Cylinder heads & head gaskets: living with the alloy V8

The root cause: two metals, one cooling system

The Stag V8 pairs an iron block with aluminium cylinder heads: without a corrosion-inhibiting coolant kept in good condition, that pairing sets up electrolytic corrosion inside the coolant passages — narrowing them, restricting flow, and letting the heads run hotter than they should. Everything in this article follows from that one fact.

How heads warp

An overheating Stag will, over time, end up with a warped cylinder head — the aluminium moves under heat in a way the design didn't allow much margin for, and the Stag gained a reputation for this happening at low mileages, sometimes as little as around 25,000 miles, on cars whose cooling wasn't kept in proper order. A warped head seats a gasket unevenly, and a blown gasket usually follows.

The skim limit: 0.010 inch, and no more

When a warped head has blown its gasket, skimming (resurfacing) the head face can sometimes rescue it — but only within a firm limit: no more than 0.010 inch (ten thousandths) can safely be removed before the head must be replaced rather than machined. This is often not enough to fully correct a badly warped head, which is why prevention matters so much more than cure on this engine. If a head is skimmed, it must also be checked and straightened first if needed, because machining the face can leave the camshaft bore misaligned if the head wasn't true to begin with — a Stag-specific complication worth knowing about before you agree to a skim.

Seized studs: the other alloy-head problem

Aluminium heads on steel studs set up their own electrochemical reaction over time, and it is common — not exceptional — for head studs to seize solid in the block. Owners doing their own head removal should expect this as a real possibility, not a surprise: some studs come out fine, others snap. The practical response, at reassembly, is to dress the threads and use a good anti-seize compound (copper- or aluminium-based) on the studs before refitting, and — as cheap ongoing insurance — to check and re-torque the head fasteners periodically rather than assume they'll stay put for decades untouched. Confirm the correct torque figure and sequence for your engine against the Workshop Manual or a Stag specialist; don't guess at it.

Symptoms worth acting on

A failing head gasket on any engine tends to show some combination of: a persistently creeping temperature gauge with no obvious external leak, coolant loss you can't account for, a sweet smell from the exhaust, white or "mayonnaise" contamination where oil and coolant have mixed (in the header tank or on the dipstick), or one bank running noticeably hotter than the other. Our cooling-upgrades guide already makes the point that a creeping gauge should never be ignored on a Stag — this is exactly the failure that advice is protecting you from. Catching it early is the difference between a gasket job and a head replacement.

Living with it: what actually prevents this

There is no trick beyond the fundamentals, and the fundamentals work:

• Correct corrosion-inhibiting coolant, kept in condition and flushed annually — see cooling-upgrades for the full picture.
• A properly working thermostat and the right pressure cap, so the system runs at the temperature and pressure it was designed for.
• Don't ignore a creeping gauge or a rattle from the front of the engine (see our timing chains piece — both failures share the same root cause of a neglected, overheating engine, even though the mechanisms are different).
• If you're buying a Stag, run it up to temperature and check both banks read evenly and the radiator is as hot at the bottom as the top — an easy pre-purchase check that can catch a problem before you own it.