The introduction of unleaded fuel created a level of panic and misinformation rarely seen in the car world. Suddenly everyone needed to know whether their car would run on it without problems and firms offering miracle potions and other snake oil cures sprang gleefully out of the woodwork. Even now, 12 years after all new cars in the UK have had to be able to use unleaded from new, there is still a lot of confusion about the issue. Millions of pounds a year are wasted by people buying conversions they don't need and bolt on gadgets that don't work. They are preyed on by firms who rely on the subject being complicated and frightening enough that most potential customers don't really understand it and therefore waste their money for "peace of mind". To understand the topic fully you need to be an fairly well versed in physics, chemistry, metallurgy, engine design, combustion theory and Samurai sword making. Since that description only applies to about three people in the entire world, everyone else might as well carry on reading.

The Early Petrol Engine

One of the first things that early engine designers noticed was that valves and valve seats wear out. Hardly earth shattering news because all highly loaded mechanisms wear out of course but exhaust valves and seats tended to do it rather fast. Early engines were made out of cast iron which is a pretty decent structural material but not ideally suited to the temperatures and loadings experienced inside a combustion chamber. Valves were, and still are, made from various grades of steel. Valve seats were machined directly into the cast iron parent material of the cylinder head. The problem that became apparent was that the material comprising the valve seat gradually wore away leading to the valve sinking deeper into the "throat" of the port. This reduced and eventually eliminated the necessary valve lash clearance in the valve opening mechanism leading eventually to the valve not closing at all and the seat then burning out very quickly indeed. Research into this wear, or "erosion" revealed what was actually happening.

The Mechanism Of Valve Seat Erosion

The rate of wear was far higher than pure frictional or mechanical considerations alone could account for. If one were to construct part of a dummy engine consisting of just a valve and a cast iron seat and cycle it open and shut millions of times on a test bench, very little wear would take place. What this fails to account for is the temperatures that are reached inside a running engine.

Components inside the combustion chamber of a running engine are exposed almost continually to the heat of the burning air/fuel mixture. In the absence of any cooling this would be enough to melt the hardest steel. As it is, most of the heat is transferred away into the cooling system or flushed out as hot exhaust gas and the steady state temperatures of the various internal components obviously stay well under their melting points. The hottest component is the exhaust valve and its seat. All the heat picked up by the valve head has to be conducted away though the seat and to a lesser extent through the valve stem into the guide. The thinner the valve seat the less area there is for this heat to be conducted away - something many race engine builders ought to bear in mind. Temperatures in these critical seat areas can reach 800c which is enough to make steel glow red. This is still well below the melting temperature of steel (or iron) but enough to soften it somewhat and significantly reduce its strength. Part of the increased wear rate can be explained by this reduction in the material strength but it still only accounts for a fraction of the wear experienced. To understand the rest we need to make a quick trip to Japan.

In conventional welding it is necessary to melt both the parent material and the filler rod. A "weld puddle" is formed directly under the torch or arc in which the two materials melt, run together and then cool as one. There is another way of joining metals without melting them though. When a sword is made it is heated till it glows yellow and then hammered into shape. As the strip gets thinner and wider it is folded over onto itself and then heated and hammered again until the two layers join together and the process repeats until many layers have been rolled over and forged into one. The process is one of heat and pressure which together achieve a similar result to that of the higher temperatures of "melt welding".

Now back to valve seats. The temperatures over a seat won't be the same everywhere. High spots and minute flaws in the material can reach higher temperatures than their surroundings because they are less well cooled although they still won't be anywhere near melting. Under the pounding of the valve these imperfections form microscopic pressure welds between the valve and its seat just like the forging of a sword which then get burst apart next time the valve opens and a speck of material is lost. Each weld might be too small to see but over millions of cycles they combine to form erosion which wears away the seat and pits the valve.

The higher the temperatures inside the combustion chamber the more these welds are created and the erosion speeds up. So a very important factor is how hard the engine is used. At low rpm and small throttle openings there might be very little wear even when the materials are not ideal because valve and seat temperatures stay low. Under more severe operating conditions the wear rates can increase exponentially. What might be deemed acceptable wear rates then depend very much on the expected life of the engine and its operating conditions. Inlet valves are not generally a problem in any case because they run at much lower temperatures than the exhaust ones.

Solving The Wear Problem

The first and most obvious thing to do is use tougher materials for the valve and seat to resist the wear. Exhaust valves already had to be made of a very tough steel just to withstand the operating temperatures and it was the cast iron seat in the head that was the primary concern. Machining a recess into the seat area and pressing in a tougher steel insert provided an easy solution to the problem but was expensive. So research took place into additives to the fuel to find out if anything would help resist the erosion process more cheaply without modifying the cast iron seats. This also came about as part of the research into raising the octane number of fuels to enable higher compression ratios to be used without detonation and hence increase power and improve fuel economy. A substance that stood out as being very effective in both raising the octane number of fuel and also preventing seat wear was tetraethyl lead (TEL). How exactly it reduces the erosion is not clear but it seems that a thin coating of this material on the seat prevents the microscopic welds from forming. TEL was added to petrol in amounts up to about 4 grams per gallon depending on the level of octane boost required. As concern about lead emissions grew, the amount of TEL per gallon dropped until finally unleaded fuel became mandatory.

Aluminium Cylinder Heads

Aluminium has gradually replaced cast iron as the material of choice for cylinder heads until nowadays almost all petrol engines have them. Aluminium is much lighter and easier to machine but is far too soft for a valve to run directly on it. So all aluminium heads have to have separate seat inserts by definition. How well such inserts resist erosion depends on what they are made of but as a general rule any modern aluminium head will have inserts that are tough enough to resist erosion many times better than plain cast iron.

What Is Meant By "Suitable For Use With Unleaded Fuel"?

Here we start to get into one of the areas of confusion when people are trying to find out, perhaps from the OE manufacturer, if their engine is ok to run on unleaded fuel. Let's firstly be clear about one thing. There is no such thing as an engine where the valves and seats don't wear at all because these are after all heavily loaded components. What is really being asked here is "will the valve seats last an acceptable period of time on unleaded fuel?". The answer to that is really "acceptable to who?". It depends on the view about engine life that the OE manufacturer took in the first place. You might perhaps expect a Mercedes to be built from higher quality materials than a Ford and have a longer design life to start with. There are hundreds of different materials that can be used for valves and valve seat inserts and every car manufacturer will use different ones from different suppliers depending on cost and design constraints. Even when leaded fuel was the only choice there were engines that lasted a long time and engines that didn't. All we can really say therefore is that a car sold after 1989 in the UK will run on unleaded for as long as that manufacturer decided was a minimum design life for the engine. A car built before 1989 might run for a perfectly acceptable period of time on unleaded whether the manufacturer says it is ok to do so or not. And don't forget that there is very little incentive for an OE manufacturer to say that a particular pre 1989 engine type is perfectly ok for use with unleaded because if something wears out there might be a claim against them. The safe option is to say no. So there are plenty of vehicles that in my own opinion run fine on unleaded with very little wear despite the manufacturer saying otherwise.

When I'm cutting valve seats it's fairly easy to tell how hard the material is by how easily it cuts. There are certainly major differences in the types of insert material used by different car manufacturers or between different engine models from the same manufacturer.

The same question applies to someone taking their cylinder head to a machine shop for a "conversion to unleaded". What quality of materials is the machinist going to use? A higher quality insert might last longer than a cheaper one but in practice either of them is probably going to be fine for normal use.

It's worth restating that all this hinges greatly on how hard the engine gets used. Even a plain cast iron valve seat might last perfectly well if you never use the car hard. A supposedly "unleaded suitable" insert might wear out very quickly in race use.

So in summary there is really no such thing as "suitable for unleaded". There are just a range of materials which will have longer or shorter service lifetimes. Only with plain cast iron can one really say that it is unsuitable for unleaded in anything other than very gentle usage.

Can I Use Unleaded In My Engine?

All engines made after 1989 can run unleaded with no reduction in service life. For pre 1989 engines we can summarize all of the above into two rules - one for cast iron heads and one for aluminium.

Cast Iron - A cast iron head without exhaust inserts is not suitable for unleaded except for very gentle use.

Aluminium - Any modern aluminium head (let's say post 1975) will have inserts that are able to run on unleaded petrol, even when the OE manufacturer says it is unsuitable. What they mean by "unsuitable" is that they are unwilling to guarantee the same 200,000 mile or so service life that they would expect from a post 1989 engine. They do not mean that the engine will suddenly and immediately fail just because you run it on unleaded petrol. In practice it will have no impact on the average car owner at all given that very few people keep their cars that long. If you have an aluminium head from a major manufacturer (Ford, VW, Peugeot, Vauxhall, Fiat, BMW, Mercedes etc) then stop worrying and just run it on unleaded. The worst that can happen is that eventually, in 100,000 miles or so, the valve seats will have pitted enough to need recutting but by then the chances are that the rest of the engine will be worn out too. There is no way the inserts will just burn out though in the same way as can happen to plain cast iron.


All valves have to be made from very tough steels anyway just to resist the temperatures and stresses inside the engine. The same steels are used whether the engine is designed to run unleaded or leaded fuel so in effect it is not an issue. Nearly all modern petrol engines use the same types of steel for valves with small variations in the alloying mix. Inlet valves are generally made from a single forging of EN52B or similar. Exhaust valves have to cope with much higher temperatures on the valve head so they need a higher grade material there. Exhaust valves use EN52B for the stem part of the valve which is then friction welded to the much tougher 21/4N steel for the valve head. Why use two types of steel in one valve? Simply because 21/4N is more expensive and as it is only needed for the head of the exhaust valve there is a small cost saving in using EN52B for the stem. The saving in material cost outweighs the cost of the friction welding operation. It might be minor but when you are making millions of valves it all adds up.

The simplest test the home mechanic can use to tell which type of steel a valve is made from is with a magnet. EN52B is magnetic and 21/4N is not. If you take a magnet and an exhaust valve it is quite easy to find where the friction welded join between the two types of steel lies as the magnet will grip the stem until the point at which the material changes to 21/4N. The only important issue for use with unleaded is that the head of the exhaust valve be non magnetic 21/4N and as it is certain to be that anyway on a modern engine you don't need to change the valves for use with unleaded. If in doubt just apply the magnet test. If the valve is all non magnetic then it is one piece 21/4N but that is of no extra benefit really in a road engine. The valve stem doesn't get hot enough for it to matter which of the two types of steel it is made from.

Valve Guides

Valve guide material is not an issue with regards to unleaded petrol. You do not need bronze guides with an unleaded conversion. In fact many post 1989 engines still use cast iron guides by choice.

What Do I need Then With An Unleaded Conversion?

With a cast iron head you just need steel seat inserts fitted on the exhaust valve side.

With an aluminium head you don't need an unleaded conversion in the first place. The chances are that even if you take an aluminium head for a "conversion", absolutely nothing will get done to it except maybe the seats will get recut. If the seats finally burned out several years and many thousands of miles later, do you really think you'd have any chance of proving that the inserts didn't get changed for better ones? To do that you'd have to have the inserts machined out again, sent to a lab for chemical analysis and compared with the same analysis for known OE inserts. You'd be way past any possible guarantee period even if you bothered doing all that. So hands up anyone who thinks they can now tell me just what the incentive is for those companies that advertise unleaded conversions on aluminium heads that never needed a conversion anyway to actually waste time and money doing anything constructive with your head when they can charge you the same for doing nothing.

Ignition Timing

The first issue with unleaded is the one of physical wear of the valve and seat. The second one is whether the engine will detonate - otherwise referred to as "pinking", "pinging" or "knock" depending on where you come from. Unleaded fuel usually has a lower octane rating than leaded and also burns differently. It might be necessary to use a bit less ignition advance with it but on many cars the standard setting works fine. There's a simple rule - if the engine pinks, usually heard at low rpm and high throttle openings,  then retard the ignition timing a couple of degrees. If not then leave it alone. Pinking sounds like a light metallic rattle, a bit like a nail being shaken about in a tin can. In small doses it does little harm but in severe cases it can damage pistons and cylinder heads.

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