this is the same discussion!

Handyman

[DickieG]

Why not try using a "Thread chaser"?

[handyman]

Yes, good idea if the metal in the thread is deformed by cross-threading and the heads were sand cast in ali-bronze, as this is a very ductile material.

From experience, with Citroen or Peugeot heads, due to the quality of the metal used in the die-casting process, chasing or reforming a thread is rarely an option. Again, once you had chased a thread in, you would need a gauge to see whether it was 'on size' or not.

The reasons why threads in many die-cast heads fail, is due primarily to the grain structure of the metal and the flow of the molten metal in the casting. The forming of the threads in the blank head usually cuts across the grain of the metal, thereby leaving a very weak form. The ideal in this type of material is either a rolled thread, difficult to achieve in an internal thread or a wire thread insert made of a tougher material.

Handyman