Timing belt failure is the usual cause of major engine failure. There is a lot of confusion over when it needs changing. When I enquired at the Ford garage they said every 70,000 miles. Most people, including many magazines, believe it should be done more regularly than that. Some say as often as 36,000 miles but I would suggest that as a good compromise you change it every 50,000 miles.
The belt itself is very cheap. I recommend you buy the proper one from Ford to be on the safe side. Last time I paid £8.50. The Haynes manual tells you to drain the cooling system but I have found that this step is unnecessary. First job is to remove the fan belt by loosening off the three retaining bolts on the alternator and pushing it inwards to allow the belt to come off the alternator pulley. Now remove the cam belt cover which is held in place by several 10mm bolts. With a bit of jiggling, it should lift out of the engine quite easily. You will now be able to see the old cam belt which twist and turns its way down from the overhead cam, connecting with the toothed distributor cog and lower crankcase pulley shaft.
Before we go any further, it is important to stress that the belt connects each pulley in a precise way and the new one should be fitted without 'jumping teeth'. To help you I suggest you remove the distributor cap and then rotate the engine by hand until the rotor arm points to where no. 1 spark plug lead fits. At the same time, note that the pulley on the camshaft has a pointer and a 'dot' that should be aligned. Also note that the crankcase pulley (at the bottom of the engine) will have the ignition timing marks aligned.
You will notice that it is impossible to remove the belt without first removing the lower crankcase pulley. This can be very difficult to get off; some people even resort to using a 'puller' tool, but I have never had any major problems until the most recent time when I did have to use a small puller. Once you have purchase done it is easy enough to work out how it goes on the pulley and how to use it. Loosen off and remove the large pulley bolt. It is torqued to 40 pounds foot so it will need some effort (or some leverage!). If you are lucky, the pulley will pull off in your hands. What is more likely is that you will have to work it off slowly using two long screwdrivers. You will see that the pulley shaft is 'notched' so that it is impossible to put it back on the wrong way.
The cam belt is now fully exposed. To remove it, simply undo the tension pulley on the left hand side. As soon as you do, the belt will become very slack and you can carefully remove it off the other pulleys.
Inspect the belt for signs of damage by bending it back; you will probably see cracks and frays. I would be especially concerned if the belt is very oily as the rubber degrades in the presence of engine oil.
If the replacement belt has writing on it, it should go into the car so that the writing is the right way round as you look under the bonnet. Before you fit the belt consider this: the belt is driven by the lower cog only and this pulls the belt clockwise onto the distributor and cam shaft pulleys. In other words, the right hand side of the belt is under tension and the left hand side (where the tension pulley is) is 'slack'. With this concept in mind fit the belt at the bottom first. Stretch it back to the distributor shaft where the teeth should match, and then up to the cam shaft pulley, again keeping the belt taught at all times. You can then wrap the rest of the belt around the tension pulley. If it is looking good, then tension the belt by pushing hard on the tension pulley whilst clamping it tight.
Now before we go any further, try rotating the engine a few times and watch all the components. Do all the timing markers still match up ? We can adjust the distributor later to get it spot on so don't worry about minor variations. It is surprisingly easy to align the engine 180 degrees out which would produce some disastrous results. If you are still worried, then remove the no.1 spark plug and watch the valves through the hole. They will be both closed and the piston visible when the cylinder is on its compression stroke. This should coincide with the distributor rotor arm pointing to where no. 1 spark plug lead fits. Remember that the crankcase pulley spins twice for every time no. 1 piston is fired. If everything is still OK, make sure the belt adjuster is as tight as you can get it and then re-fit the lower crankcase pulley, torqueing the bolt to 40 pounds foot. The fan belt should be re-fitted at this point.
For peace of mind, you may want to check the engine running before putting the cam cover back on, but be very careful as this is potentially lethal (especially if you have long hair!). With the engine running it is easy to see why these belts wear out when you consider the belt forces the cam shaft to rotate against all those valve springs.
You must re-time the ignition after replacing the cam belt. For this you will need a xenon strobe. The procedure is listed in my service guide page. Good Luck !