Envisioned as a luxury sports car, the Stag was designed to compete directly with the Mercedes-Benz SL class models. All Stags were four-seater convertiblecoupés, but for structural rigidity – and to meet proposed American rollover standards of the time – the Stag required a B-pillar "roll bar" hoop connected to the windscreen frame by a T-bar. A body color removable hard top with defrost wires on the rear window, full headliner and lever operated quarter windows was a popular factory option for the early Stags, and was later supplied as a standard fitment.
The car started as a styling experiment cut and shaped from a 1963–64 pre-production2000 saloon, also styled by Giovanni Michelotti. His agreement was, if Harry Webster, Director of Engineering at Triumph, liked the design, Triumph could use the prototype as the basis of a new model. Webster loved the design and took the prototype back to England. The end result, a two-door drop head (convertible), had little in common with the styling of its progenitor 2000, but retained the suspension and drive line. Triumph liked the Michelotti design so much that they propagated the styling lines of the Stag into the new T2000/T2500 saloon and estate model lines of the 1970s.
Triumph gave new projects four letter development code names (e.g. Bomb for Spitfire) and interestingly, Stag was the only Triumph to take its development code name into production.
The initial Stag design was based around the saloon's 2.5-litre six cylinder engine, but Webster intended the Stag, large saloons and estate cars to use a new Triumph-designed overhead cam (OHC) 2.5-litre fuel injected (PI) V8. In 1968, under the direction of Webster's successor, Spen King, the new 2.5 PI V8 was enlarged to 2,997 cc (3 L; 183 cu in) to increase torque. To meet emission standards in the US, a key target market, the troublesome mechanical fuel injection was dropped in favour of dual Zenith-Stromberg 175 CDSE carburettors. In common with several other manufacturers, a key aim of Triumph's engineering strategy at the time was to create a family of in-line and V engines of different size around a common crankshaft. The various configurations Triumph envisaged would enable the production of four-, six-, and eight-cylinder power plants of capacity between 1.5 and 4 litres, sharing many parts, and hence offering economies of manufacturing scale and of mechanic training. A number of iterations of Triumph's design went into production, notably a 2.0-litre slant four-cylinder engine used in the later Dolomite and TR7, and a variant manufactured by StanPart that was initially used in the Saab 99. The Stag's V8 was the first of these engines to be fitted to a production car. Sometimes described as two four-cylinder engines siamesed together, it is more strictly correct to say the later four-cylinder versions were the left half of a Stag engine.
It has sometimes been alleged Triumph were instructed to use the proven all-aluminium Rover V8, originally designed by Buick, but claimed it would not fit. It is unclear if Triumph meant the Rover engine would not physically fit inside a Stag's engine bay. It can in fact be made to fit the space, but the decision to go with the Triumph V8 was probably driven more by the fact that the Buick's different torque characteristics and physical weight would have entailed substantial re-engineering of the Stag when it was almost ready to go on sale. Such a substitution would also have required a rethinking of the wider engineering strategy, both of which were important "fit" considerations beyond the comparatively trivial matter of the relative dimensions of the two engines. Furthermore, Rover, also owned by British Leyland, could not necessarily have supplied the numbers of V8 engines required to match the anticipated production of the Stag anyway.
The car was launched one year late in 1970, to a warm welcome at the various international auto shows. The Stag rapidly acquired a reputation for mechanical unreliability, usually in the form of overheating. These problems arose from a variety of causes.
First, the late changes to the engine gave rise to design features that were questionable from an engineering perspective. For example, the water pump was set higher on the engine than is usual. If the engine became hot in traffic, and coolant escaped from the cooling system via the expansion bottle, the reduced volume of fluid left when the engine cooled down again fell below the level of the pump, which would eventually fail as a result. Water pump failures sometimes occurred due to poorly-hardened drive gears, which wore out prematurely and stopped the water pump.
A second cause of engine trouble was the lack of attention to corrosion inhibitor in the coolant. The block was made from iron and the heads from aluminium, a combination that required the use of corrosion-inhibiting antifreeze all year round. This point was not widely appreciated by owners or by the dealer network supporting them. Consequently, engines were affected by electrolytic corrosion and white alloy oxide sludge collected in radiator cores, reducing radiator efficiency and causing overheating. The result was head gasket failure due to cylinder head heat distortion, a very expensive repair. Owners would usually get their repaired cars back with the radiator still clogged, leading to repeat failures.
A third cause of trouble was the engine's use of long, simplex roller link chains, which would first stretch and then often fail inside fewer than 25,000 miles (40,200 km), resulting in expensive damage. Even before failing, a stretched timing chain would skip links and cause valves to lift and fall in the wrong sequence, so that valves hit pistons and damaged both. This fault may have been worsened by poor quality chains.
Another problem with the cylinder heads was said to be the arrangement of cylinder head fixing studs, half of which were vertical and the other half at an angle. Anecdotally, this arrangement was used to reduce production costs, as the cylinder head mounting studs and bolt were all accessible with the rocker covers fitted. This allowed the factory to assemble the cylinder head completely before fitting to the engine. The same arrangement worked well enough on the 4-cylinder engines, but in the V8 the angled and vertical studs, when heated and cooled, expanded and contracted in different directions sufficiently to give rise to sideways forces that caused warping of the engine block.
Finally, although pre-production engines cast by an outside foundry performed well, those fitted to production cars were made inhouse by a plant troubled with industrial unrest and inadequate quality control. Engines are still being discovered with casting sand inside, blocking the coolant passages and causing overheating.
This combination of manufacturing and maintenance flaws caused a large number of engine failures. Time magazine rated the Triumph Stag as one of the 50 worst cars ever made.
British Leyland never materially reengineered the Triumph 3.0 litre OHC V8 to address these issues, other than introducing a high-pressure cooling system that boiled over at a higher temperature. Another problem was that the Stag was always a relatively rare car. British Leyland had around 2,500 UK dealers when the Stag was on sale and a total of around 19,000 were sold in the UK over seven years. Thus the average dealer sold only seven or eight Stags during the car's whole production run, or roughly one car per year. This meant that few dealers saw defective Stags often enough to recognise and diagnose the cause of the various problems.
A number of owners replaced the troublesome engine with units from other cars, such as the Rover V8, or the Triumph 2.5-litre engine around which the Stag was originally designed. The number of such conversions undertaken is not known, but as at July 2017, 91% of Stags known to DVLA had a 3-litre engine, according to www.howmanyleft.com. It is not clear how many of these are original Stag engines and how many are Ford 3-litre Essex units. The once-popular Rover V8 conversion powers fewer than 4% of surviving Stags.
Perhaps thanks to the Stag engine's reputation for unreliability, only 25,877 cars were produced between 1970 and 1977. Of this number, 6,780 were export models, of which 2,871 went to the United States. Several variants were produced, indicated by the factory as initial production, followed by 1st, 2nd, 3rd and 4th "sanction" changes, as noted only in changes of the production numbering sequences. These have become unofficially designated as "Early" Mark I 1970, Mark I (1971–72/3), Mark II (1973) and "Late" Mark II or Mark III (1974–77).
The most notable differentiating features between Mark I and Mark II Stags are the sills and tail panel, which are body-colour on the earlier car and low-gloss black on the later. The addition of twin coachlines is an indication of a Mk II variant. Inside, early cars had a slightly different warning light cluster and the instrument dial needles point down rather than up. They also had two courtesy lights at the top of the B posts rather than the later configuration of one in the T-Bar, and also had a map-reading light fitted to the door of the glove box, whereas later cars had a light inside. Very early production cars had a three-quarter window in the soft top, which was quickly deleted as it tended to become trapped and then to split when stowed. Mark III cars returned to having body-colour sills and tail panel, but some acquired a stainless steel sill cover. This item was originally fitted to US Stags, and the surplus stock of these that had accumulated when the Stag was withdrawn from the US was used up on cars for other markets. Stags thus fitted are usually considered Mark III Stags, though it is not clear that all had them, and many early Stags have since had them retrofitted. Mechanically, late Stags fitted with the slightly longer BW65 automatic transmission had a correspondingly shorter propshaft to compensate. A high-pressure cooling system was introduced and fitted to later Mark II Stags.
Cars for export markets such as the USA comprised unique combinations of features specifically required for compliance with various states' requirements, or simply for marketing purposes. Cars factory-designated as "Federal specification" included features such as vinyl-covered hard tops, Federal Department of Transportation compliant lighting, and a wide range of anti-smog emissions changes not found on other market vehicles.
Whilst official Triumph parts manuals may differentiate variants by commission plate ranges, owners often find that minor parts for the old variant are present in early production examples of the new variant, possibly due to the manufacturer making use of existing stocks of parts. For example, Mark II cars have been known to have Mark II wiring looms or door latches, and certain combinations of body and trim colour persisted past after they were supposed to have been withdrawn.
The majority of cars were fitted with a Borg-Warner Type 35 3-speed automatic transmission, which on the last few vehicles produced gave way to a Type 65. The other choice was a derivative of the ancient Triumph TR2 manual gearbox which had been modified and improved over the years for use in the TR4/A/IRS/TR5/250/6. The first gear ratio was raised and needle roller bearings were used in place of the bronze bushings on the layshaft. Early 4-speed manual transmission models could be ordered with an A-type Laycock overdrive unit and later ones frequently came with a J-type Laycock unit. The overdrive option is highly desirable as the engine RPM drops significantly with this option in 3rd and 4th (top) gears.
Other than the choice of transmissions, there were very few factory-installed options. On early cars, buyers could choose to have the car fitted with just the soft-top, just the hard-top (with the hood stowage compartment empty), or with both. Later cars were supplied with both roofs. Three wheel styles were offered. The standard fitment was steel wheels with Rostyle "tin-plate" trims. The wheels are secured with the usual four bolts, but the Rostyle trims have five false bolts. Five-spoke alloy wheels were an option, as were a set of traditional steel spoke wheels with "knock-off"' hubcaps. The latter were more commonly found on Stags sold in North America on Federal specification vehicles.
Electric windows, power steering and power-assisted brakes were standard. Options included Delaney Galley air conditioning, a luggage rack, uprated Koni shock absorbers, floor mats, and Lucas Square Eight fog lamps. A range of aftermarket products, most of which were dealer-installed as optional accessories could also be fitted. Leather upholstery was also a listed option, but its actual existence is problematic as no surviving car is known to have original factory leather. Rather unusually for a 4-seat touring car, the accessory list included a sump protector plate that was never produced. This was probably included as a slightly "gimmicky" tribute to Triumph's rallying successes.
The Stag was never directly replaced. British Leyland planned an equivalent model to follow the Stag in the form of a derivative of the Triumph TR7 sports car which was codenamed the Lynx. The Lynx used the TR7 platform with an extra 12 inches in the wheelbase to accommodate a rear seat and had fastback coupe bodywork. Power came from a 3.5-litre Rover V8 and the gearbox and rear axle were lifted from the Rover SD1. The Lynx was very close to production being scheduled for launch in 1978. However the sudden closure of the Triumph factory in Speke, Liverpool where the car was to be built and new policies implemented by BL's new chief executive, Michael Edwardes, led to the Lynx's cancellation.
Triumph planned a coupé version of the Stag to complement the open-top tourer, in the same manner as the smaller GT6 coupé was based on the Spitfire. In 1970 Michelotti converted two of the prototype Stag shells into two-door coupés with very clear styling cues from the GT6, including the shape of the rear windows and roofline and the provision of air vent 'gills' in the C-Pillar. In 1971 Triumph built a one-off 'production' Stag coupé, called the Fastback, to Michelotti's design with minor detail differences to match the production Stag more closely. Although the design was considered successful and "more useful than an ordinary Stag", British Leyland did not continue with the project, reportedly because they feared the Stag fastback would take sales from other actual and planned vehicles in the BL range. The prototype survives.
A number of Stags were built with four-wheel drive using the Ferguson Formula developed by Ferguson Research and pioneered on the Jensen FF. One Stag was fitted with the FF system by Triumph itself during development but was either scrapped or converted back to standard specification. Two more 4WD Stags were built in 1972 by FF Developments, a company separate from Ferguson that had licensed the technology for converting existing road cars. The cars (one with manual transmission, one with automatic) were commissioned by GKN for development and testing work. Both had automatic locking differentials actuated by a viscous coupling and the same Dunlop Maxaret mechanical anti-lock braking system as used in the Jensen. The cars differed visually from standard by having a broad bulge in the centre of the bonnet; the engine had to be mounted slightly higher in the engine bay to accommodate the drive to the front wheels. Both these cars survive.
A lightweight Stag was tested which was essentially a standard Stag with most of the sound-deadening material removed. This did not proceed. A 32-valve Stag was rumoured to have been built and tested, featuring Dolomite Sprint 16-valve heads, but no documentary or other trace of any such actual vehicle having been built has come to light and it is unclear whether this was an actual test vehicle or simply a proposal. Any such engine would have required mirror-image cylinder heads to be made, as otherwise the camshaft would have been at the wrong end of the engine.
FF Developments also converted a number of Triumph 2000s (saloons and estates) to 4WD, including at least one 'Triumph 3000 estate' which received a Stag engine and gearbox as well as the Ferguson 4WD drivetrain.
The Triumph Stag has sizeable club and owner support and a number of specialist suppliers. According to the main enthusiast club in the UK, approximately 9,000 Stags are believed to survive in the United Kingdom. According to DVLA data, in 2018 just over 7,000 survive in the UK either taxed or under SORN