Overheating and head gasket failure. What goes on?

TrickyWicky

Hi all

I think many of us know that overheating can cause head gasket failure but I'm wondering exactly what happens in this process?

Having pulled out a failed gasket moons ago from a renault I remember there being rotten bits that had rotted away allowing the oil and coolant to mix etc but there was no sign of melting etc.

I had a theory recently though and I'd like to find out if I'm right. Heat makes metal expand / grow right? So I'm thinking that when an engine overheats the head bolts stretch slightly allowing oil and/or coolant to breach the slightly decompressed gasket. Presumably this then allows fluid to continue seeping through even after the engine has cooled down to normal levels again?

Any experienced mechies care to comment?

Reason I ask is that I have a 'bomb proof' volvo lump that I over heated recently. It's knocked itself out with an electrical glitch but I'm wondering if it would be better to replace the lump or just try running it.

colino

If a car has an aluminium head, and most have, and its been overheated, as well as the partly sacrificial head gasket acting as a blow off valve, the head itself can deform quite significantly, and while we talk about a head bending like a banana, its never that bad. Aluminium is used because it is a great thermal conductor, is easy to machine and it's light, but when things go wrong, the fact that it expands at more than one and a half times the rate of cast iron, you get problems. In an overheat situation, you can imagine the issues the poor hg is facing, separating but sealing these two dissimilar metals and being squeezed between both by banks of mild steel head bolts. Good engineering dictates that the problem after heat failure can be resolved. However with modern engineering tolerances there frequently isn't enough metal on the head to take enough of a skim to make it tolerably flat and some you can't get thicker or double head gaskets for anyway and the compression ratio would be a future problem.
A lot of cars with hgf are binned because of the apocryphal tales of them never being right again (K Series for example) and it is really down to the repairer using inferior parts, not cleaning down well enough, failing to fix the original cause etc., etc.
Head bolts form their clamping action by being torqued correctly and while some are truly supplied as stretch bolts and shouldn't be reused, most can tolerate a far wider heat range than the head itself and would take a lot to degrade significantly.
In short, Volvos, even with their ally heads, tend, up until the hybrids, to be properly engineered and after checking the head with nothing more scientific than a good straight edge (a decent metal ruler) a torch and a set of feeler gauges, a decent diyer can check across all orientations, that the head is acceptably flat or not. Even then, there is usually enough metal to take a light skim and build it back up with a new gasket set. As ever though, find out why it had hgf and fix that. Normally hg deterioration is just the pricey result of a fault elsewhere.

forgotmyname

If that was the case the overheating may have been the symptom on the headgasket corroding away.

Lack of antifreeze or regular changes. etc cause corrosion.

When an engine overheats it expands and then cools at an uneven rate depending on what caused the overheat. Thats what causes the gasket to fail also.
When you inspect the head in minute detail it wont be flat anymore. Banana shape.

Common cause is lack of maintenance and a low water level. Then it gets hot and overheats and people pour lovely cold water into the engine.
As the water enters on one side and not evenly it hits the hot metal rapidly cooling that spot.

Richard53

Can't add much to what colino says above, except to say that the head bolts are designed with exactly this situation in mind The elasticity of the studs/bolts and the torque setting of the nuts will allow for any reasonable amount of overheating without allowing the HG to breach. Failure is usually a result of the head itself warping due to heat, and the HG cannot seal a gap which has become much wider than it was designed for. Until the head is skimmed flat, the HG will keep leaking.