1.0 Background & specifications

The Triumph GT-6 was produced from 1966 to 1973. Conceived from the idea of a 'fastback' version of the popular Spitfire, it used many parts currently in production at Triumph to assemble onto a modified Spitfire chassis the running gear & suspension of Triumph's two-litre 6 cylinder Vitesse sedan. The bodywork was designed by Michelotti.

The car was intended for sale not only in the UK, but export markets. Variations for the cars manufactured for these different markets were for the most part accommodations for right-hand drive or left-hand drive; and particular UK, USA, German, & Sweden requirements.

Three basic versions of the GT-6 were produced over time: the original GT-6, the GT-6 Mk2 (sold slightly modified as the GT-6+ in USA, & the GT-6 Mk3.

1.1   GT-6 models

The GT-6 Mk2/GT-6+ had major revisions from the original GT-6 in bodywork & suspension. Bodywork revision included addition of vents in the bonnet.

The rear suspension redesign replaced the swing-axle rear suspension with a semi-independent arrangement incorporating a rotoflex coupling on each side, & the result was improved handling.

Inside changes included a change to black-bezel gauges from the previous model's chrome-bezel gauges, & a 3-point safety harness.

The GT-6 Mk2 was not sold in the USA. The USA got the GT-6+ instead, a variation of the GT-6 Mk2. The GT-6+ differed only in engine specifications & the addition of evaporative loss equipment to accommodate the new federal emissions requirements.

• Evaporative loss control components added, comprising an expansion tank near the fuel tank, charcoal canister in engine compartment, fuel vapor line running from fuel tank & filler cap to expansion tank, & from expansion tank to the charcoal canister; vacuum connection from charcoal canister to line from valve cover to emissions valve
• Exhaust valves, Stellite faced valves are fitted to maintain effective valve seating between servicing intervals
• Cylinder head, A modified cylinder head is fitted, giving (via thicker head casting) a compression ratio of 8.5:1
• Camshaft, A new camshaft with 10-50--50-10 timing is fitted to give better control of emissions during idle & low-speed cruising
• Ignition distributor, A special distributor with a vacuum retard component which retards the ignition at idle & during deceleration
• Carburettors, Stromberg CDSE 150 carburettors which each have an additional TC thermostatic compensating valve assembly & 'starter box' starting valve assembly (these features operate only when cold), throttle by-pass valve assembly (opens to admit air during deceleration), fixed non-adjustable (except via special tool) jet assembly & biased needle, leak balancing screw for setting all carburettors to a common datum during manufacture & is then sealed.
• Air cleaners, with additional holes matching CDSE carburettors
• Choke cables, special twin choke cable to operate chokes on both carburettors simultaneously
• Accelerator linkage, Modified to provide a positive idle stop & incorporates "lost motion" for operating a vacuum control valve when the throttles are closed
• Vacuum control valve, mechanically connected to, & operated by, the accelerator linkage, to permit normal vacuum ignition advance under part throttle condition. When the accelerator pedal is released, full manifold depression is allowed to operate on the distributor (vacuum retard module) to provide retarded ignition.
• Crankcase emission control valve, connected between the valve cover outlet & the intake manifold. Manifold depression is used to remove the blow-by gases yet facilitate satisfactory idling.
• Spark plugs, Champion UN.12.Y plugs are designed to give improved combustion.
• Pipe layout, The various emissions controls devices are connected by rigid metal pipes.

The GT-6 Mk3 had significant body changes, resulting in a car that was plainly a GT-6 but a bit more streamlined. The radiator opening was moved completely below the new front bumper, the new hood had no stamped vents, & door handles were redesigned to recess into the bodywork. The new car was an inch longer & about 30 pounds heavier than the MK2/+.

Inside, the dashboard was redesigned, & the Mk3 was fitted with a revised fuel tank having a capacity 1.25 gallons less than the previous car.

A new engine block was introduced during the Mk3's production run at engine number KE/KF/KG 10,001. The design incorporated recessed rings on the deck of the block around the tops of the cylinders, to locate a revised head gasket.

The engine cooling system became a "no-loss" design with a revised radiator neck & radiator cap rated at 13 lbs. pressure.

During the GT-6 Mk3's production, British Leyland was instituting changes in the manufacture of its cars to reduce parts variations & standardize similar engines across its cars.

The GT-6 engine derived from the 1.6 litre inline 6 engine which was originally in the Triumph Vitesse at the start of its production in 1962.

Triumph increased this engine's displacement to 2 litres, & the new engine was first offered when it debuted in Triumph's Vitesse 2-Litre & GT-6 cars in 1966.

The engine's development outside of the GT-6 & Vitesse cars continued by increasing displacement again, by lengthening the stroke, to 2.5 litres. This derived 2.5 litre engine was used in Triumph's later cars such as updating the 2000 series saloons (starting with the Triumph 2.5 PI in 1968) & the TR series (starting with the TR5/TR250 in 1968).

At engine number KE/KF/KG 10,001 in the GT-6, the block was redesigned with recessed rings on the deck around the tops of the cylinder bores, to locate a revised head gasket. The components of the 6 cylinder engine during the GT-6 run were also subsequently changed to allow greater interchange & manufacturing commonality across the 6 cylinder engines in all of British Leyland's car lines.

• Four-speed constant mesh gearbox with synchromesh on 1st, 2nd, 3rd, & 4th.
• Hypoid bevel gear rear axle: 3.27:1 ratio standard; 3.89:1 with optional overdrive.
• Optional overdrive operates on top two gears only. Overdrive ratio: .8:1

Master & slave cylinder specifications changed through the run of the cars.

Girling Powerstop brake servo (fitted to most late Mk3's) listed as option in parts catalogs, but not in sales catalogs.

Tandem brake system with special master cylinder & separate hydraulic circuits for front & rear in later models.

The difference in the production year & the model year is important to understand. For example, cars built, advertised, & sold as 1972 Triumph models actually began production in 1971 & continued production into 1972. A car built in December 1971 would be a 1971 production year car, & a 1972 model year car. Changes at chassis numbers are more definite references, but are not always available.

Some changes to the engine are referenced to the engine number.

A partial list of production changes is below:

USA showroom brochures listed as options SCCA-approved racing wheels of aluminum or magnesium. These wheels were not manufactured by Triumph, but provided through the dealer.

The wheels were the Silverstone model from American Racing, available in magnesium or aluminum. Magnesium is the more common. You didn't typically see them with the absurd three ear spinner! I have six of them but they're not suitable for racing! The magnesium over time weakens with internal corrosion. Buy modern Panasports for the look. - thanks to Steve Smith

By the 1970s, it was common in warmer sections of the USA to see in the Triumph dealer showrooms cars with what looked like factory-installed air conditioning. These air conditioning units were in fact installed by the dealership, as customer expectation of air conditioning in cars became general about this time. Most of the units were bought from a single aftermarket supplier which manufactured model-specific air conditioning for many makes of cars which did not originally have air conditioning.

Most of these systems were later removed by owners. Removal usually occurred when the units needed repair or recharging, as the benefit of the air conditioning system was weighed. The system didn't do much to mitigate the famous GT6 cabin heat, especially on the warmest days when it was needed most, and the penalty to performance resulting from driving the air conditioning compressor often seemed worth eliminating in the lower-power late US-spec GT6 engines. Removal of the components would additionally improve performance by saving weight.

The original kit & installation comprised:

• small condensor located in front of the radiator at bottom
• plastic engine fan replaced with metal fan
• front engine lifting eye replaced with fabricated bracket holding idler wheel and compressor
• receiver/drier located in engine compartment along the right-side cardboard engine valance
• evaporator/fan built into a unit which replaced the passenger's parcel shelf. This unit presented a closed black plastic panel of the same size & shape of the parcel shelf area & on its face an eyeball vent matching the OEM dashboard eyeball vents
• cabin air output from the evaporator/fan unit ducted into the existing dashboard vent system with additional OEM ducting & Y's
• 2 holes for the refrigerant lines punched into the firewall behind the kickplate in the passenger footwell
• small, 2-knob control panel attached to the inside corner of the driver's parcel shelf

The simple system would likely be more effective if used today, incorporating typical more efficient modern components & approaches such as improved cabin insulation, more-efficient compressors, lightweight bracket for idler pulley & compressor, & a typical A/C vacuum switch to disengage the air conditioner compressor's clutch on acceleration.