Fitting an Oil Cooler:

Fitting an oil cooler can be very beneficial if your Fiesta is subject to a lot of hard driving (most of them then :-)).
Serious applications like towing can drastically increase the working temperature of your engine, as well as other daily tasks like burning off lesser vehicles (i.e. Novas).
As we all know its critical to maintain the balance in engine temperature, too hot and you damage it, too cold and it won't run !

Safety!!
Well, I should have thought this was obvious... but. Drain all of your engine oil. Don't work on a hot engine. Don't work with the engine switched on either.
DISPOSE OF OIL PROPERLY! Don't flush it down the drain!
Empty the old oil filter into the same bucket/tray as your drained oil and take it to your local waste site where you'll find a big oil bin to put it in. This makes everyone happy, seabirds can stay a nice shade of white and the oil companies get to recycle it and sell it as cheap shite oil.
A silly point which is valid for all engine mods is: Don't work on a blustery day. The wind will bend your bonnet hinges and it'll never sit right when closed ever again, no matter how careful you are it always happens.
I had to take mine off completely and bend the actual hinges out with a big hammer and a screw driver !!!

Step 1: Finding.
Well, I haven't a clue how difficult it is to find one of these. Theoretically not at all because they are basically universal.
However there is one specific-to-car part, a thermostatic plate that sits between your engine block and oil filter.
You could, in theory, find out how many cars use a fiesta compatible oil filter and then buy a cooler kit for one of them if a Fiesta kit were not available. The kit used for this reference is manufactured by Clova (yes, the fan people) and was certainly available in the 80's !

Step 2: Fitting:
Fitting is described below in easy to follow steps. Trust me, fitting one of these really is a piece of pi$$.
Please accept my apologies for the quality of the images. If some rich person (or commercial co) were to sponsor me (BIG HINT!) I could greatly improve the site as a whole. And even 'plug' their products, if they're any good !.

Step 3: Positioning. The cooler element itself needs to be positioned where it can receive a direct flow of cooling air. This could be infront of your radiator, in the mouth (hole in between lower valance and bumper) under a bonnet vent or anywhere else exposed to cold air. In the picture the element has been placed behind a front light. If done on the passenger side you'll need to re-position the horns.
Step 4: Fitting the thermostatic plate. The sandwich plate fits between the oil filter and engine block. Remove your existing filter and disgard it. Now screw the plate in place of the oil filter (not forgetting the rubber gasket).
Step 5: Replacing the filter. Screw a new oil filter onto the end of the thermostat plate. It screws on just like it used to on the engine block only this time it's got a mate !!
Step 6: Connect up the hoses. Now, using Jubilee clips, connect up the supplied half-inch pipes. You need to run two of these from this plate to the oil cooler element. One in, one out. DO NOT run these tubes near any sources of extreme heat. Don't be a twat and run them over your manifold or you'll end up piloting a fireball. Try and run them along your inside wing or clip the hoses to other hoses already present. ie. The brake pipes or vacuum hoses. Other things to avoid are kinks in the pipes or fouling by sharp objects. Never forget these pipes will actually be bursting with engine oil during use !
Step 6.5: Re-fill your car with oil !
How does it work? Well, the sandwich plate you just fitted operates just like your cooling system thermostat. During normal operation oil will flow past this little device, into the filter and then back again just as normal. When the oil temperature starts to rise the thermostat starts to open allowing oil to flow through some thick pipes, into the oil cooler and then back again to the filter. The oil is cooled by cold air whilst in the cooler. This should keep your oil at the recommended operating temperature all the time, thus prolonging the life of your engine and ehancing its performance. An easy way to grasp all of this is to think of the oil cooler as being a mini radiator.