Keeping the MG T-Series Cool: A Practical Guide to Cooling & Overheating

The short answer

An MG T-Type that runs hot is rarely suffering from a fundamental design flaw, it is almost always fixable. The XPAG and XPEG engines were laid out in the 1930s and 1940s for a temperate British climate and moderate road speeds, so their standard cooling package has little in reserve for modern traffic, hot summers, or long climbs. Work through the causes in the right order and you will get the temperature under control:

1. Prove the ignition timing and mixture are correct.
2. Service the cooling system itself, coolant, hoses, thermostat, water pump.
3. Fit an uprated radiator to cure high temperatures at cruising speed.
4. Fit a lightweight multi-blade fan for low-speed and idle cooling.
5. Fit a manifold heat shield if the car suffers vapour lock after a hot stop.
6. Tidy up with a properly-tensioned fan belt and a coolant overflow tank.

The rest of this guide explains each step, why it works, and where the pitfalls are.

Not sure where to start? Work through the interactive flowchart above, it asks the same questions a specialist would, in the order they would ask them.

1. Tune before you spend

Before you buy a single cooling part, prove that the engine is correctly tuned. On an XPAG or XPEG, the most common single cause of running hot is not the radiator, it is retarded ignition timing. An engine whose distributor is not advancing fully, or whose static timing is set too late, will dump excess heat into the cooling system all day long, and no amount of new hardware will fix it. A lean or otherwise poor mixture will do the same. See our guides to ignition and charging and SU carburettor tuning before you go any further.

While you are at it, rule out the "boring" cooling faults that live outside the scope of this article:

• Old coolant that has never been changed, and the silt and rust that come with it.
• Furred waterways in the block and head, very common on cars that have run on plain water.
• A weak, stuck or wrong-temperature thermostat.
• Perished or collapsing bottom hoses (they can suck flat under load).
• A slipping fan belt, a tired water pump, or a leaking core plug.

Everything that follows assumes the engine is in tune and the cooling system itself is clean, sealed and correctly filled. If it is not, start there.

2. The radiator, the biggest single improvement

Once the engine is tuned and the system is clean, the standard T-Series radiator is usually the next thing to look at. Its capacity is modest by modern standards, and in hot weather or on a long, hard drive it simply cannot shift heat as fast as the engine makes it. This shows up as a temperature that climbs steadily on a fast run and refuses to come back down, the classic symptom of a radiator that is at its limit.

The single most effective cure is an uprated, high-efficiency core. A good radiator specialist can either re-core your existing shell with a modern three-core (or high-density two-core) matrix, or supply a complete replacement built to fit. Owners who make this change routinely report a substantial drop in cruising temperature, often enough on its own to turn a marginal car into a relaxed one.

A few practical points worth passing on:

• Keep the original overall dimensions (core height, width and depth) so the shell, the mounting brackets and the original filler cap all still fit. A specialist who knows T-Types will do this as a matter of course.
• Retain the original external appearance where authenticity matters, the visible slats and shell can be preserved even with a modern matrix behind them.
• Ask for a correct filler neck so your original cap continues to seal properly. A pressure cap that does not seat is as good as no cap at all.
• Have the specialist flow-test the old core before you commit, it may already be partly blocked, in which case even a good clean and re-core is a big step forward.

A healthy, uprated radiator sorts most cruise-speed overheating on its own. Above roughly 30 mph the fan is doing very little; airflow through the core is doing almost all of the work. If your car runs cool on the open road but climbs in traffic, the radiator is probably not your problem, the fan is.

Standard vs uprated radiator

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3. The fan, for idle, traffic and low speed

The original steel fan fitted to the T-Series is a small, four-blade item. At road speed it barely matters, because the car's forward motion is doing the work. At a standstill, or crawling in traffic, it is doing almost all of the cooling, and it isn't doing very much of it. This is why a T-Type that is happy at 50 mph can start climbing towards the red at the first long queue on a hot day.

The popular, sympathetic upgrade is a lightweight multi-blade plastic fan of the MGB type. Compared with the original steel fan it moves significantly more air at low rpm, weighs a fraction of the steel item (which reduces load on the water-pump bearings and the belt), and, because its blades are unevenly spaced, is noticeably quieter. Just as importantly, it removes a real safety concern: an old steel fan whose blades have quietly work-hardened over decades can, and occasionally does, fatigue and throw a blade. That blade will destroy the radiator, and often the bonnet and paintwork with it. A modern plastic fan simply does not fail in the same way.

Standard steel fan vs MGB-type plastic fan

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Fitting notes

• The job is far easier with the radiator out, a good moment to combine the fan swap with a re-core.
• Offer the new fan up to the water-pump pulley and confirm that it centres correctly on the boss and that the mounting holes line up. Do not force it.
• Use lock washers on the mounting bolts and tighten evenly.
• Once the radiator is back in, rotate the fan by hand through a full turn and check clearance to the radiator core, the top hose, and any wiring or brackets.
• On TD and TF cars the fan-spacer and hub stack sit slightly differently to the earlier cars, and clearance to the radiator can be tight. Check carefully; the spacer may need adjusting. If you are not sure, get a T-Type specialist to eyeball it before you close everything up.

If your original steel fan has been on the car for decades, treat it as a candidate for retirement even if it still looks fine. A plastic replacement is cheap insurance against a very expensive failure.

4. Vapour lock and manifold heat-soak

There is a distinctive T-Series fault that isn't really an overheating problem at all, though it feels like one: you drive hard, stop for ten minutes, and when you restart the car it fires up, then coughs, stumbles and bucks for the first minute or two before settling down. That is vapour lock, and it is a cooling and heat-management issue even though the temperature gauge may look perfectly reasonable.

The mechanism is straightforward. On the T-Series the exhaust manifold sits very close to the SU float chambers. When the car is moving, the airflow keeps the carburettors reasonably cool. When you stop, that airflow disappears but the manifold is still radiating heat, and the fuel sitting in the float chambers boils. The engine then tries to run on vapour instead of liquid petrol, and it will not do so cleanly until fresh, cooler fuel has worked its way through.

The fixes are well-established:

• Fit a manifold heat shield. A machined aluminium shield between the exhaust manifold and the carburettors reflects radiant heat away from the float chambers and helps duct hot air out of that area. It is the single most effective cure. Older shields sometimes used an asbestos layer against the manifold, do not disturb an old shield of unknown vintage without proper precautions; modern shields use safe insulating materials and are the right choice for any new installation.
• Improve under-bonnet airflow. Anything that helps hot air escape after a stop, clear vents, correctly-fitting side panels, helps. Some owners find that simply lifting the bonnet for a couple of minutes after a hot run is enough on cooler days.
• Check the throttle linkage. After fitting a heat shield, make absolutely sure nothing fouls the throttle linkage, the choke, or the return springs. It is easy to nip a cable route without noticing.

Because vapour lock and hot-starting problems overlap, it is worth reading this alongside our guides to SU carburettor tuning and the SU electric fuel pump. If the car is hard to start hot as well as running roughly, the fuel pump and its wiring often deserve attention at the same time.

5. Finishing touches

Two small measures round out a well-sorted cooling system:

• A correct, good-quality fan belt, sensibly tensioned. The belt only has to drive the dynamo and the water pump, it is not a competition-car component. Over-tightening chews out the dynamo and water-pump bearings; under-tightening lets it slip in the wet. A rough guide is about half an inch of deflection at the midpoint of the longest run. Use the correct section and length for your model.
• A coolant overflow / expansion tank. The standard T-Series set-up simply overflows onto the road when the coolant expands past the cap. A small overflow tank plumbed to the radiator overflow catches that coolant and lets it return as the system cools, so the radiator stays properly filled between top-ups. On long, hot runs, especially in traffic, this makes a real difference to reliability and stops the tell-tale coolant trail on the driveway.

Neither of these is a headline upgrade, but together they turn a cooling system that works into one that keeps working.

6. A sensible order of attack

If you are staring at a T-Type that runs hot and you are not sure where to start, work through this list top to bottom. Do not skip stages, every one of them can, on its own, be the answer.

1. Confirm ignition timing and mixture. Nothing else matters until this is right.
2. Service the cooling system. Fresh coolant, a proper flush, good hoses, a correct thermostat, a healthy water pump.
3. Fit an uprated radiator (or have the original re-cored to a higher spec). This is your biggest cruise-speed gain.
4. Fit a lightweight multi-blade plastic fan. This is your biggest low-speed and traffic gain, and it removes a safety concern.
5. Fit a manifold heat shield if the car suffers vapour lock after a hot stop.
6. Tidy up with the correct fan belt at the right tension and a coolant overflow tank.

A properly-sorted T-Series will sit happily in summer traffic and cruise all afternoon without drama. If yours is not doing that, one of the stages above is the reason, and every one of them is well within reach of a careful owner and a decent specialist. For radiator re-cores, fans, heat shields and belts, our Parts & Specialists directory lists UK suppliers who know these cars.

Related reading in the Technical Library: Ignition and charging · SU carburettor tuning · SU electric fuel pump, diagnostics · Storing your MG T-Series.