Am i really going to type all this lot out again? Well, maybe a summary any way.
HGF affects, as has been presumed earlier in the thread mainly 1.6, but primarily 1.8 K engines - there are literally dozens of potential causes, but it is worst by far in the MGF and there are a number of reasons for this too.
The head gasket fails as a consequence of another problem - it does not go on it's own, it is therefore critical that when replaced, the original cause is found and dealt with to prevent a repeat failure.
Known causes (list incomplete as i'm working off the top of my head):
1) Differential thermal expansion of (oil cooled) block vs (water cooled) head.
Water coolant temperature is variable due to overenthusiastic opperation of thermostat. Oil temp stays fairly constant once up to temperature. Therefore block expands when hot and stays constant, head expands and contracts in a cyclical pattern due to thermostat opening and closing. Worse on an F due to long coolant run between radiator and engine, therefore coolant temp is a lot colder (additional cooling under the car).
This was largely cured by swapping plastic dowells in the head for steel ones - they basically help reduce movement (but do not stop it). The uprated gasket also contributes to this cure, the rubber bead is now pinned through the gasket as opposed to simply being stuck on - this helps to keep the bead in place, but again isn't 100% effective. TF has a revised thermostat and the 160 version also has an oil cooler - these are attempts to stabilise the problem and work well.
This is the main reason in my opinion, this opinion can be bourne out by the statistics on the Hall Of Shame website. This site has been recording MGF/TF HGF incedents since 1996 and runs a statistical analysis on them - there is a marked drop off in occurences post 2000, i.e. when the new gasket and dowells were introduced.
Hall of Shame can be found >> here <<
for those who are interested. To date, this site remains the only
free to all data on the subject. The data if flawed as the demographic it covers is quite small (MGF/TF owning PC/internet users who have had HGF and found the site and were bothered to fill it in!) But there is enough data there to begin to see some trends.
2 ) Water leaks.
Numerous sources of leakage, failed radiator expansion tank cap pressure release valve, failed hose, failed hose spring clip (these are totally rubbish and go all the time), rusty radiator (MGF rads have a 5 year life span before corrosion becomes an issue), sheared radiator bleed screw (is plastic and if overtightened by a grease monkey will shear resulting in a leak), on an F, corroded underbody coolant pipes.
Any leak will result in air being drawn into the system causing localised boiling and hot spots on the gasket.
3 ) Low capacity, presurised coolant system.
This is linked to the leaks, once pressure is lost the boiling temperature of the coolant is lowered, because the system is low capacity it doesn't take long for a large amount of coolant (poportionately) to be boiled off and the gasket then fails
4 ) Anti-corrosion inhibitors.
Coolant doesn't just contain anti-freeze, it also contains anti-corrosion inhibitors. These also break down over time, some people have said that they break down quicker than the anti-freeze elements (typically 15-18 month lifespan apparently). This means that if you miss a coolant change, or if the muppet dealership simply tops up the coolant rather than replace it, your engine is unprotected.
Why is this a problem?
Well, the block and head are alloy and the gasket is steel - two materials that don't really get on. The only thing preventing eletrolytic corrosion is the inhibitors in the coolant - QED.
Personally, i blame sloppy dealerships for this one, if the schedule is followed properly then it shouldn't be an issue. Likewise, not bleeding the system properly will also cause HGF and also overtightening the rad bleed screw - muppet dealerships i feel are responsible for at least a third of all HGFs in my opinion.
OK, so why does the MGF suffer more than any other car?
Well, firstly because it was the first car to get the 1.8K engine, the engine was developed for the F and then spun off onto other platforms later. Secondly, the mid-engined layout means the coolant systems is inherrantly larger and more complicated than on a front engined car (radiator at the front, heater matrix at the front of the cabin, engine in the rear). And lastly, although this is not a cause per se, if you get HGF in an F/TF/Elise, the steam is behind you and is fairly easy to miss (don't rely on the temp guage, it is so damped as to be almost useless, buy the time the needle starts moving most of the coolant is gone!) HGF on a front engined car is reasonably obvious, the stem is right in the vision of the driver and therefore the car is normally stopped fairly quickly before any serious damage is done. HGF on an F tends to go unnoticed for a while, thus the coolant boils away and the result is a new engine £££££££ kerching! ££££££££
There's loads i've missed, but the main points are here give or take. HGF is an issue, but it is no where near as bad as it was and the gasket itself is not the problem, it is a myriad of other things that result in HGF.