The head temp sensor came with the Trail Tech Vapor speedo.You remove the washer on your sparkplug and it's basically a copper fitting with a wire coming off it which goes up to the speedo to give a temp reading.
On the XT it doesn't fit down the plug hole very well,so after a few times removing and putting the plug back in,it eventually breaks.
It can prove useful in letting you know just how hot the engine is,normal range (NZ conditions here) is around 180-220 degrees.

As for the coil,it turns out once the shop had opened the L/H casing up,that two pickup coil wires had hit the flywheel and were shorting out.All due to the little tab that holds the wires down in that case had somehow come off.The tab had also done some damage to the stator,great!.

So once I get a new or repaired stator back in the bike,it's going to be sold.It's cost me far too much money over the last year and a half and I can't afford to have much else go wrong....Which I don't think much else can go wrong as I've pretty much replaced every worn out part now.
And don't forget,you can only learn by asking questions. ;)

Hey, don't belittle yourself! No bloke was born knowing this stuff either.

Many things, none of them good.

1. Not enough cooling air (obviously)
2. Fuel mixture too weak (caused by, hey, not enough fuel - blocked filter or jet - or too much air - air leak to inlet, mismatched exhaust)
3. Wrong timing (spark at wrong time causes engine to use energy heating itself up rather than powering the bike)
4. Mechanical drag, e.g. binding brakes, causing engine to work too hard
5. Lack of oil (oil prevents heat building up by lubricating, but also carries heat away from the vital bits). It's why oil level is important - less oil = hotter oil.

I'm sure there are more, but those are the most common reasons for overheating. The paradoxical thing is that more fuel (i.e. an over-rich mixture) makes an engine run cool rather than hot. All those little unused molecules of fuel vapour absorb heat if they don't burn. No. 2 is why so many people get problems with aftermarket exhausts. The exhausts flow the air more freely, drawing more air through the engine and weakening the mixture, and making the bike overheat, especially at high speeds. Rejetting the carbs to allow more fuel to balance the extra air is the solution, but it's too much hassle for some, and they end up with holed pistons after a fast run on a hot day. The thinnest part of the piston, the centre, simply melts.

And don't apologise. Questions are what makes groups like this interesting. I've learned a lot in the short time I have been here.

Thanks guys, for explaining the 'hotter' thing - however - how does that tie in with Yobbo's diagnoses of a faulty coil?

Which one of the several causes for overheating, was caused by the electric fault?

And how?

(As I understand it, the coil is in charge of the spark -- so if its faulty, would it cause scenario no. 3?)

The stator was going bad,which is the diagnosis I got from the shop before they'd pulled the bike apart - just doing voltage testing.It wasn't till they pulled the cover off they found two or the pickup coil wires (the little black box that senses the two magnets on the flywheel) had come in contact with the flywheel and were shorting out.With it doing that it's caused the bike to run very advanced by giving a false signal.

Back to my No. 3 above - wrong ignition timing. If the spark occurs at the wrong time, it can cause overheating. Basically, there is a chunk of energy stored in the fuel/air charge that is drawn into the cylinder. If that energy isn't used in providing motion to the piston, it is wasted as heat. If the spark occurs at the right time, the majority (well, 30% or so, but what the heck) is used in moving the piston and the heat produced is less. Too early or too late, and the kinetic energy produced is less, and the heat energy more, giving a) a lack of power, and b) too much heat to get rid of.

One small point in addition. The ignition is timed so that the spark occurs before TDC. In an ideal world, it would occur at TDC to allow the burning fuel to give maximum thrust to the piston. But the fuel takes a fraction of a second to burn fully after it is ignited, so the spark is timed a couple of degrees before TDC so that the fresh charge is starting to burn as the piston comes over TDC, rather than chasing the descending piston down the bore.