Question 1

When the head gasket needs replacing

Accepted Answer

The head gasket is a metal-elastomer structure clamped between the cylinder block and cylinder head that seals the combustion chambers, coolant passages and oil passages from each other. When the gasket fails, combustion gases enter the cooling system (bubbles appear in the expansion tank with the engine running), coolant enters the combustion chamber (copious white vapour from the exhaust), or — the worst case — oil and coolant mix and the dipstick shows a light-brown emulsion. The engine must not be started in that state, because lubrication capability has collapsed.

The most common causes are repeated overheating (leaking radiator, faulty thermostat, coolant loss), cracks in the cylinder head from earlier damage, or a poorly executed performance tune that has raised boost and combustion pressures beyond the gasket's tolerance. Catching the problem early saves thousands — a delayed repair can melt the crankshaft bearings and force a full engine swap.

Question 2

How a head gasket is replaced

Accepted Answer

The engine is stripped down in stages: the timing belt or chain is unlocked with reference marks, the rocker cover is removed and the cylinder head bolts are undone in the prescribed order (usually a spiral from the centre outwards). The head is lifted off and both its surface and the block's are cleaned of old sealant material. The cylinder head's surface flatness is measured with a straightedge — if the warp exceeds the manufacturer's tolerance (usually 0.05 mm), the head is sent out for machining. At the same time the valve seats, valve guides and camshaft bearings are inspected.

The new gasket is always fitted on dry surfaces the right way up — on modern gaskets there is a TOP marking and manufacturer part number. Head bolts are typically single-use (torque-to-yield) and always renewed. The bolts are tightened in three or four stages using a torque-angle gauge to the manufacturer's specified values. The engine is reassembled, the cooling system is filled and bled, and the engine is run without load for about 20 minutes to stabilise temperature. During the test drive coolant temperature, the pressure tank and exhaust colour are monitored.

Question 3

Why do head gaskets fail?

Accepted Answer

Overheating is by far the most common cause of gasket failure. The metal-elastomer structure tolerates only so much heat, and a single serious temperature spike can soften the gasket enough that it leaks after just a few kilometres. Gasket failure is often the result of a longer-term problem, though — for example, a slowly leaking radiator whose coolant loss means the engine constantly runs hotter than normal. A third common cause is a cracked cylinder head: internal cracks in aluminium heads aren't necessarily visible from outside, but they allow combustion gas to penetrate coolant passages. That's why head straightness measurement and pressure testing for cracks are always part of the repair — otherwise the fault recurs soon after a new gasket is fitted.

Head Gasket Replacement in Vantaa