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The K Series engine is maligned for head gasket failure, yet this is almost invariably the effect rather than the cause of a problem - gasket failure is the result of the engine being overheated.
(Exception: early engines had plastic dowels between the head and block which were prone to failure; later engines/replacement gaskets have steel dowels) On many occasions, head gaskets have needlessly been replaced because the mechanic (or owner) has leaped to the wrong conclusion because of hearsay.
The coolant level on the MGF is critical. If it gets too low, (and it doesn't have to be all that low) then an air lock develops which stops the coolant circulating through the radiator - the engine can be imitating a kettle, pushing coolant out everywhere yet the radiator remains stone cold as no coolant is going through
it. The pressurising in such cases is simply good old fashioned boiling. Heaps of pressure, coolant coming out everywhere, but just driven by boiling rather than compression pressure.
A further common cause of MGF overheating is the thermostat failing to open, either because of failure or because of the marginal recirculation system which keeps it open. Its position at the intake to the water pump means that it has cooled coolant from the radiator flowing over it, so it is very reliant on this bypass flow. For a reason I have yet to fully understand, this seems to go on the blink in a few cases.
If overheating is caught soon enough, no damage occurs - simply top up, bleed the air out of the system (most important) and off you
go. We've done this successfully to a few cars now which we were confident had not gone past the point of no return in coolant loss/temperature rise and have monitored the resulting job over a significant period time. No further problems have evolved.
Unfortunately most people don't notice the high temperature in time, not the least because the engine is behind you so there is no telltale trail of coolant coming out of the bonnet onto the windscreen. The result is all/most of the coolant get boiled off, and the head overheats and softens, leading to indentations from the fire ring and a loss of clamping pressure. No headgasket can cope with a collapsing head.
The give-away is the fact that the indentation is worst round the exhaust valve, where it is hottest. If you want to confirm, get the hardness of the head checked. Starts off Brinell 100+,
serviceable above 90, (though the gasket makers say 75). Soft heads can be down in the 50 range in the worst spots, possibly lower.
We have checked out quite a number of heads now, and have yet to find fire ring indentations on any head still retaining its correct hardness, regardless of
mileage. The softer the area of the head is, the deeper the indentation goes. And the only thing that causes the softness is serious overheating.
Giving the car a good thrashing round a track won't cause a problem so long as the cooling
system works properly and carts the heat away as it should - the villain is coolant loss. Without coolant, nothing stops the head from reaching seriously unpleasant temperatures.
We have successfully re-hardened such heads here, the guinea pig head being the current one on our K Midget which has done a couple of years really hard graft with no ill effects. (There was nothing wrong with the old head, just wanted a good test bed to verify if we could sell such a process to our customers with confidence, and the K Midget works harder and is easier to
work on than our F)
A number of split hoses are triggered by overheating arising from gradual cooling loss from a slow leak, leading to air lock, circulation failure, boiling, excess pressure and thus a tired hose gives up.
However a temperature gauge which shows normal when the engine is boiling indicates sudden coolant loss, as the gauge monitors the coolant temp. With the coolant gone, it has to wait for conducted heat from the head, and there is usually a lag in this showing up. This is a strong indication that the failed hose caused the overheating rather than vice-versa. The gauge could of course be faulty, but the lag in reading is very characteristic of sudden coolant loss.
(I might be showing my age, but the first time I experienced this was when I was stranded in the dark out in the countryside in my old Morris Oxford van when the heater suddenly when cold while the temp gauge still went normal ... )
As I noted before I've seen high mileage heads which show no problem, and every head I've seen with the problem has been hot at some stage. A lesser degree of cook-up seems to result in the damage taking longer to appear, but it's always instant when the overheating is major.
A good guide as to how hot the head got is to check the inner cam belt cover - if it has melted (and I've seen that on a quite a few!) then the head got seriously hot.
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