For an MG T-Series the enemy in storage is moisture, not cold — and it matters more than for most classics, because the coachbuilt body has an ash timber frame that rots when damp. The goal is to keep relative humidity low (ideally below about 55%) and stop condensation forming. This guide covers winter and summer storage, heating and dehumidifying, portable garages and their base, and airflow bubble systems.
Why storage matters — the humidity principle
Corrosion is the threat most owners think of first, but a T-Series has a second, quieter danger that outranks it: rot. The body is a coachbuilt shell of steel panels tacked to an ash timber frame, and damp air soaks the timber, separates the joints and eventually softens the frame from within. See the T-Series Bodywork article for how the frame is put together and where it fails.
The controlling factors are relative humidity and the dew point. When the metal of the car is colder than the surrounding air's dew point, moisture condenses on it — and inside it, in the door bottoms, under the wing beading, along the tack rails and up into the ash. Warm, humid air on a cold car is worse than cold, dry air on a cold car.
The practical target is a stable, dry environment — relative humidity below about 55% — rather than a warm one. Stability matters as much as the number: repeated wet/dry cycles work moisture into timber joints in a way that a steady 55% never will.
The secondary risks are the familiar ones: flat-spotted tyres, seized brakes, degraded fuel, a flat battery, perished rubber, and mould in the trim. On a T-Series, all of those sit below the ash frame in importance. Get the humidity right and most of the rest becomes a matter of routine.
Winter storage — the main UK season
Winter is when a T-Series does most of its sitting still. Work through this in order; the humidity control at the bottom is the piece most owners under-do.
- Wash and thoroughly dry the car, and clean the interior, before storage. Nothing wet, nothing dirty, goes into the lay-up.
- Protect the paintwork with a good wax and feed the leather so it doesn't dry out over months of stillness.
- Fill the fuel tank to reduce the air space that would otherwise breathe moisture in and out with each cold night. Add a fuel stabiliser. Modern E10 petrol absorbs moisture and is unkind to older fuel systems, so E5 "super" unleaded plus a stabiliser is preferable — see the SU carb tuning and SU fuel pump articles for why the T-Series fuel system is particularly sensitive to modern petrol.
- Change the oil before laying up. Old oil is acidic and will corrode bearings if left to sit; fresh, classic-spec ZDDP oil is what these flat-tappet engines need. See XPAG/XPEG oil pressure for the ZDDP argument in full.
- Confirm the coolant has the correct corrosion-inhibiting antifreeze strength for a cast-iron block and brass radiator — winter storage is a bad time to discover it does not.
- Battery: these are positive-earth cars. Either remove the battery and store it somewhere frost-free on a maintenance charge, or leave it in the car on a trickle charger or conditioner that is safe with positive-earth wiring. Not every modern charger is.
- Tyres: over-inflate slightly to reduce flat-spotting, and — where practical — move the car a fraction periodically. Wire-wheeled cars are especially prone to flats where the tyre sits hard on a limited contact patch.
- Leave the handbrake OFF and chock the wheels. A handbrake left on for months is one of the classic ways to seize the rear brakes.
- Deter rodents. Bung the exhaust tailpipe and air intake (with a bright reminder to remove them before starting), remove any food sources, and set traps or deterrents. Mice in the wiring and trim are a real problem in rural garages.
Summer storage
Summer storage is not just winter with the heating off. The threats are different.
- UV light fades paint, perishes rubber and tyres, and cracks trim. Keep the car out of direct sunlight.
- Heat and trapped moisture under a cover encourage mould in the trim and hood.
- Warm days and cool nights produce condensation cycles that soak the timber just as thoroughly as winter damp.
- Pests are more active — insects, mice and, in some regions, martens looking for wiring.
- Fuel evaporates faster, so a stabiliser and a properly-sealed cap still matter.
Portable and temporary garages can overheat badly in summer. Interior temperatures under a PE/PVC cover in full sun can climb well above the outside air; ventilation is not a nice-to-have but essential.
Heating in winter
The point of heating a storage garage is not to make it warm, but to keep the car above the dew point and hold humidity low, while avoiding large temperature swings — the swings themselves cause condensation, and they are particularly harsh on a wood-framed body.
Gentle, consistent background heat is what works. Options include:
- Tubular greenhouse heaters run along a wall — low wattage, very steady.
- A frost-thermostat oil-filled radiator that only cuts in as temperatures approach freezing.
- Dedicated frost heaters designed for outbuildings.
Whichever you choose, insulate the space first. Heating a leaky, uninsulated garage is expensive and futile; sealing draughts and insulating the door often does more good than the heater itself.
There is an honest trade-off here, and both camps are right:
- Warm and dry: a well-sealed garage held at a steady, mild temperature keeps everything above the dew point.
- Cold and dry: a stable, cold garage — cold air holds less moisture — with a good dehumidifier running is often just as effective and cheaper to run.
For a T-Series, either works; stability and dryness matter more than temperature.
Dehumidifying
For most owners in a typical UK garage — and especially for a T-Series with its ash frame — a dehumidifier is the single most effective piece of kit you can add. It attacks the actual problem: airborne moisture.
There are two main types, and the difference matters in an unheated garage:
| Type | How it works | Best when |
|---|---|---|
| Refrigerant / compressor | Cools coils so moisture condenses out | Warmer conditions; loses effectiveness as temperatures drop toward freezing |
| Desiccant | Absorbs moisture into a desiccant wheel | Works well in cold winter garages; often preferred for classic storage |
Practical rules:
- Target relative humidity below about 55%.
- Use a continuous drain to outside where possible, so you're not emptying a tank in February.
- Seal gaps and insulate the door for efficiency — a dehumidifier fighting an open garage is a losing battle.
- Add moisture-absorber crystals inside the cabin as a second line for enclosed space.
- Always use a breathable car cover. Never non-breathable plastic — it traps condensation against the panels and drives it into the timber.
Portable / temporary garages
Portable garages — a steel tubular frame with a heavy-duty PE or PVC cover (Clarke's units and similar) — are a legitimate option where there is no permanent garage. They are affordable, they go up on a weekend, and a good one will keep the weather off the car.
They also have real weaknesses, and being honest about them is what makes them safe for a T-Series.
- Ventilation and condensation. The cover is essentially a big waterproof bag. Moist air gets in, meets a cold car, and condenses. Airflow matters — pair the outer cover with a breathable car cover, a dehumidifier, or an airflow bubble (see below) inside. For a wood-framed car this is not optional.
- Anchoring. These frames catch wind. Anchor them properly to whatever base you build.
- The base is critical. More on this below.
The base — the piece most owners get wrong
Never stand the car on bare earth or grass. Ground moisture rises constantly, condenses under the car, rusts the underside from below and soaks straight up into the frame.
In descending order of preference:
- A concrete slab — ideal.
- Paving slabs on a properly prepared base — very good.
- At minimum, a heavy-duty damp-proof membrane or vapour barrier laid over compacted hardcore or well-drained gravel.
A proper floor vapour barrier dramatically reduces internal humidity inside a portable garage. It is arguably the single most important detail after the cover itself.
Airflow bubble systems (Carcoon and similar)
An airflow bubble is an inflatable enclosure — a base mat plus a clear cover — with an Active Airflow system: low-wattage fans that continuously recirculate filtered air around the car. That constant gentle airflow isolates the car in a clean, stable, ventilated micro-environment that resists condensation and protects the underside as well as the panels. Most run a drying mode first, then settle into a lower-power storage mode.
Variants:
- Bubble covers (Carcoon-style) with no rigid frame — the cover inflates around the car.
- Framed drive-in versions with a rigid arch — easier to walk around.
- Similar products such as the Airflow Airchamber that use the same active-airflow principle.
Combining solutions
The best T-Series storage set-ups usually combine two of these ideas:
- A portable garage on a damp-proof base, with a dehumidifier and a breathable cover inside.
- A Carcoon inside a dry garage — the bubble does its job in the environment it was designed for.
- A permanent garage with insulation, a frost heater or dehumidifier, and a breathable cover.
Match the combination to your building, your climate and how often you get to the car. What all of them have in common is dry, stable air around dry, protected metal and timber.
FAQ
How do I store an MG T-Series over winter?
Wash and dry the car thoroughly, protect the paint, feed the leather, fill the tank with E5 super plus a stabiliser, change the oil to a classic ZDDP grade, confirm the coolant has proper corrosion inhibitor, keep the battery on a positive-earth-friendly conditioner or remove it, over-inflate the tyres slightly, leave the handbrake OFF and chock the wheels, and deter rodents. Then keep the space dry — a dehumidifier matters more than heat — and avoid brief cold starts.
What humidity should a classic car be stored at?
Aim for relative humidity below roughly 55%, held steady. Above that, condensation forms on cold metal and moisture soaks into the ash frame; big humidity or temperature swings are worse than a cold but stable environment.
How do I stop my MG's ash frame rotting in storage?
Keep the air around it dry. Use a dehumidifier to hold RH under about 55%, seal draughts, stand the car on a damp-proof base rather than earth or grass, use a breathable cover — never plastic — and keep the hood and side screens weatherproof so water doesn't get inside the body.
Do I need to heat my garage to store a classic MG?
Not necessarily. The goal is to stay above the dew point and keep humidity low, not to be warm. A well-sealed, gently heated garage works, but so does a cold but stable garage with a good dehumidifier — and for a wood-framed T-Type, avoiding large temperature swings matters more than warmth.
Are portable garages good for classic cars?
Yes, provided you address ventilation, the base, and anchoring. The cover can trap condensation, so pair it with a breathable car cover or a dehumidifier, never stand the car on bare earth, and anchor the frame properly against wind. For a T-Series, a solid damp-proof base is essential.
Should I start my MG during winter storage?
Not for short cold runs. Brief cold starts push moisture and unburnt fuel into the oil and exhaust and do more harm than good. Either leave the car properly laid up, or run it fully up to operating temperature occasionally.
Frequently asked questions
Why does my tuned XPAG misfire under load?
Misfire under load is almost always ignition on an XPAG, not carburation. Check the plug gaps and colour first — sooty plugs mean the mixture is rich or the ignition is weak; white or blistered electrodes mean it is lean. Then confirm the points gap and dwell, the rotor arm and condenser, and that the coil is producing a strong blue spark. On a Stage 1 or Stage 2 engine, also check the static timing has not crept too far advanced; a pinking engine often feels like a misfire and will eventually burn a hole in a piston. Finally, balance the SUs again — an unbalanced pair can leave one cylinder running lean enough to misfire at wide throttle.
Why does my XPAG hesitate when I open the throttle?
Hesitation on pick-up is the classic SU weak-mixture signal. The piston in the carburettor is lifting too fast for the fuel to follow, usually because the jet is set too lean, the needle is worn, or the dashpot oil is too thin. On a Stage 1 car with standard 1.25 in SUs, start by checking the damper oil level and weight — SAE 20 is correct — and that the piston rises smoothly without sticking. Then richen each jet by one flat, test again, and confirm with the lifting-pin method. On a Stage 2 car with 1.5 in SUs the same rule applies, but the needle profile is more critical; a worn or mismatched needle will hesitate whatever the jet setting.
Why is my tuned XPAG hard to start when hot?
Hot-starting problems on a tuned XPAG usually come down to fuel evaporation or flooding. A free-flow exhaust and a hot engine bay can boil the fuel in the float chamber and carburettor body, making the mixture over-rich. Check the float level first — it should be at the workshop-manual figure, not simply "looks about right". If the car starts cold but floods hot, try easing the throttle fully open while cranking to clear the cylinders, then start with no throttle. On Stage 2 engines with raised compression the starter also has to turn a higher-compression engine against a hot block; a good battery and clean earth straps matter more than on a standard car.
My Stage 1 XPAG pings on acceleration — what do I adjust?
Pinking means the combustion pressure peak is arriving too early. On a Stage 1 tune running on modern 95 RON petrol, retard the static timing by one or two degrees first and re-test on a warm day under load. If it still pings, check that someone has not already advanced the distributor beyond the Stage 1 figure, verify the compression ratio has not been raised by a previous skim, and make sure you are not running the engine below about 1,500 rpm in a high gear. Do not simply richen the mixture to mask pinking — it helps a little, but the real fix is timing or compression.
After fitting 1.5 in SUs my Stage 2 idle hunts — what is wrong?
An uneven idle after a Stage 2 carburettor swap is usually one of three things: the throttle spindles are worn and drawing air, the inlet manifold is not matched to the head ports and is causing reversion pulses, or the jets and needles are not centred and matched. Re-check the manifold face and gasket match, recentre both jets with the piston-drop test, and confirm both carburettors are drawing the same airflow at idle. Stage 2 engines with a fast-road cam also have less manifold vacuum at idle, so the idle speed may need to be set a little higher than standard — typically 800–900 rpm — to keep the SUs stable.
How do I know if my tune is running lean?
Lean running shows up as a flat spot under load, popping in the exhaust on the overrun, white or grey spark-plug insulators, and elevated exhaust-manifold colour after a run. On a rolling road the air/fuel ratio will read above 14.5:1 at cruise or above 13.0:1 at wide throttle. Fix it by richening the mixture a flat at a time on the SUs, checking for air leaks around the manifold and balance pipe, and confirming the fuel pump is delivering at least 1.5 psi at the carbs. Lean mixture is the fastest way to burn exhaust valves on a tuned XPAG, so do not ignore it.
Related reading
- MG T-Series Bodywork — how the ash frame and steel panels go together, and why damp is the greatest enemy.
- Cooling the XPAG and XPEG — coolant condition matters at lay-up too.
- XPAG/XPEG oil pressure — the case for fresh classic ZDDP oil before storage.
- SU carb tuning and SU electric fuel pump — why E5 plus a stabiliser is kinder than E10.
- Buying Guide — how a car's storage history shows up when you come to buy one.
- Technical Library — the full index.