Envisioned as a luxury sports car, the Triumph Stag was designed to compete directly with the Mercedes-Benz SL class models. All Stags were four-seater convertible coupés, but for structural rigidity – and to meet new American rollover standards at the time – the Stag required a B-pillar “roll bar” hoop connected to the windscreen frame by a T-bar. A removable hardtop 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 1963-4 Triumph 2000 pre-production saloon, which had also been styled by Michelotti, and loaned to him by Harry Webster, Director of Engineering at Triumph from the early to late 1960s. Their agreement was that if Webster liked the design, Triumph could use the prototype as the basis of a new Triumph model. Harry Webster, who was a long time friend of Giovanni Michelotti, who he called “Micho”, absolutely loved the design and spirited 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.
It has been alleged that internal politics meant that Triumph intended, but were unable, to use the proven but old technology Buick designed all aluminium Rover V8. The no-fit story is probably a myth as Rover, also owned by British Leyland, simply could not supply the numbers of V8 engines to match the anticipated production of the Stag. “Brand loyalty” between Triumph and Rover was high as they were former rivals. Triumph engineers preferred their new design as it was lighter using aluminium cylinder heads and the superior overhead cam design.
Harry Webster had also 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 V8 marketplace. Under the direction of Harry Webster’s replacement, Spen King in 1968, the new Triumph OHC 2.5 PI V8 was enlarged to 2997 cc (3.0 litre) to increase torque and the troublesome fuel injection dropped in favour of dual Zenith-Stromberg 175 CDSE carburettors to meet emission standards in one of the target markets the 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.
As in the Triumph 2000 model line, monocoque construction was employed, as was 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 car was launched one year late in 1970, to a 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 were related to design problems in the engine. These included:
British Leyland never provided sufficient budget to correct the few design issues of the Triumph 3.0 litre OHC V8. Many owners adopted a popular conversion of the car fitting a Rover V8, Ford Essex V6, Buick 231 V6, or Triumph 6-cylinder engine but now such conversions fetch lower prices than a genuine Stag V8-engined car.
However, renovators over the years did iron out the V8 problems, rather than Triumph engineers.
This all adds up to an engine that now runs very well. If these glitches had been eliminated by Triumph, the engine could have been used in many other models and not only the Stag.
Perhaps thanks to such a reputation for its unreliable engine, only 25,877 cars were produced between 1970 and 1977. Of this number, 6,780 were export models, of which only 2,871 went to the United States. Several variants were produced, noted only in changes of the production numbering sequences, and these have become unofficially designated as “Early” Mk I 1970, the Mk I (1971–1972/3), Mk II (1973) and “Late” Mk II (1974–1977). The addition of twin coachlines is an indication of a Mk II variant.
Whilst official Triumph parts manuals may differentiate variants by commission plate ranges, it is common (from owner’s observations) that minor parts for the old variant may turn up in early productions of the new variant.[4] For example Mk2 cars have been known to have Mk1 wiring looms or door latches. Triumph either took the opportunity of clearing out the parts bin or quality control was not their best attribute.
Most cars were fitted with a Borg-Warner 3-speed automatic transmission. The other choice was a derivative of the ancient Triumph TR2 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 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. On early cars buyers could choose to have the car fitted with just the soft-top, just the hard-top (with the hood storage compartment empty) or with both. Later cars were supplied with both roofs. Three wheel styles were offered. The standard fitments were steel wheels with Rostyle ‘tin-plate’ trims. 5-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.
Electric windows, power steering and power assisted brakes were standard. Options included air conditioning, a luggage rack, uprated 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 could also be fitted. Rather unusually for a 4-seat touring car, the accessory list included a sump protector plate. This was probably included as a slightly ‘gimmicky’ tribute to Triumph’s rallying successes.
The Triumph Stag has sizeable club and owner support and a number of specialist suppliers. About 9,000 Stags are believed to survive in the United Kingdom. The car’s popularity is due to its performance, comparative rarity and its Michelotti styling. The problems associated with the car over the years have been solved by those enthusiast clubs supporting the Stag, elevating this classic to its intended place in popularity envisioned by its designers.
The Stag engine has proved to be its great weakness. Some people have even christened it the “Triumph Snag”. The key problem with the Stag is its cooling system. To test it, you should let any potential purchase idle for 15 minutes (preferably on a hot day) while keeping an eye on the temperature gauge. Check that the radiator is as hot at the bottom as it is at the top. If it’s not, there may be problems with coolant circulation and expensive damage may have already been done. In addition, check that the thermostat (in the intake manifold) hasn’t been removed to mask some chronic cooling problem, and make sure that the oil pressure light is connected. If not, leave well enough alone.
An overheating Stag will eventually end up with a warped head. Like the TR7 (which uses half of the Stag V8), Stag heads have a bad reputation for warping in service, often with as few as 25,000 miles. The blown gaskets which soon follow can lead to damaged head faces which will need skimming if they are to be rescued. The trouble is that Triumph only allowed 0.010″ for removal before the head had to be replaced. In some cases, this is still not enough to save the head. A blown head gasket can also cause engine coolant to seep between the head and the studs. An electrochemical reaction between the aluminum head and the steel studs causes the head studs to seize in place, making head removal almost impossible since the studs and bolts used to attach the head to the block are not parallel. Some people have had to lift the car with an engine hoist and pound on the head studs with an air hammer, using the weight of the car to pull the studs through the head. Internal corrosion of the coolant passages is another problem if a corrosion-inhibiting coolant was not used.
Mark II Stags originally came with a sheet metal radiator fan cowling to increase the efficiency of the radiator cooling fan. Many of the cowls were removed when the original engine mounts sagged, causing the engine driven radiator fan to rub against the cowl. Removal of the cowl solved the noise problem, but may have created an overheating problem. If your Stag doesn’t have a radiator fan cowling, new composite cowls are now available. Visit the Triumph Stag Composite Radiator Cowl page for more information. Be aware that while these parts were modeled from a genuine MkII Stag cowling, they are being produced from composite polyester/fiberglass instead of the original sheet metal, and are not British Motor Heritage approved. If you have a 16.5″ metal radiator fan, you should plan on replacing both of the front engine mounts to eliminate the possibility of the fan striking the cowl or the lower radiator hose. This is a major job – replacing the right engine mount requires you to remove the alternator, which requires removal of the anti-sway bar and automatic transmission oil cooler. Changing the left engine mount will require you to remove the exhaust manifold.
Timing chains are another source of concern. The single-roller chains wear quickly and should be replaced every 25-30,000 miles. If they break or skip a tooth, the valves will embed themselves in the pistons. The hydraulic timing chain tensioners are actuated by the engine lube oil, so you must keep the idle rpm up high enough to maintain sufficient oil pressure. If you hear the timing chains rattle when you start the engine, but the noise goes away a few seconds later as the oil pressure comes up, plan on replacing the timing chains within the next 3000-5000 miles. If the rattle doesn’t go away at idle, don’t drive the car any further or you’ll be facing a major repair bill in the very near future. You don’t want to see the oil light come on at idle, since it means you have less than 5psi of oil pressure.
Along with the TR7 engine, the Stag engine has a jackshaft that runs in the block. This shaft drives the water pump, distributor and oil pump. Both engines have shown a tendency to wear the bottom of the front jackshaft bearing surface in the block. This results in reduced oil pressure, and eventually, a seized jackshaft. A preliminary symptom of excessive front bearing wear is the failure of the water pump drive gears due to insufficient lubrication splashing out from the front jackshaft bearing. Some people simply replace the jackshaft and water pump, not realizing the root cause of the problem is the worn front jackshaft bearing. The only fix for this is to completely strip the engine and have the jackshaft bearings align-bored and resleeved. This wear is not apparent until the engine is torn down, and even then, some experienced engine rebuilders have missed it. The lack of an engine oil pressure gauge makes this problem even harder to spot.
There was a brief period when crankshaft bearings wore out prematurely due to a temporary machining problem which resulted in the wrong surface finish on the crankshaft journals. If you have to regrind the crankshaft, make sure that it is properly hardened afterwards or you’ll have a disaster in a very short time. Since the original factory hardening process only penetrated around 10-15 thousandths below the surface, you may remove all of the hardened surface with the first regrind. Premature wear could also occur as a consequence of poor maintenance, or if the engine (and hence, the oil) was running too hot as a consequence of cooling problems.
The source of any fluids found on the top of the engine can be checked by cleaning the top of the engine with a pressure washer (or at a self-service car wash). Drive the car until it is hot enough to evaporate any water left on top of the block. When the engine is dry, place a couple of white paper towels between the intake manifold and the top of the block and let it sit for several hours. Coolant (green) may appear from a number of sources. A leaking water pump seal will cause coolant to leak out of an opening below the waterpump cover, on the flywheel side of the waterpump housing. The intake manifold gaskets are supposed to seal the fuel/air mixture flowing from the manifold into the heads, as well as coolant flowing from the heads into the manifold. These gaskets are notorious for allowing coolant leaks. Other coolant leak sources could be from the thermostat cover, bypass hose, or heater return hose. The water pump cover gaskets are well designed and rarely, if ever, leak. Fuel leaks may be from improperly adjusted carburetor floats or leaking needle valves. Oil leaks are probably from the camshaft cover gaskets.
Test for suspension bushing wear in the usual manner by shaking each wheel sideways while a helper applies the brakes. If there is play, the suspension bushings may need to be replaced; if there is play with the brakes off, then suspect a wheel bearing.
A trait common to Triumphs with semi-trailing arm rear suspension (Triumph 2000/2500, TR4A IRS, TR5, TR250, TR6) has developed the name “Triumph Twitch”. This is a curious sideways lurch at the rear when you let off the throttle in the middle of a turn. This lurch doesn’t always happen, but when it does, it’s caused by the splines of the telescopic drive shafts sticking under power and then freeing in coast, thereby locking, then releasing the rear suspension. The official word from Triumph was that this is a lubrication problem that should not arise, though they did admit that it sometimes does occur. Contrary to popular opinion, the lurch is not caused by worn semi-trailing arm bushings. Several Stag parts houses in the UK have developed replacement halfshafts that use modern constant velocity joints that provide articulation, as well as compression and extension without the use of sliding splines. These new halfshafts eliminate the Triumph Twitch, but at a price – expect to pay $600+ for a pair!
Like the TR6 that shares its manual gearbox, the Stag has a reputation for wearing out its synchromesh in second and third gears. They also tend to suffer from gearlever buzzes after a lot of use. The automatic transmission (Borg-Warner Type 35) tends to engage Reverse and Drive with a lurch if the idle speed is set too high. This problem can also be accentuated by worn driveshaft U-Joints. Reluctance to shift at speed can be caused by a poorly adjusted downshift cable. A metallic ping from the propellor shaft can be heard if the differential input flange bolts are loose. The differential has a good reputation, although they tend to whine as they age. Most new differentials whine from day one because a recent batch of ring & pinions appear to have been improperly hardened. Another source of drive line noise can come from the quill shaft bearing in the differential’s pinion extension housing. If the differential pinion oil seal fails, the differential oil will flow into the extension housing and wash away the grease for the quill shaft bearing. A visible oil leak from the pinion flange area could mean future trouble.
Power steering leaks may occur, but no more than on any other power-steered car. Inspect the rack gaiters for signs of oil leaks. Type F Automatic Transmission fluid should be used to top up the reservoir. If the rack leaks, it’s best to have it professionally rebuilt. Replacement seal kits are available, but it seems that DIY repairs don’t last long enough to be worth the trouble of doing it yourself. Note that the power steering pump must be unbolted to remove the battery, although the hoses stay connected.
Brake troubles are rare. The convertible storage hold can leak, and if the top was not stored correctly, it can be very difficult to erect. Before the top is lowered into the hold, the rear section must be locked in the vertical position using the two latches provided. Failure to do this will cause the rear section to become trapped in the hold, although instructions are provided in the owner’s manual on how to extract it.
The tinworm can and does attack the Stag, and you should look for rust damage in the floorpan, fenders (inner and outer), rear fender/sill joints, rear subframe mountings, and the outriggers. Check these areas very carefully, for while replacement panels are available, they are expensive. It seems that post-1974 Stags are more susceptible to rust problems than are the earlier models due to the factory switching to a different sheet steel supplier.
This section was written by William Mayo and Walter Holliday,
originally appearing in Issue 38 of The Vintage Triumph magazine (currently out of print)
Section in brackets [] added by Tim Buja, [email protected]
The Stag needs all the help it can get in order to prevent boiling over. Most Stags have the familiar hexagon-shaped brass plug on top of the radiator; early models had the American style pressure relief cap. During periods of peak pressure, coolant is allowed to run into the plastic collection bottle. Because most of these bottles were fitted with pressure caps rated at only 13 lbs. per square inch, we strongly recommend the use of 20 lb. caps which became standard on later model Stags. If your dealer can provide one, it is Unipart # GRC124.
Equally important is the use of a good thermostat. Never attempt to operate the car without one. The principal behing the “pump-assisted, thermo-siphoned” cooling system is that coolant remains in the radiator long enough to be cooled before returning to the engine. The thermostat regulates this flow. In addition, it provides a very necessary build up of pressure because the water pump works best when it assists the flow of coolant already in motion.
[Be sure that you have the correct type of thermostat. To improve drivability and shorten the engine warm-up time, the Stag has a bypass port behind the thermostat which connects the water pump suction line to the intake manifold coolant chamber. Since it collects all of the coolant flow from the engine through both of the cylinder heads, the intake manifold coolant chamber is the hottest part of the cooling system. The heated manifold prevents fuel from condensing on its inner walls and causing drivability problems like lean stumble. The bypass port allows the manifold to heat up faster by allowing coolant to flow into the water pump via the intake manifold coolant chamber as the engine warms up. In addition to the manifold chamber passageway, coolant can also flow from the left cylinder head through the heater control valve and heater core to the water pump. If the heater valve is closed, there won’t be any flow through this path, leaving the bypass port as the only path for coolant flow into the water pump when the engine is cold.
As the engine comes up to normal operating temperature, the thermostat should close off the bypass port as it opens the main discharge line to the upper radiator hose (hot side of the radiator). Most thermostats do not have the blanking disk that closes this bypass port. If you use one without the blanking disk, the water pump will take suction from both the hot and cold sides of the radiator. In addition to raising the temperature of the coolant pumped back into the engine, this will reduce the volume of coolant flowing through the radiator.
This picture shows the Unipart GTS 101 (left), Robertshaw 412-180 (center), and for comparison, a “normal” thermostat as used in Triumph six cylinder OHV engines, Spitfire four cylinder engines, and the Rover V8 (right). Both of the thermostats on the left, along with the Stant 35398 (BT 339 180) thermostat (not shown) have the proper blanking disk to close off the bypass port at normal operating temperature. One disadvantage of using the aftermarket (Robertshaw & Stant) thermostats is that these units are not equipped with the small bleed hole and “jiggle pin” that is used in the Unipart thermostat. You can see the plastic part of the jiggle pin just below the flat mounting face of the Unipart thermostat in the top picture.
This picture is a top view of the Unipart GTS 101 (left) and Robertshaw 412-180 (right), with the vent hole and metal part of the jiggle pin plainly visible in the upper right area of the mounting face. The Stag Repair Operations Manual shows the jiggle pin in the thermostat in the illustration in section 26.45.09, but it does not mention anything about it in the thermostat installation procedure. The Triumph TR8 ROM explicitly states that the thermostat should be installed “with the jiggle pin uppermost at 12 o’clock”. It appears that the bleed hole & jiggle pin is there to assist in venting air from the cooling system as you fill it, and also for reducing the peak cooling system pressures that may develop in the engine before the thermostat opens. You can modify the Robertshaw and Stant thermostats by drilling a 1/8″ hole in the stationary part of the thermostat to aid in filling & venting your cooling system. As mentioned in the TR8 manual, the thermostat should be installed with the jiggle pin (or vent hole) at the “12 o’clock” position.]
The mating of aluminum and iron castings in an engine poses the special problem of corrosion, and as local water sources often contain a high concentration of minerals, coolant composition should not be overlooked. We recommend a mixture of Prestone II antifreeze and distilled water at all times. The correct ratio is 50/50. In that strength, your car is drivable from a cold start down to -33F, and, after minimal warm up, as cold as -53F. Whenever adding coolant, be sure to use the same mixture.
The Stag cooling system holds 22-1/2 US pints. If you can’t fit that volume into a dry engine [and heater core], then there is probably a trapped air pocket somewhere. Coolant should be added with the engine running and warm. Make sure the heater control is in [the full hot] position. It’s been suggested that having the front end of the car raised when filling will eliminate air pockets. As a final step, hold the overflow bottle higher than the radiator when filling.
Back flush the cooling system once a year after cleansing with a suitable cleaning agent. If you have any doubt about the condition of your radiator, it is best to have it “rodded” and cleaned in a tank by a competent radiator shop. Make sure that the radiator is not painted by the shop afterward, as most types of paint can clog the thin metal fins. If you must have the cosmetic touch, we suggest a light coating of the type of paint used on barbecue grills.
It really is important to keep the radiator full at all times. Replacement heads are costly items. The high-mounted water pump makes the balance of water between the heads delicate at best. As many Stag owners know all too well, the heads must be properly cool[ed] to prevent warpage… after skimming off 10 to 15 thousandths, about all you can do is buy expensive, extra-thick gaskets.
Crankcase oil functions as a coolant, too, so keep it clean and up to capacity. Proper torque settings of head bolts are exceptionally important. We are looking into the rumored advantage of increasing the manufacturer’s recommended setting (55 [ft-]lb.) by additional 5 [ft-] lb.
The intake manifold, which allows coolant to flow from head to head, while at the same time keeping the carburettors warm, uses gaskets on either side that have very narrow bands of material surrounding the inlets. These are sometimes the source of leaks. Universal type radiator hoses, of the “accordion” type, are not recommended as they offer little flexibility and can cause cracks where they attach to the radiator fittings. Use properly suited, [reinforced,] molded radiator hoses. Exhaust headers, available for the Stag, are thought to act as heat sinks, thereby eliminating a certain amount of heat.
In summing up, it is fair to say that practically anything can be the cause of overheating. On that list you might include: ignition timing, exhaust valve clearance, vacuum hoses to distributor or air cleaner, air leaking in past the “O” ring on the base of the carburettors, a cracked block, and sticking brakes. One final suggestion: If you live in an excessively hot area and wish to have every measure of protection available, you might want to consider installing one of the imported, heavy duty radiators. These units are 4-row, staggered core affairs, and reputed to be of very high quality.
