Exhaust leak via spark plug

[stantondavies]

The engine on my 3 litre had developed a slight tick more noticeable under load. On tickover I could hear an exhaust leak around the manifold and set about checking the manifold nuts and then manifold to exhaust pipe without improvement.
The situation deteriorated and I discovered the leak was coming from No.6 spark plug. This was out on the road, so I had to use the car's toolkit and to remove the plug I had to take off the breather. (Despite being careful, I subsequently knocked the retaining bolt into the engine which took ages to locate and recover but that is another story)
Having cleaned everything up I refitted the plug but the problem persisted although much improved.
Back at home I replaced the plug with the old (still in very good condition) plug to eliminate the possibility that the plug was at fault in some way. The problem was still there.
Has anyone come across this problem before? I assume that leak is via the threads and possibly I may have inadvertently cross-threaded at some time; I copper-greased the plug threads - something I don't normally do. The plug is firmly screwed down.
I have tried to ensure that the plug washer is seating well without anything fouling the seal, but No.6 is very difficult to see clearly without a mirror. Has anyone any ideas?
Incidentally, the last time before the problem occurred, the plug had been replaced by a garage, not myself.

[Phil Nottingham]

May well be cross threaded - see if it will hold otherwise a helicoil insert is required which will be awkward in situ on no 6 but should be possible

[stevew]

Hi stantondavies, a quick check you can make to help determine where your leak is, try this.
With with a small tube and funnel, dribble a small amount of water around the sparkplug base with the engine running. The water will make a temporary seal and the noise you are hearing should stop if the gasses are leaking from this area. If the leak is very bad it will blow the water away. Be carefull and dont use too much liquid as it may get sucked in and cause damage, just a litttle is sufficient. If the noise continuse, then the leak is in another area. Good luck, hope this helps

[stantondavies]

Thanks Phil. The odds are that the plug is cross-threaded but I wondered if a crack or fault with the head might be the cause. I have five helicoils in so far so this must be the last one. The plug is in tight but I don't want to overtighten. I had thought of trying two washers on the plug to see if it seated better, but that might encourage the plug to blow out, which I've had before.
The head and sump have to come off shortly as compression is down on 4 & 5, so will examine the head then.
Thanks for the interesting tip stevew. I am certain that the leak is from the plug area; from inside the car it sounds like a tick on each up-stroke. At one stage it was bad enough to feel the leak with my fingers. I replaced the plug to eliminate the possibility that the original one was faulty. However, your tip is an interesting academic exercise to try - would oil be a better medium and easier to apply from a long spouted oil can?

[stevew]

Yes, oil will do the job quite effectively, however it will smoke and smell when the engine reaches operating temperature, if you do use water it will simply evaporate with no smoke etc. How about putting water instead of oil in you long spouted oil can,? Good luck