Sensible Swap: 1973 Triumph Stag – Sold?

March 10, 2022, Update – We just confirmed the listing for this “Classifind” expired, so with no replacement found we’re assuming this ride “Sold?” While this one got away, please reach out either by email or call us directly if you’d like to be informed when we come across something similar.

Virtually every engine swap ever performed had one goal in mind: more power.  In the case of the rare Triumph Stag, the removal of the car’s original 3.0 Liter V8 in favor of a 2.5 Liter Inline six from a donor TR6 ended up being performed by many enthusiasts for the sale of keeping their Stag roadworthy.  This red-over-black 1973 Triumph Stag originally listed in February 2022 on Craigslist in St. Charles, Illinois (Chicago) features a donor engine from a ’76 TR6 as well as a non-color matching removable hardtop.

Currently offered for $9,500, comparing that price against the Hagerty Insurance Online Valuation Tool confirms the private seller has their Stag priced between this guide’s #4 “Fair” (Daily Driver) estimate of $7,400 and its #3 “Good” appraisal of $14,000.  Interestingly, the  Collector Car Market Review Online Tool reveals the seller’s ask falls between this guide’s #3 “Good” estimate of $9,250 and its #2 “Very Good” appraisal of $14,100.

Under British Leyland ownership, Triumph built the V8-powered, 2+2 Stag convertible from 1970 through 1978.  Envisioned as a luxury sports car, the Triumph developed the Stag to compete directly with the Mercedes-Benz SL class models. All Stags were four-seater convertible coupés, 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 hardtop with defrost wires on the rear window, full headliner, and lever-operated quarter windows was a popular factory option for early Stags that eventually became standard equipment.  Designers developed the Stag with a unitized body featuring fully independent suspension: MacPherson struts in front, semi-trailing arms at the rear. Braking was by front disc and rear drum brakes, while steering was power-assisted rack and pinion.

The initial Stag design was based on the company’s 2.5-Liter inline-six engine, however, Triumph management intended the Stag, large saloons and estate cars to use a new Triumph-designed overhead cam (OHC) 2.5-Liter fuel-injected V8. The V8’s design parameters changed to 3.0 Liters during development to increase torque. To meet emission standards in the US, a key target market, engineers dropped troublesome mechanical fuel injection in favor of dual Zenith-Stromberg 175 CDSE carburetors. 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 sizes 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-liter slant four-cylinder engine used in the later Dolomite and TR7.  The Stag’s V8 was the first of these engines to be fitted to a production car. Sometimes described as two four-cylinder engines sandwiched together, it is more strictly correct to say the later four-cylinder versions were the left half of a Stag engine.

Urban legend has it that British Leyland management urged Triumph the proven all-aluminium Rover V8, originally designed by Buick, however, engineers claimed that engine would not fit.  It can be made to fit the space, but the decision to go with the Triumph V8 was probably more due to the Buick’s different torque characteristics and 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.

Triumph launched the Stag one year late in 1970, to a warm welcome at the various international auto shows. Unfortunately, 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.

The third cause of the 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, 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 it 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 in-house 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.  Consequently, many owners replaced the troublesome engine with units from other cars, such as the Rover V8, or the Triumph 2.5-Liter inline-six engine around which the Stag was originally designed.

Jeremy Clarkson’s modern review of the Triumph featured on the BBC Studios YouTube Channel is not only a great review of the Triumph Stag, but Clarkson also points to the car as a symbol of everything that went wrong with the British automobile industry:

Finding a running example such as this 1973 Triumph Stag in any condition stateside is a rare treat, and the fact this one features a proven TR6-sourced engine a hint of reliability over the original V8.

Here’s the seller’s description:

“Selling my 1973 Triumph Stag MKII, 4-speed manual. Rare car, estimates are that there are only about 1,400 surviving imports of this car. It does not have the original (V8) engine—it was swapped with the mid-70s (1976 I believe) TR6 motor. Car runs and drives well.

Has some issues—speedo doesn’t work, wiper motor needs to be replaced, a small leak in the differential (I have an extra one), needs the softcover restored or replaced. The paint is okay—there are some problem areas, but no rust spots. I have the removable hardtop (despite it being a different color than the car, see photo).

I restored the dash myself, did some interior work, got some electrical issues repaired. It has new rims (I still have the original Stag rims but they aren’t in the best condition) and the tires are in good shape. Overall mechanically sound, runs, drives, no major body/rust/structural issues.

I also have a TON of extra parts for the car. Too much to list. They all come with the vehicle. Asking $9,500 OBO. No calls. I will respond to email or text.“

Do you have a Triumph Stag story to share?  If so, comment below and let us know!