Triumph Stag

1970–1977 · 25,939 built · a Triumph V8, not a Rover one

Triumph’s own V8, used in no other car — and the cooling reputation, explained rather than repeated.

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About the Triumph Stag

The Stag engine is a 2,997cc Triumph V8. It is not a Rover V8. The Rover unit is an all-alloy pushrod 3.5 that came from Buick; the Stag’s is Triumph’s own design with a cast-iron block, aluminium alloy heads and a single overhead camshaft per bank driven by chain, 86 × 64.5mm, twin Zenith-Stromberg carburettors, around 145bhp. Lewis Dawtrey designed it under Spen King, and it went into no other car. It was conceived in 1963 alongside the slant four as one modular family sharing tooling — which is where the “half a Stag engine” shorthand comes from, though the two are related by design rather than interchangeable. Worth checking before you order engine parts: around six per cent of surviving Stags have had a different V8 fitted, more often a Ford Essex than a Rover.

The cooling reputation is real and it has specific causes, which means it has specific fixes. The water pump sits unusually high in the vee, so the system can run with air trapped and the pump uncovered. Coolant without adequate corrosion inhibitor sets up electrolytic corrosion between the iron block and the alloy heads and silts the radiator. Some engines left the factory with casting sand in the coolant passages. Cylinder head studs were mismatched, which distorts heads. And the timing chains stretch and want replacing every 25,000 to 30,000 miles. None of that is terminal — a properly rebuilt and correctly maintained Stag V8 will run past 150,000 miles. The practical answer is corrosion-inhibited antifreeze, a radiator re-core roughly every ten years, and chains on schedule. One other thing: the T-bar and the fixed roll hoop are structural on every Stag, not an option — the car was designed around them. The removable hard top was the option, so check which the car has before quoting hard-top parts.

Triumph Stag — specification & changes within the model

Built: 1970 – June 1977, 25,939 built. Final chassis LD45722.
Engine: 2,997cc TRIUMPH V8 — not a Rover V8. Cast-iron block, aluminium alloy heads, single overhead camshaft per bank, chain driven, 86 × 64.5mm, twin Zenith-Stromberg. Around 145bhp. Used in no other car.
Not the Rover V8: The Rover unit is an all-alloy pushrod 3.5 litre from Buick. Different engine entirely. About 6 per cent of surviving Stags have had a non-Triumph V8 fitted — more often a Ford Essex than a Rover. Check under the bonnet.
The cooling story: The water pump sits high in the vee, so the system can run with air trapped and the pump uncovered · coolant without corrosion inhibitor causes electrolytic corrosion between iron block and alloy heads, silting the radiator · casting sand left in some coolant passages · mismatched head studs distorting heads · timing chains stretch and want replacing every 25,000–30,000 miles.
What actually fixes it: Corrosion-inhibited antifreeze, a radiator re-core or replacement roughly every ten years, and chains on schedule. A properly rebuilt and maintained Stag V8 will run past 150,000 miles.
The T-bar: Structural on every Stag — the fixed B-pillar roll hoop joined to the windscreen frame. Not an option; the car was designed around it.
Hard top: A factory option, body-colour, with heated rear screen, full headlining and lever-operated quarter windows. Some cars have both hood and hard top, some only one — check before quoting hard-top parts.
Transmission: Four-speed manual with Laycock overdrive, optional then standard from October 1972, or Borg-Warner Type 35 three-speed automatic, later Type 65.
Suspension: MacPherson struts at the front, semi-trailing arm independent rear.
Relation to the slant four: Conceived in 1963 as one modular family sharing tooling — an inline four and a V8 on common lines. Related by design, not interchangeable. The slant four was in a Saab before it was in a Triumph.