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Unleaded Fuel

Leaded fuel may have been unavailable since 2000, but there is still some confusion concerning what can and can’t be used. Here is a list (as published by Ford Motor Company) of engines which can be continuously run with unleaded petrol.

Cosworth YB Engines

All heads have hardened valve seats fitted. However 95 RON unleaded fuel should only be used in standard green or blue top engines. Red top engines and all modified engines must run at least 97 RON super unleaded fuel and/or octane booster.

CVH Engines

CVH Engines: Those built up to 12/84 may only be run on unleaded fuel if ‘LPG’ is stamped on the boss of the head above No 1 exhaust port. From 10/85, all engines produced are suitable for running on unleaded fuel.

DOHC Engines

All engines are suitable for running on unleaded fuel.

Essex V6 Engines

All engines are unsuitable for running on unleaded fuel.

SOHC Pinto Engines

All engines produced after 01/89 are suitable for use with unleaded fuel. For all other engines, an identification mark (adjacent to No 4 spark plug) must be present to designate suitability for unleaded. These are:
* 1.6 - M, MM or N, NN
* 1.8 - S, SS
* 2.0 - P, PP or R, RR

Cologne V6 Engines

All 2.4 and 2.9 are suitable for unleaded fuel. 2.3 and 2.8 must have the following codes stamped in the centre of the cylinder head exhaust flange to designate suitability for unleaded:
* 2.3 - B or F
* 2.8 - D or E

X/Flow & Pre-X/Flow Engines

All engines are unsuitable for running on unleaded fuel.

Why Can’t Older Engines Use Unleaded Fuel?

By far the biggest problem arising from using unleaded fuel on older engines is a condition known as valve seat recession (VSR). Without the protective lead coating (previously provided by leaded fuel) on the exhaust valve seats, the intense heat (650°C) and hammering effect of the valves opening and closing, causes iron deposits from the valve seat to become micro-welded to the valve edge. Left unchecked, this continual tearing away of metal particles will result in the exhaust valve digging a deeper and deeper hole for itself into the cylinder head. Eventually, and often within only a few thousand miles, the engine will breakdown completely and it will require an expensive overhaul. To combat this problem, modern petrol engines have special hardened valve seats which can withstand this harsh environment.

Solution A: Use Lead Replacement Fuel (LRP).

Although this fuel may have replaced leaded fuel at the pump, it is by no means a direct replacement. Petrol manufacturers only recommend that it is used for light duty applications. It is not suitable for regular motorway use, towing or high performance applications because the fuel burns much hotter than its predecessor. Its octane rating also seems to be an unknown quantity.

Solution B: Use Anti-Wear Additives.

All additives will have one of the following four compounds as parts of its constituent - potassium, phosphorous, sodium or manganese. All these compounds have been advocated by various companies as being the most suitable substitute for lead petrol. The truth is that none of these compounds will totally replicate the unique properties of lead and certain engine types and driving conditions may be better suited to one formula than another. Even the most suitable additive will only delay the amount of wear unleaded fuel cause to an engine’s valve seats.

Solution C: Fit Unleaded Valve Seat Inserts.

This is the only permanent solution to valve seat wear. At Burtons we can provide this service for just about any petrol engine type. The operation involves the cutting of a recess around the exhaust seat to accept the interference fit of a suitable valve seat insert. Special heat treated alloys are used in the production of these inserts, the two most common being nickel/chromium and vanadium/molybdenum. Both the machining and the fitting processes are critical for ensuring a trouble free service life and we pride ourselves in being able to provide the expertise and specialist machinery required for this precision work. And finally, the answers to the three most asked questions concerning unleaded conversions:

First the old valve seat is milled out.

After careful cleaning, the recess and insert are checked for fit.

The new hardened seat is then pressed into position.

The new seat is then cut to the correct angle for the valve face.

Finally the valve and the seat are lapped to ensure a perfect seal.

Why do you only fit hardened valve seats on the exhaust side?

Only the exhaust valve seats have to be replaced since they run at a much hotter temperature than the inlet valves.

Do the valves have to be changed for stainless steel types?

No, provided your existing valves are in good condition, they can be refaced and lapped into the new valve seat. You do not need a valve made from a harder material.

Do the valve guides have to be replaced with bronze types?

No, this is not true. Both cast iron and bronze guides are suitable for use with unleaded fuel. However, we may need to replace the valve guides if they are excessively worn. This is because the spindle of the valve seat cutter centres on the valve guide and any excessive movement will make machining impossible.


Once an engine has been converted for unleaded use, remember that only super unleaded fuel is a suitable replacement for leaded fuel as far as octane ratings are concerned. Use of premium unleaded fuel will require changes to ignition timing due to a lower octane rating. We recommend that turbo engines only use super unleaded fuel to reduce the risk of detonation.