MG MGB, Known Issues and Common Problems
Overview
The MGB was built in vast numbers (over 500,000) over an 18-year production run, which means its problems are extremely well documented. The car’s engineering is fundamentally sound — the B-series engine is tough, the driveline is simple, and the suspension is basic but effective. The MGB’s enemies are rust, age, and the accumulated neglect of half a century of ownership.
This guide covers the patterns — the problems that affect most MGBs and that every owner or prospective buyer should understand.
Body and Structure
Sill Corrosion (Inner and Outer)
What happens: The outer sill skin bubbles, softens, and eventually perforates. In advanced cases, the outer sill crumbles when prodded. The inner sill — the structural member — rots from inside the box section and loses its load-bearing capacity.
Why it happens: The sills are box-section steel with drain holes at the bottom. These holes block with dirt and underseal, trapping water inside the cavity. Corrosion works from inside out, so by the time you see exterior rust, the inner sill may be compromised. The sills are structural — they carry bending loads and support the door hinges.
How to fix it: Replace both inner and outer sill sections. This requires stripping the car to bare metal in the sill area, cutting out the corroded metal, and welding new panels. Quality repair panels are readily available from Moss Motors, British Motor Heritage, and others. Cost: $1,500—3,000 per side for professional work. DIY is possible but requires competent welding skills.
Severity: Critical. Compromised sills affect structural integrity, door alignment, and crash safety.
Floor Pan Corrosion
What happens: Soft or perforated floor pans, particularly in the footwells and under the seats. Wet carpet is a telltale sign. In severe cases, you can see the road through the floor.
Why it happens: Water enters through deteriorated door seals, windscreen seals, and from below via road spray. Carpet retains moisture against the metal. The floor pans are thin and have minimal factory corrosion protection.
How to fix it: Repair sections are available for all floor areas. Weld in new metal. Cost: $500—1,500 per section depending on extent.
Severity: Urgent. Structural floors contribute to body rigidity and occupant safety.
A-Post Corrosion
What happens: Rust at the base of the windscreen pillars where they meet the scuttle panel. This area can be invisible until advanced.
Why it happens: Water runs down the windscreen surround and collects at the base of the A-post. The double-skinned construction traps moisture.
How to fix it: Cut out and fabricate new sections. This is skilled work that requires correct jigging. Cost: $1,500—3,000 for professional repair.
Severity: Critical. A-posts are primary structural members.
Door Skin Corrosion
What happens: Rust along the bottom edge of each door, working upward from the inside.
Why it happens: Drain holes in the door bottoms block, trapping water inside the door shell. The steel corrodes from inside out.
How to fix it: Replacement door skins are available. The skin is peeled off the frame, the frame is treated, and a new skin is crimped on. Cost: $300—600 per door.
Severity: Cosmetic, but indicates neglect that may extend to structural areas.
Rear Spring Hanger Corrosion
What happens: The mounting points for the leaf spring rear suspension corrode, crack, or tear away from the body.
Why it happens: Road spray and mud accumulate around the spring hangers. The metal thins and weakens under the constant load of the spring.
How to fix it: New spring hanger assemblies are available. Weld in replacements. Cost: $300—800 per side.
Severity: Critical. A failed spring hanger means the rear axle is no longer properly located. Dangerous.
Engine
Bottom End Bearing Wear
What happens: Low oil pressure at idle when warm (below 15 psi). A deep rumbling or knocking sound from the lower engine, particularly at idle and on acceleration. This is the main and big-end bearings wearing out.
Why it happens: Age and mileage. The B-series engine is tough but the white-metal bearings wear over 100,000+ km. Infrequent oil changes, overheating episodes, and oil starvation accelerate wear. The 3-main-bearing engines (1962—1964) are more susceptible.
How to fix it: Engine rebuild. The crankshaft is reground to the next undersize, new bearings are fitted, and the engine is reassembled with new gaskets and seals. Cost: $3,000—6,000 for a full rebuild.
Severity: Urgent once oil pressure drops below safe levels. Running on worn bearings causes crankshaft damage that requires more expensive machining.
Overheating
What happens: Temperature gauge climbs into the red, particularly in traffic or on hot days. Coolant loss through the overflow.
Why it happens: The MGB’s cooling system is marginal for Australian conditions. The original radiator loses efficiency with age. The thermostat sticks. The water pump impeller corrodes. The electric fan (rubber bumper cars) or mechanical fan (chrome bumper cars) may not move enough air at low speeds.
How to fix it: Replace the radiator with an aluminium upgrade ($300—500). New thermostat ($15—30), new water pump ($60—100), and ensure the fan system works properly. A supplementary electric fan is a sensible upgrade for chrome bumper cars. Total cooling system refresh: $300—700.
Severity: Critical. Overheating causes head gasket failure and head warping, turning a $300 fix into a $2,000 problem.
Oil Leaks
What happens: Oil residue on the engine, drips on the ground, burning smell from oil on the exhaust manifold.
Why it happens: Every gasket and seal on a B-series engine eventually leaks. The rocker cover gasket, timing cover gasket, rear main seal, oil pan gasket, distributor shaft seal, and tachometer drive seal are all culprits. The rear main seal is the most significant — it’s a rope-type seal that wears and cannot be properly replaced without engine or gearbox removal.
How to fix it: Systematic reseal. Replace all gaskets and seals in one session. Rocker cover gasket ($10—20), timing cover gasket ($15—30), sump gasket ($15—25), rear main seal (rope type: $20—40, requires gearbox removal). Full reseal: $200—400 DIY, $600—1,200 at a workshop.
Severity: Needs attention. Oil on the exhaust manifold is a fire risk. Oil on the clutch causes slip.
Cylinder Head Cracking
What happens: Coolant consumption with no visible external leak. Milky residue on the oil filler cap. White smoke from the exhaust. Overheating.
Why it happens: The B-series head can crack between the valve seats, particularly on engines that have been overheated. The head is cast iron and can be repaired, but prevention (maintaining the cooling system) is far better than cure.
How to fix it: Remove the head, check for cracks (pressure test), and either repair or replace. Remanufactured heads are available. Cost: $800—1,500 for head replacement including gasket and machining.
Severity: Urgent. Continuing to drive with a cracked head causes further engine damage.
Gearbox
Second Gear Synchro Wear
What happens: A crunch or grind when shifting quickly from 3rd to 2nd gear, especially when the engine is cold.
Why it happens: Second gear takes the most punishment in normal driving — it’s the gear you downshift to most often. The brass synchro ring wears thin and can no longer match shaft speeds effectively.
How to fix it: Gearbox rebuild with new synchro rings. Cost: $800—1,500 for a rebuild. A used replacement gearbox from a specialist is $400—800.
Severity: Needs attention. A worn synchro is not dangerous but gets progressively worse. Double-clutching on downshifts extends the life of the synchro.
Overdrive Failure
What happens: The overdrive fails to engage, engages intermittently, or drops out under load.
Why it happens: The Laycock overdrive is an electro-hydraulic unit. Common failures include a faulty solenoid ($80—150), worn wiring or switch ($20—50), low oil level in the gearbox (the overdrive shares gearbox oil), or worn internal components (clutch plates, pistons).
How to fix it: Start with the simple items: check the wiring, switch, and oil level. If the solenoid clicks but the overdrive doesn’t engage, the internal hydraulics need attention. A full overdrive rebuild: $600—1,200.
Severity: Not urgent but highly desirable to fix. A working overdrive transforms highway driving.
Electrical
Lucas Alternator and Charging Failures
What happens: Dim lights, slow cranking, dead battery, charging light on the dashboard.
Why it happens: The Lucas alternator’s brushes wear, the voltage regulator fails, and the bearings give out. On early cars with generators, the commutator wears and the brushes make poor contact.
How to fix it: Rebuild or replace the alternator. A modern replacement alternator ($100—200) is a practical upgrade. Ensure all earth connections are clean and tight — poor earths cause more electrical problems on MGBs than component failures.
Severity: Needs attention. A failed charging system strands you.
Earth (Ground) Connection Corrosion
What happens: Intermittent electrical failures, dim lights on one side, instruments reading incorrectly, indicators flashing at wrong speed.
Why it happens: The MGB relies on the body as the earth return path. Corrosion at earth strap connections — engine to body, body to frame, battery earth — creates resistance in the circuit. This is the single most common cause of electrical problems on the MGB.
How to fix it: Clean every earth connection to bright metal, apply dielectric grease, and retighten. Add supplementary earth straps if necessary. Cost: $0—30.
Severity: Needs attention. Poor earths cause cascading electrical problems that can be maddening to diagnose.
Ignition Component Failure
What happens: Sudden engine death, misfiring, no-start condition.
Why it happens: The points-type ignition system relies on a condenser, contact breaker points, rotor arm, and distributor cap — all of which wear and fail. The condenser is the most common sudden-failure item. Modern aftermarket condensers are often poor quality.
How to fix it: Carry spares. Alternatively, fit an electronic ignition conversion (Pertronix, Powerspark, or Accuspark — $80—150) that eliminates the points and condenser entirely. This is one of the best upgrades you can make to any MGB.
Severity: Stranding risk. Carry a spare condenser and rotor arm at all times.
Suspension and Steering
Front Kingpin Wear (Chrome Bumper Cars)
What happens: Clunking from the front end, wandering on straight roads, uneven tyre wear.
Why it happens: Chrome bumper MGBs use kingpins rather than ball joints for the front upright. The kingpin bushings wear, introducing play. The grease nipples must be serviced regularly — many owners forget.
How to fix it: Replace the kingpin bushings and thrust washers. The kingpins themselves are usually reusable. Cost: $200—400 per side including parts and labour.
Severity: Needs attention. Worn kingpins make the car feel vague and cause accelerated tyre wear.
Leaf Spring Fatigue
What happens: Rear of the car sits low. Sagging stance. Poor ride quality and reduced handling.
Why it happens: The leaf springs lose their temper (metallurgical, not emotional) over decades. Overloading and fatigue cycling flatten the springs.
How to fix it: Replace with new leaf springs. Parabolic spring conversions are available and offer significantly improved ride quality. Cost: $300—600 per set for standard, $500—800 for parabolic.
Severity: Needs attention. Sagging springs change ride height, which affects handling and can cause contact between the rear axle and body.
Cooling and Fuel
Fuel Pump Failure
What happens: Engine starves for fuel under load, particularly at high RPM or when the tank is low. The engine sputters and dies, then restarts after a few minutes (vapour lock).
Why it happens: The SU electric fuel pump (mounted at the rear near the tank) uses contact points that corrode and pit. Modern ethanol-blended fuel accelerates deterioration of the pump’s internal diaphragm.
How to fix it: Rebuild the pump ($40—80 for a kit) or replace with a modern solid-state fuel pump ($80—150). The electronic replacements are more reliable and quieter.
Severity: Needs attention. Fuel starvation at speed is dangerous. Carry a small hammer — tapping the SU pump body can temporarily restore contact and get you home. This is a genuine MGB ownership technique.
Carburettor Wear and Maladjustment
What happens: Rough idle, hesitation, poor fuel economy, rich or lean running.
Why it happens: The twin SU carburettors (chrome bumper) or single Zenith-Stromberg (rubber bumper) have numerous wear points. SU throttle shaft bushings develop play (causing air leaks), needles and jets wear, damper oil level drops, and the mixture drifts with age.
How to fix it: SU carburettors respond beautifully to a proper service. New needles and jets ($30—50 per carb), new bushings if worn ($50—100 per carb), correct damper oil level, and careful mixture adjustment. A well-set-up pair of SUs is a thing of beauty. Cost: $80—200 DIY for a full service kit.
Severity: Needs attention. Poorly set carburettors waste fuel, reduce power, and can cause overheating through lean running.
Preventive Maintenance
To keep an MGB reliable and enjoyable:
- Inspect for rust every 12 months. Sills, floors, spring hangers, A-posts. Catching it early saves the car and your wallet.
- Maintain the cooling system. Aluminium radiator, fresh hoses, correct thermostat, working fan. Non-negotiable in Australia.
- Service the SU carburettors every 10,000 km. Top up damper oil, check mixture, lubricate the linkage.
- Grease the kingpins (chrome bumper cars) every 5,000 km. This is the most commonly neglected maintenance item.
- Fit electronic ignition. It eliminates the most common cause of roadside breakdowns for under $150.
- Check and clean every earth connection annually. Five minutes of work prevents hours of electrical diagnosis.
- Change engine oil every 5,000 km with 20W-50 mineral oil. The B-series engine rewards frequent oil changes.
- Check the overdrive oil level (if fitted). The overdrive shares gearbox oil and is sensitive to low levels.
Loading comments...