Section 04
Buying Guide
A sorted Triumph Stag is a rewarding car. A neglected one will cost you many times its purchase price. The difference is almost always told in the first fifteen minutes: a proper cold start, a hot radiator top and bottom, no bubbles in the expansion tank, no mayonnaise under the filler cap, no rattle from the timing chains, and honest rust in honest places. Do the checks below, in order, before you talk price.
Golden rule: insist on a stone-cold start. Never buy a Stag that was warmed up before you arrived.
Cold-start routine
The single most important half hour of any Stag viewing. Every check below happens with the car started, from cold, in front of you.
01.Arrive to a stone-cold engine
Major- How to check:
- Insist the car has not been run before you arrive. Feel the exhaust manifolds and the top radiator hose — both should be cold to the touch.
- Red flag:
- A warm engine on arrival is the classic way a seller hides a difficult cold start, a smoke on start-up, or an early overheat.
02.Start from cold and idle for at least 15 minutes
Major- How to check:
- Start the car yourself. Let it idle, undisturbed, for a full 15+ minutes. Watch the temperature gauge climb, stabilise, and hold.
- Red flag:
- Gauge climbs past normal and keeps rising, or the seller becomes anxious and wants to shut it off early.
03.Confirm the radiator is hot top AND bottom
Major- How to check:
- Once fully warm, carefully feel the top and bottom radiator hoses. Both should be hot. A cool bottom hose means poor flow through the core.
- Red flag:
- Cool bottom hose — the radiator core is silting up and the car is one traffic jam away from a head-warping overheat.
04.Look into the expansion tank for bubbles
Walk away- How to check:
- With the engine warm and running, look for continuous bubbling in the expansion tank.
- Red flag:
- Steady bubbling is combustion gases in the coolant — a failing head gasket. Walk away or price as an engine job.
05.Listen for the timing-chain rattle on start-up
Major- How to check:
- Stand at the front of the car on a cold start. Listen for a rattle from the front of the engine that fades as oil pressure rises.
- Red flag:
- A rattle that persists once warm, or is loud and metallic on start-up — chains are due or overdue and this is an interference engine.
06.Check under the oil filler cap and on the dipstick
Major- How to check:
- Unscrew the filler cap and inspect the underside. Pull and inspect the dipstick.
- Red flag:
- Creamy beige 'mayonnaise' residue — coolant is emulsifying with oil. Head gasket / warped head territory.
07.Confirm the thermostat has NOT been removed
Major- How to check:
- The engine should take a proper interval to reach operating temperature and then hold there. An engine that reaches temperature very slowly and never quite settles has often had its thermostat removed to mask a deeper cooling problem.
- Red flag:
- Slow warm-up, unstable gauge, and a seller who cheerfully explains that 'thermostats aren't needed on these'. They are.
08.Confirm the oil-pressure warning light is connected
Walk away- How to check:
- With ignition on, engine not running, the oil-pressure light must be lit. It should extinguish within a second or two of starting.
- Red flag:
- Light never illuminates with ignition on — the bulb, or the wire, has been disconnected. On a Stag this is almost never innocent.
Rust
Work front to back and outside to inside. Take a torch, a magnet, and be unhurried. Fresh underseal is a question, not an answer.
01.Front valance and behind the headlights
Moderate- How to check:
- Look at the valance below the front bumper. Reach up behind each headlight bowl and feel for scale and softness.
- Red flag:
- Bubbling paint, filler, or crunchy metal behind the headlights — repairs here are cosmetic-visible and hard to do cheaply.
02.Inner and outer wings
Major- How to check:
- Check the outer wing seams and the arch lips. From the engine bay, look at the inner wings and the top seam.
- Red flag:
- Rot along the inner-wing top seam or arch bubbling; inner wings are structural and expensive to repair properly.
03.Scuttle (bulkhead below windscreen)
Major- How to check:
- Open the bonnet and look at the scuttle panel below the windscreen and where the wings meet it.
- Red flag:
- Rot in the scuttle is a bad find — it is a wet trap and repairs mean glass and dash out.
04.A-posts
Walk away- How to check:
- Inspect the base of each A-post, inside the door aperture and where the post meets the sill.
- Red flag:
- Rusty A-posts compromise the T-bar structure and the door hang — a serious structural repair.
05.Inner and outer sills
Walk away- How to check:
- Look along the outer sill for waves and fresh paint. From underneath, look at the inner sill and the join to the floor. Tap suspect areas.
- Red flag:
- Filler, fresh underseal hiding fresh welding, or the tap goes 'thud' rather than 'tink' — inner sills are structural.
06.Floors
Major- How to check:
- Lift the carpets front and rear. Look at the footwells and under the seats.
- Red flag:
- Perforated or freshly patched floors — check what is underneath them.
07.Boot floor
Major- How to check:
- Lift the boot mat and the spare-wheel cover. Look into the corners and along the rear valance seams.
- Red flag:
- Water staining, rust in the spare-wheel well, or filler around the rear valance — the boot is a rain trap on a leaky car.
Hood & hard-top
Both roofs matter. A Stag with a hard-top only, and no working soft-top in the well, is a materially less valuable car.
01.Lift the hard-top
Moderate- How to check:
- Ask the seller to remove the hard-top (or at least show it can be removed) and inspect where it sits on the body.
- Red flag:
- Hard-top has clearly not been off in years, or is masking rot around the rear screen aperture and rear deck.
02.Confirm a working soft-top is present in the well
Major- How to check:
- With the hard-top off, open the hood well and confirm the soft-top frame and fabric are actually there and operate. Raise and lower it.
- Red flag:
- No soft-top at all, or a torn/seized frame — a Stag without a working hood is worth meaningfully less, and hoods are not cheap.
03.T-bar and roll hoop condition
Major- How to check:
- Inspect the T-bar and hoop for corrosion at its base and for tidy paint/trim.
- Red flag:
- Corrosion where the hoop meets the body is a structural concern on this bodyshell.
Drivetrain
Confirm what the car actually has (manual + overdrive, or Borg-Warner 3-speed auto) and drive it. The rear-end 'twitch' is a known Stag trait with a known fix.
01.Manual + overdrive: confirm the overdrive engages
Moderate- How to check:
- On the road in 3rd and 4th, operate the overdrive switch. You should hear a distinct click and feel the revs drop at a steady throttle. Off again, revs rise.
- Red flag:
- No engagement, or engagement only intermittently. Start with switch/relay/solenoid/oil-level; if those are good, it is an overdrive rebuild.
02.Automatic: Borg-Warner 3-speed behaviour
Moderate- How to check:
- The Borg-Warner 3-speed auto has NO overdrive — the car will feel higher-revving on the motorway than a modern equivalent. Confirm clean shifts up and down under load.
- Red flag:
- Slurred, flared or bangy shifts, or reluctance to kick down — auto rebuilds are specialist and not cheap.
03.Rear-end 'twitch' under power
Moderate- How to check:
- On a quiet road, apply power in a gentle bend and lift. Feel for a distinct lurch or lash from the rear.
- Red flag:
- Pronounced twitch — worn rear half-shafts / UJs. The modern CV-jointed conversion cures it but is a real spend.
Originality
Original Triumph V8 cars command a clear premium over engine-swapped cars. Neither is 'wrong' — but the price should reflect which one you are buying.
01.Confirm the engine is the original Triumph 3.0 V8
Moderate- How to check:
- Check the engine number against the commission plate and the car's paperwork. The Triumph V8 has a distinctive appearance — twin Zenith-Stromberg 175 CDSE carbs, alloy heads, cross-over inlet manifold.
- Red flag:
- Any Rover V8, Ford Essex V6, Buick 215 or other transplant — value implications are significant (see below).
02.If converted, assess quality and reversibility
Major- How to check:
- A well-engineered Rover V8 conversion with cooling, mounts, driveshafts and paperwork correctly done is a usable car. A rough transplant with hacked wiring and mismatched ancillaries is not.
- Red flag:
- Bodged conversion, missing original parts, no paper trail — priced as a project regardless of how it drives on the day.
03.Paperwork, provenance and commission number
Major- How to check:
- Cross-check V5C, commission plate, engine number, service history, previous invoices, and any Stag Owners Club (SOC) history.
- Red flag:
- Mismatched numbers, no history, no invoices for a recently 'restored' car — assume the worst until proven otherwise.
Values (2026, indicative)
The figures below are indicative UK ranges for 2026. They are not a valuation of any specific car. A single major fault — rotten sills, a snapped timing chain, a missing soft-top — can move a car a full band, and a documented history is worth real money against a car without one.
| Condition | Indicative range | Notes |
|---|---|---|
| Concours / fully restored | £25,000–£35,000 | Original Triumph V8, matching numbers, documented restoration, working hood and hard-top. |
| Good usable driver | £15,000–£22,000 | Sorted cooling and chains, tidy body, honest history — the sweet spot for a first-time owner. |
| Project / needs work | £5,000–£12,000 | Running or nearly running, but with rust, deferred engine work, or an incomplete history. |
| Rover / other-V8 conversion | Typically 20–35% below equivalent-condition original | A well-done conversion drives well, but originality drives value on a Stag. |
Indicative only. Cross-check against recent SOC classifieds and auction results before offering.