Unleaded Fuel and Ethano

The standard B-series cylinder head was not designed for unleaded petrol so if you want to avoid damage over long-term use, either a lead-replacement compound needs to be added to the fuel or the head needs to be modified.

Additives like Castrol or Millers are generally available from Halfords and independent motor factors. Once you have chosen a brand it is best to stick to it to avoid any unitended chemical “cocktails” in your tank. They do not all use the same active ingredients. If you cannot trace a local source, then they can also be obtained by mail order. The cost may seem high but a bottle of additive will last for a number of tank refills and, compared to the cost of an enforced cylinder head rebuild, is very tolerable.

If you are taking the head off anyway for maintenance, then it is best to have it converted before refitting. The required work is to have hardened valve seats inserted in the exhausts and aluminium bronze valve-guides and Stellite exhaust valves. If you are using a post 1977 18GV 1800cc engine, the valve seats were harder and more tolerant of unleaded fuel, but it is still best to play safe and have the work done.

95 Octane fuel is OK up to a compression ratio of 9.5 to 1 and the standard Magnette 1500cc high compression engine had a ratio of 7.15:1 up to car 18101 and 8.3:1 thereafter. This will have been raised a bit if it has been refaced a few times and metal planed from the gasket face.  You cannot assume what your compression ratio is merely by referring to your car number because the engine may have been modified over the years and the factory only altered the ratio by fitting pistons with tops that were dished to a greater or lesser extent. Flat-topped pistons will raise the ratio considerably. Be careful also if you fit a non-standard head. Some 1800cc B-series engines had different combustion chamber sizes and if you fit a smaller capacity, you will increase the compression ratio.

Modern unleaded fuels also have additives that were not foreseen in the Magnette design. In particular, be aware of:

  • Damage to rubber hoses. Older rubber compositions are vulnerable to attack by unleaded fuel and may harden, perish and crack. It is a good idea to replace any old flexible fuel lines with modern equivalents. Steel braid on hoses may mask the problem until the hose fails totally.
  • Erosion of lead. Lead is dissolved by modern fuels, so soldered joints in brass fuel line components may fail. If there is any sign of damage, re-solder the joints with modern lead-free solder. Remember, also, that the floats in your carburettor may be solder-jointed.
  • Volatility. In hot conditions, especially those experienced in the engine compartment, vaporisation can occur in fuel lines and the float chambers. In these conditions (e.g. when the car comes to a halt after a long fast run) the engine may splutter and stop unexpectedly which at best is inconvenient and at worst could be dangerous. Do whatever you can to separate the heat from the fuel. The MGA carburettor heat-shield can be fitted, the manifold wrapped with heat-proof blanket and fuel lines wrapped with insulating bandage (all generally obtainable from MG specialists). In modern cars, unused fuel is recirculated back to the tank, so the circumstances do not occur.


Ethanol or ethyl alcohol is used as a fuel additive but it can have damaging side effects if the concentration is too high. Up to a 10% dose is believed to be OK for our engines but be aware that in higher concentration it attacks metal, including steel, aluminium, brass, copper and cast iron. The carburettor body is vulnerable, being made of an alloy known as mazak.

At the time of writing, petrol quality standards mean  that normal fuels do not contain more than 5% ethanol but with growing pressure from the environmental lobby to increase use of the less polluting bio-ethanol fuels, this situation may change and the classic car movement will need to be vigilant. The Federation of  British Historic Vehicle Clubs acts on behalf of the vintage and classic car movements in representing our interests to legislators, regulators and other bodies and is already monitoring the ethanol issue. Discussions are now in hand about the proposal in the UK to introduce "E10", which is oetrol with 10% bio-ethanol content. This has bee sold in the USA for a while and the recommended self-protection measures are:

1. Teflon-lined fuel hoses.

2. Viton seals between float chambers and carb body

3. Teflon rings on the main jets.

4. Protect the carbs from heat. Fit the MGA-type heat-shield and wrap the manifold if you have serious vapourisation problems.

With more alcohol in the fuel, the risk is that bubbles will form more easily in hot conditions and the fuel mixture goes very lean. If you are hit by this in traffic a short-term fix is to pull the choke out to redress the balance until the carbs cool again.

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