What is Valve Seat Recession?

Valve Seat Recession (VSR) is the tendency for the valve seat sealing face to shrink upwards into the cylinder head. When this occurs, the valve will be pulled further up into the cylinder head to meet the shrunken (recessed) valve seat by the powerful springs which are used to close it. Eventually, the point is reached where the valve head and stem have moved up so far that the top of the valve stem is touching the cam follower, tappet or whatever for all of the time. There should be a clearance gap here which normally ensures full and correct sealing of the valve. If there is no clearance gap, the valve may be held slightly open by the contact with the cam follower or tappet. If this happens the cylinder involved cannot be truly gas tight. The condition may result in the affected cylinder producing far less than its normal power or produce no power at all. This is because the affected cylinder will have low (or no) compression due to the leakage at the valve.

What are the causes of VSR?

The short version(s);

There are at least 3 reasons why engine valves recess into their seats;

1. Brinelling (flowing or change of shape of the metal that makes the valve heads and seats)

2. Erosion (through friction, resulting from the rotary motion of the valves when closing, caused by the helical valve spring(s)

3. Microwelding (ask your local Metallurgist or research on the 'net)

Note that these factors and their combined effects are present when an engine runs on ANY fuel.

A breakdown of these effects;

Brinelling (factor 1).

Brinelling is well demonstrated by a Blacksmith who heats the metal he wants to re-shape in order to make it flow (Brinell) more easily under his Hammer. The metal being worked on is said to become 'softer' (more Ductile) when it is heated. The more the metal is heated (up to the point where it becomes molten or liquid), the softer it will become. The Blacksmith could re-shape the metal when it is cold although it would take him far longer. From this we can see that more heat can accelerate the rate of Brinnelling.

In addition, the rate of change in shape or flowing of the metal depends on its hardness and ductility at any given temperature. Lead, Copper or Gold would be easy to work when cold, whereas Tool Steel would have to be glowing bright yellow with heat before a heavy hammer could make any dent in it. Metal hardness is therefore a factor directly affecting the rate of Brinelling.

In a Piston engine, the valve heads are taking the Blacksmith's place, doing the hammering on the valve seats. The softer those valve seats are, the more they will change in shape as their metal flows. As in the Blacksmith's Forge, it is clear that the rate of Brinelling can be accelerated by more heating of the valve seat and valve steam /head.

To carry the useful Blacksmith analogy a little further, the effect of Brinelling can be increased by using more force or a heavier hammer (which amounts to the same thing). Engine designers would like to make their valves as light as possible in order to reduce the force although they have to be made of enough metal to do their job adequately. One way of reducing the force exerted on a valve seat by the valve head (the Hammer) is to use two valves to do the job of one. This will almost halve the weight of individual valve heads. This solution is demonstrated in engines having 3 or 4 valves per cylinder (called Desmodromics). Many think that 3 or 4 valves per cylinder are used to improve the 'breathing' or gas flow of an engine where, in fact, the method is used to reduce the weight of the valve heads and thus reduce the force exerted on their valve seats. If this design solution is used it will minimise the effect of Brinelling and retard the onset of VSR.

Erosion (factor 2)

This effect cannot be avoided by using any pure fuel, it is purely a mechanical problem which can only be overcome by stopping the valve heads and stems rotating. Efforts to resolve this problem are made by engine designers, the most common solution is to use two springs with opposing helixes. Because one spring must be smaller in diameter than the other (one spring must fit inside another in an axial fashion) then the turning forces cannot be totally balanced and valve head/stem rotation will still take place but it will be reduced.

Another partial solution for Erosion is found in lubrication of the valve head and seat. If we apply lubrication to surfaces that are eroding each other then less molecules will be worn off each surface. Lead used to be added to petrol because it provided a form of lubrication for the valve head and seat sealing faces. Nowadays, Lead is not added to Petrol so that lubrication benefit is gone.

Microwelding (factor 3)

Metallurgy shows us that any two similar metals that are truly clean (right down to a molecular level) will begin to stick back together if left touching for any length of time. The longer they are left this way (when the engine is not running), the more similar the metals are and the cleaner their surfaces are, the more quickly they will begin to Microweld.

When two Microwelded surfaces are pulled apart, molecules of metal are pulled from both, resulting in surface damage. After prolonged surface damage due to Microwelding the valve head and seat may not produce a gas tight seal any more. The simplest way to prevent or retard Microwelding is to add 'Dirt' to the surfaces of the two pieces of metal. That 'Dirt' could be the Oil we use to stop things siezing up, it could be Polish or Paint of some kind, it could also be the Soot produced by burning some fuels. It is convenient that Petrol combustion produces lots of carbon (Soot) whereas it is a little inconvenient that LPG burns much cleaner than Petrol and does not leave Carbon deposits. Thus, the cleaner combustion of LPG and other fuels (CNG for instance) can actually promote Microwelding.

Why do some engine types suffer VSR when others do not?

If the factors above have been understood it is clear that there are two differences between running an engine on Petrol or LPG.

Microwelding

In essence, valve wear will occur slightly more quickly when running on LPG simply because it burns much more cleanly. There is no Soot or Dirt to prevent the occurrence of Microwelding. Don't let that worry you, this effect alone is very small. We are still talking about Valve life that is often longer than the life of the car.

Microwelding can be avoided, at least in part, by having the valve head and seat made out of metals that are as dissimilar as possible. Carbon steel valves and Stellite seats provide a good solution here. Another weapon in our anti-Microwelding arsenal is provided by fuel additives such as 'Flashlube'. These come in the form of light Oil which, if introduced in the correct dosage can lubricate the valve head/seat faces and prevent Erosion. Also worth mentioning is that the Flashlube oil will eventually burn. When it does, it will replace the missing Soot (Carbon) that can do a lot to prevent Microwelding. Injection of these tiny amounts of oil must have some cooling effect on the valves (like petrol does) although it is probably so small as to make no difference to valve temperatures.

A word of warning here - If a modern engine is Overdosed with Flashlube or similar, its exhaust emissions may well be compromised and damage to the catalytic converter will probably result.

Running Temperature of the Valves and Seats

The most common misconception you'll hear about LPG is that it makes an engine run hotter than Petrol. This is no more than myth.

LPG has only 85% of the calorific value of the same amount of petrol so it is impossible for it to create more heat. It can only make 85% as much.

There is one factor missing though - The cooling effect that squirting wet petrol mist into the cylinder has on the valves. When running on LPG the gas is injected (most often) as a vapour and does not have quite as much cooling effect on the valves. As a result, LPG running will see very slightly higher valve temperatures. This is why 'soft' valves will tend to recess into their seats when run on LPG. (Go back to the Blacksmith analogy in 'Brinelling' above to refresh yourself on the effect of more heat on the process)

Having established that fact, slightly higher valve temperatures only present more risk of VRS if the valves are too soft for the job. Engines that suffer the most are some of the Fords, Hondas, Jaguar V6's (actually a Ford engine) etc., although over 90% of engines made today are just fine. You'd be wise to check with the manufacturer of your engine and your LPG installer before conversion to see if your engine has suitably hard valves.

What kind of engine is least suitable for LPG conversion?

The worst kind of engine for LPG conversion would have badly designed (soft) valves which will not resist Brinelling and Friction as well as harder valves would. In addition, it is known that the more similar the metal that makes the valve heads and seats are, the more Microwelding will occur. Weight of the Valve Head and Stem is also as important as the hardness of the valve seats. An engine with 4 valves per cylinder is more likely to resist VSR than an engine with only 2 per cylinder, although if those 4 valves are soft there will be no benefit. If the engine does not have hard valves it is wise to begin by having the valves re -engineered to the required standard using parts made of much harder material (often 'Stellite'). In addition, an engine with single helical valve springs will suffer more Erosion due to Valve rotation.

Can an Engine with Hard Valves be damaged?

Valve damage can occur when running on any fuel.

If the valves are made to run too hot due to incorrect fuel/air mixtures this may make Brinelling and resulting VSR much more likely. Incorrect fuel/air mixture, especially if too weak, can overheat the valves and result in accelerated Brinelling. It is important for the correct mixure to be supplied to the engine whether it is being run on Petrol, LPG or any other fuel.

Fitting Harder (normally Stellite) Valve Seats

This needs the services of a specialist engine re-manufacturer (re-conditioner). It is a wholly seperate business, requiring a large investment in machinery and the skills required. We cannot offer this service and it is unlikely that any other LPG conversion company will be able to do so. If you decide to go ahead with conversion of an engine with soft valves it would be best to begin by having the valve work done prior to conversion. A search for local engine re-manufacturers (perhaps in Yellow Pages) would be the way to go.

The Upshot

It would be wrong to heap the blame for VSR on LPG. If an engine's valves are truly fit to run on modern unleaded petrol (for any reasonable length of time) they are good enough to run on LPG with no additional problems. It must be remembered that all of the effects described above still happen when running an engine on Unleaded Petrol. The plain fact is that ALL engine valves will reach the end of their useful life at some point. Nothing lasts forever, although harder valves and seats made of quite different materials will definately last longer when running on any fuel.

Steven P. Sparrow BSc (Hons)

February 2008

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