There are times when an engine will miss-fire, use excessive oil, overheat or be just plain unhealthy, when all that needs to be done is to overhaul the cylinder head. It is here that most D.I.Y. people go wrong. We will deal with this under 4 headings:
• Valves, Guides and Springs
• Head Gaskets
• Hydraulic Cam Followers
Valves, Guides and Springs
It is commonplace to have a number of configurations of valves in four cylinder engines. 8 valve (4 exhaust and 4 inlet valves); 12 valve (4 exhaust and 8 inlet valves); 16 valve (8 exhaust and 8 inlet valves) and sometimes 20 valve (8 exhaust and 12 inlet valves). The basic overhaul procedures stay very much the same. The only problem that arises is the amount of space available, particularly in multi-valve engines, for them to operate in. The basic premise is to replace worn or burnt valves, warn guides and ineffective stem seals. To ensure that a valve seals the combustion gasses properly, you either need some very sophisticated equipment or really know what you are doing. By this we are not saying that from a D.I.Y. point of view this cannot be done, but that unless it is done correctly the problem won’t be solved. The following areas need to be checked and measured:
- Valve stem wear
- Cotter groove wear
- Guide bore wear
- Configuration of the entire valve train
- Valve seats
Valve stems do wear without the valve head burning. This is caused mainly by lack of regular maintenance and foreign matter in the oil. If the clearance between the valve stem and the guide is too big then oil will seep through into the combustion chamber and cause the engine to “smoke” and use oil. Irrespective of whether the seals are new or not. Cotter groove wear can be very dangerous. If a cotter comes off, there is nothing to stop the valve from dropping onto the top of a rapidly rising piston. Very expensive damage can occur. Guides are made to be sacrificial. In other words they are meant to wear, not the valve stem. They often wear oval. This is because the tappet is pushing it down sideways instead of straight down. This is one of the reasons that the entire valve train configuration should be checked.
But why should this change, you ask? Generally speaking, a head is only overhauled because 1) the engine has been over heated; 2) there is lack of compression on one or more cylinders and 3) the engine is smoking and using oil.
There are many cases when, in fact, the first has caused the other two!! They are not necessarily independent of one another, particularly in overhead camshaft (OHC) engines. Picture the scene – OHC heads are generally made of aluminum and camshafts are made of cast iron. Expansion rates cannot be the same. All the parts consisting of the valve drive train are situated and fixed to the head except the overhead camshaft, which runs in what are known as “saddles”. The camshaft won’t bend. The head will. Miss-alignment then occurs. Valves are not pushed straight down, causing the seals to distort and the guides to wear oval; and because the valves are not seating properly, they burn. The re-alignment of the valve train has to be checked and rectified with special machinery, before any valve, guides, etc. are replaced.
Valve springs are generally overlooked when cylinder heads are overhauled. Did you know that in the design of the valve train, manufacturer’s engineers have specifications for the speed in which a valve must retract? Crazy, but true. In order for this to happen correctly, there are measurements for what is called the “free”, or uncompressed length, as well as the measurement of pressure needed to compress them. Ever heard of “valve bounce”? Weak springs or springs without the correct length cause this.
Cylinder head gaskets are made to seal the joint between the head and the block and are quite technical in their make-up. A head gasket not only seals off the combustion chambers but also seals off water and oil galleries. The material that a head gasket is made of, cannot do this alone. It requires what we call “clamp load” as well. This is the pressure on the gasket exerted by the cylinder head when the bolts have ”torqued” down correctly. Should you require the torque settings for the cylinder head bolts on your engine – consult the manufacturers care manuals. If your engine takes what we call “stretch” bolts, please replace them. They are not costly and are generally available from any of our branches.
You need to make sure of the following when replacing a cylinder head gasket:
- Is the surface of the head flat? Does it need machining? If your engine has overheated then it will most likely need to. Take it to your local motor engineer to check.
- Does the surface finish comply with the gasket manufacturer’s specification?
- Is the “deck” or surface of the block flat? Not many people check this, but it is important to do so.
- Remember, as in most things, you get what you pay for. How much did you pay for your head gasket?
- Have you set the tappet settings? It is generally easier to set these with the head off the engine.
- Before you insert the bolts, whether they are stretch bolts or not, put oil on the threads and under the bolt head. DO NOT SQUIRT OIL INTO THE BOLT HOLES. Ensure bolt threads and holes are clean.
- Are you replacing the cam belt? Ever seen a head after a cam belt has broken? It’s quite a mess. Save yourself a lot of time and money – REPLACE IT.
- Do not use any oils, grease, silver paint, copper slip, sealants or glue to the gaskets – The warranty will be void by the manufacturer.
If your engine did not need a thermostat, the manufacturer would not have put one there. Do not run your engine without one. They are not expensive parts and your engine will run a lot better with a good one. In order for an engine to operate optimally, it needs to have a constant temperature. The rings, bearings, gaskets and mainly the oil need to run at a specific, constant temperature. If this does not happen then the rings loose their tension against the bore walls and the bearings suffer premature wear because the oil is too thin and will have lost its viscosity and the oil seals will harden and leak. It doesn’t pay. If your engine overheats with a new thermostat, then you have a problem elsewhere. Check your water pump and have your radiator cleaned by a professional radiator business.
Hydraulic Cam Followers
Hydraulic cam followers can be an absolute pain. The main thing to remember is that they do not work too well on old or thin oil. The rattle that you get first thing in the morning is not serious. They are just waiting for the oil to get to them. Just don’t rev the engine. If they rattle all the time, even on new oil, then we would suggest that you replace them, because they are not opening the valves as they should. We carry a fair range of good quality hydraulic lifters. In some cases you can revive them by getting all of the oil out of them. Do not use an oil solvent.