Section 02
The V8 Engine
Triumph's own 2,997 cc 90° V8 defines the Stag. It is a smooth, oversquare, single-cam-per-bank engine that gives 145 bhp and a distinctive off-beat exhaust note. It is also the reason the car earned its early reputation for unreliability — and, sorted properly, the reason a well-kept Stag will now cover 150,000 miles without drama.
What it is
A Triumph-designed 90° V8 with a cast-iron block and aluminium-alloy heads, one overhead camshaft per bank, twin Zenith-Stromberg carburettors and a distinctly oversquare bore-to-stroke ratio.
| Specification | Detail |
|---|---|
| Configuration | 90° V8, single overhead camshaft per bank |
| Capacity | 2,997 cc |
| Bore × stroke | 86 × 64.5 mm (oversquare) |
| Block / heads | Cast-iron block, aluminium-alloy heads |
| Compression ratio | 8.8 : 1 |
| Fuelling | Twin Zenith-Stromberg 175 CDSE carburettors |
| Power | 145 bhp @ 5,500 rpm |
| Torque | 170 lb·ft @ 3,500 rpm |
| Valvetrain | SOHC per bank, chain-driven (single-row) |
| Character | Interference engine |
The slant-four family — a common myth, corrected
It is often repeated that the Stag V8 is “two Dolomite fours welded together.” The chronology is the wrong way around.
The Stag V8 came first as a complete engine in its own right. The Triumph slant-four that later appeared in the Dolomite and the TR7 is, in effect, half of the Stag V8 — one bank of it, developed into a standalone four-cylinder. The relationship is real, but it runs from the V8 down to the four, not from the four up to the V8.
Why it earned its reputation
The V8's problems are well documented and, individually, undramatic. It is the combination — a marginal cooling system feeding a dissimilar-metal engine with single-row chains and a temperature gauge that reads only one bank — that gave the Stag its early reputation. Each fault below is treated on its own so owners can cite them individually.
Cooling & electrolytic corrosion
The alloy heads and iron block form a dissimilar-metal pair. With neglected coolant, electrolysis attacks the aluminium — waterways corrode, deposits build, and the cooling system's already-slim margin disappears. Most Stag engine failures ultimately trace back to a cooling system that was allowed to run tired.
Casting sand in the waterways
Early blocks left the foundry with casting sand still trapped in the water jackets. Over time it migrates, blocks passages and encourages hot-spots — particularly around the rear cylinders. Modern rebuilds flush and reverse-flush the block to clear what the factory did not.
Warped alloy heads
Once the cooling system loses control, the alloy heads warp. They can be skimmed, but the accepted limit is 0.010″; beyond that the head is scrap. Many surviving heads have already been skimmed at least once, which matters when planning a rebuild.
The two-angle head-stud blunder & seized studs
The head studs were specified with a shallow change of angle along their length — a manufacturing detail that made them prone to seizing in the block. Removing seized studs without shearing them is one of the classic Stag workshop nightmares, and one of the reasons an engine-out job can escalate quickly.
The single-head temperature sender
The factory temperature sender reads from one cylinder head only. If the other bank overheats — a real possibility given the marginal cooling — the driver sees a normal gauge until damage is done. The standard modern remedy is a second sender feeding either a switch or a second gauge.
Water-pump cavitation
At sustained high rpm the original water pump can cavitate, momentarily losing flow just when the engine most needs it. Combined with the cooling system's other weaknesses, this contributed to the engine's early reputation for boiling in traffic and cooking itself on motorways.
Single-row timing chains on an interference engine
The V8 uses single-row chains to drive its two overhead camshafts. They stretch with age and mileage, and because the engine is interference, a snapped or jumped chain means bent valves and, often, a rebuild. Timing chains are a service item on a Stag, not a fit-and-forget.
Undersized main bearings
The bottom end was specified with main bearings on the small side for a 3-litre V8. In a well-maintained engine this is not the failure point owners worry about — but it is one of the reasons the standard engine does not tolerate abuse, overheating or long-term oil neglect.
How it's fixed today
Every one of the faults above has a well-understood remedy, and the Stag community has been refining the package for decades. A modern-fixes rebuild typically addresses cooling first: a properly flushed block, an uprated radiator core, a re-cored or improved water pump, an electric fan, and — critically — a second temperature sender so both cylinder heads are monitored. Head studs are replaced with better material and correctly torqued; chains and tensioners are renewed; the cooling system is bled properly and kept on fresh inhibited coolant.
The bottom line owners now quote is straightforward: a correctly sorted Triumph V8 will do around 150,000 miles and is reliable. The engine's bad name belongs to the 1970s car, not to the well-sorted 2020s car.
Timing chains
≈ £600
Indicative cost for chains & tensioners done properly.
Full engine rebuild
£6,000+
Indicative starting figure for a specialist rebuild.
Figures are indicative only and vary considerably with specialist, specification and the condition of the core engine.
Frequently asked
Is the Triumph Stag engine really that bad?
No — but its reputation is earned. As delivered in 1970 the V8 had a marginal cooling system, casting-sand contamination, seizing head studs and a single temperature sender that could hide a hot bank. Left standard and neglected, it fails. Sorted with the well-understood modern fixes and looked after, it is a smooth, willing and reliable engine.
How often do Stag timing chains need changing?
The V8 drives its two overhead camshafts by single-row chains, and because the engine is interference a snapped or jumped chain will bend valves. Owners treat the chains as a service item rather than a fit-and-forget component, and budget for replacement — indicatively around £600 — well before symptoms appear. Chain condition should form part of any pre-purchase inspection.
Why do Stag heads warp?
The alloy heads sit on an iron block with a cooling system that was marginal from new. Once coolant is neglected, electrolytic corrosion attacks the aluminium and hot-spots form; the heads then warp. They can be skimmed, but only to a limit of 0.010″. Beyond that, the head is scrap. Cooling discipline is the single most important thing a Stag owner can offer the engine.
What coolant should a Stag use?
Because the V8 pairs alloy heads with an iron block, coolant is not optional and is not a top-up-with-water job. Owners use a good-quality inhibited antifreeze mix year-round, changed on schedule, to keep the corrosion inhibitors live. Fresh coolant is the cheapest single thing anyone can do to protect a Stag engine from the failure mode that made its reputation.