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 1970's.
It has been alleged that internal politics meant that Triumph intended, but were unable, to use the proven but old technology General Motors designed all aluminium Rover V8. As we know today, the no-fit story is probably a myth. Rover, also owned by British Leyland, simply could not supply the numbers of aluminium V8 engines to match the anticipated production of the Stag. We know that "brand loyalty" between Triumph and Rover was high between these former rivals. Besides, Harry Webster had already started development and testing of a new unique, all Triumph designed overhead cam (OHC) 2.5 litre fuel injected (PI) V8 to be used in the Stag, large saloons and estate cars.
The vision was to allow Triumph to compete in the highly desired V8 marketplace. Under the direction of Harry's replacement, Spen King in 1968, the new Triumph OHC 2.5 PI V8 was enlarged to 2,997 cc (3.0 litre) to increase torque and the troublesome fuel injection dropped in favour of dual Zenith-Stromberg 175 CDSE carburetors to meet emission standards of the target market - USA. The Triumph Slant-4 engine shared the same basic design as the Triumph V8, consisting of a single overhead cam cast iron block with aluminium heads. However the cylinder heads of the two engines do not share the same footprint on their respective engine blocks. This same engine manufactured by StanPart was initially used in the Saab 99. Using a gear driven water pump, the Slant 4 could be easily installed in a front wheel drive car. This same water pump design was used in the Stag V8.
The car was launched one year late in 1970, to a fairly warm welcome at the various international auto shows, which soon turned sour after delivery to the market with reports of engine problems. Some of these were due to the perennial problem of poor build quality, endemic to the British motor industry of the time, while others related to what some feel were the fatal design problems in the engine. These included:
- long simplex roller link chains combined with inedaquate engine maintenance and factory specified 7,500-mile (12,070 km) oil change intervals. The chains could last less than 25,000 miles (40,200 km) resulting in expensive damage when they failed;
- inadequately sized main bearings in the early OHC 2.5 litre V8 design with short lives, changed in the 3.0 litre design;
- aluminium head warpage due to poor castings, head gaskets which restricted coolant, leading to overheating;
- water pump failures relating to poor drive gear hardening, prematurely wearing out the gear and stopping the water pump.
Most cars were fitted with a Borg-Warner 3-speed automatic transmission. The other choice was a derivative of the ancient TR-2 gearbox which had been modified and improved over the years for use in the TR4/A/IRS/TR5/250/6. First gear ratio was raised and needle roller bearings were used in place of the bronze bushings on the layshaft. Early 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 is excessive without it. Other than the choice of transmissions there were very few factory installed options. Some early cars came with just the soft-top and some with just the hard-top but most ended up with both. Electric windows, power steering and power assisted brakes were standard. Options included air conditioning, chrome wire wheels, luggage rack, Koni shock absorbers, floor mats and Lucas Square Eight fog lamps, and a range of aftermarket products, most of which were dealer installed as optional accessories.