Thanks for the suggestions guys.
Robodestro, that's certainly a possibility, but there don't appear to be any score marks down the bores.
Frank, it's a 52 plate - 2002 I guess - it also has coil packs.
I had been wondering if it could be caused by sticking valves. My reasoning was that if the valves had been slightly bent by the debris from the old plugs, one or more might stick as it rotates and that would produce the apparent randomness of the rattling. I reasoned that the impact causing the noise might be the cam hitting the valve (or shim) because of the excessive gap. There seemed to be no development of the noise to a catastrophic failure, so I thought the pistons hitting the valves would not be likely. So, to begin today, I thought I would check the compressions again before I removed the valve cover and tried to manually depress the valves. On removing plug no 3 I saw that the outer electrode had been pushed on to the central electrode, closing the gap completely. This, no doubt, reduced the engine to 3 cylinders. Now, I am assuming there can be only three possibilities for this:
1. There is some debris in the chamber that has been wedged between the piston and the spark plug. That was my initial assumption before I removed the head. The head was thoroughly cleaned before reassembly and there was no debris in the cylinder, so I am discounting that.
2. The spark plugs are too long and fouling the pistons. The problem with this is it has only happened on cylinders 2 and 3 and the rattling is intermittent. Also, I would expect the gap to be closed straight away and the engine to sound pretty sick more or less immediately. There have been two makes of plug used: NGK BKR5E and Bosch FR8DCX (as best as I can read from the plug). Both types of plug measure approximately 23 mm from the sealing washer to the outside of the outer electrode. The NGKs were sourced using the registration details and this showed the engine and chassis numbers on the shop database matched those on the car.
3. Somehow the pistons in cylinders 2 and 3 are occasionally travelling beyond their normal top dead centre limit and fouling the plugs when they do. I am totally at a loss as to how this could happen, but removed the sump today to see if there is anything amiss at the bottom end. I can't see anything unusual and applying pressure to the bottom of the con rod and underneath of the piston doesn't seem to produce any unusual movement or reveal any excessive play.
To go any deeper might involve just as much work and cost as obtaining another engine, but I would like to know what the cause is before I junk the engine.
The symptoms are exactly the sames as when I started on the car.
I really am open to suggestions and help.
