SETTING THE SEAL
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Oil and water leaks are not only annoying but can cause MOT and driveline failures as well. Phil Reed talks to gasket and jointing compound manufacturers to find what to use where and why engines still leak.
Gaskets? Not much to them, they're just pieces of paper or whatever you put between joint faces to stop leaks. So the popular myth goes.
So why do we still get oil leaks on engines, and why do head gaskets blow?
Two leading gasket makers, Cooper Payen and James Walker, say that the majority of gasket problems are due to using cheap, unsuitable items, incorrect fitting, or a lack of understanding of a gasket's role.
A gasket will not work properly unless it is made of the right material for the job. As Walkers points out, a gasket maintains the tension in the bolts holding the joint faces together, and to do this it has to have a resilience, which cheap gaskets may lack. It also has to be made of a suitable material to resist the liquid or gas that it's containing and any heat encountered.
This is particularly important if you have to make a gasket from a sheet of material. Gaskets in a cooling system, such as a thermostat housing, use the smooth type of gasket material, usually called "brown paper" — though it's very different from parcel wrapping paper. It is made to withstand high temperature antifreeze and anti-corrosion agents in a cooling system. In more general use, where there is oil splash but not high pressure, a softer fibrous type of material should be used. This often contains rubber and other compounds which make the gasket swell when it comes into contact with oil and forms an efficient edge seal inside the joint. Always store this type of gasket or material in a clean, dry place. If you get oil or grease on it before use it will already have swollen up and will not seal the joint properly.
Obviously, because a gasket is thin, it can only do its job properly if the mating faces are true and the material is clamped over its whole area. Fitters appreciate that if a cylinder head face is warped then the gasket is not going to last very long, but not all realise the importance of clean joint faces. When you take apart a joint there is always an accumulation of dirt, oil and general muck round the bottom of the studs even after wiping the face clean.
Casket makers realise this, that is why they allow a generous clearance in the holes round the studs. However, this also enables the gasket to move before the joint is tightened, so it is possible for one edge of a clearance hole in the gasket to be tight on the side of its stud. If there is dirt round that stud, the gasket creases up, does not make proper contact and leaks.
Its obvious that the brutal removal of old gaskets or compounds with a scraper or cold chisel can damage a soft joint face very easily, but some fitters unwittingly provide a leak path when they try to make sure that the joint face is flat and clean. Many put a length of emery cloth on a flat surface and rub the joint face across it until it is clean. It is a good way of doing the job, but you should always rub with a circular motion, not back and forth, because that can create grooves across the face of the joint allowing seepage.
If joints are held by bolts, you can find that the tension of the bolt has raised a small ring round the threaded hole. If this happens, relieve that edge of the hole lightly with a countersink bit. On items like sheet steel covers, it is common to find that the area round the bolt clearance hole is slightly domed on the joint face. This is often caused by not using a flat washer under the bolt head to spread the load. You should always check these joint faces and, if necessary, dress them down flat with a hammer and dolly.
Compress liquids
Threaded bolt holes on a joint face can be a real trap for the unwary. When you take a head off and clean the block face you are always going to get oil, water and dirt down the bolt holes — you cannot avoid it. But what you can avoid is leaving it there. You cannot compress liquids, so if the hole has oil or coolant left in it, the bolt will bottom on this before clamping the gasket. It takes only a few minutes to go round the bolt holes with an air line and blow them clean, but do wear goggles. Check also that the threads are not tight because this can also give you a misleading torque reading. If they are tight, run a tap down them.
To get a gasket properly seated and clamped between the joint faces, it is essential to tighten the bolts in the proper sequence. Most people know this about cylinder heads and workshop manuals give diagrams, but it is just as important on all other joint faces. Tightening "round the clock" puts an uneven clamping pressure on the gasket so that it stays tight one side and frets on the other. So the bolts loose their tension where the gasket frets, and do not hold the joint tight. Where there is not a tightening sequence for joints in the workshop manual make sure the gasket is evenly clamped by thinking or drawing a Union Jack. You make an upright cross first, and then a diagonal one. Tighten the bolts round a cover like this, repeating it until they are fully tight, and you will not go far wrong.
Jointing compounds
What about jointing compounds? Not surprisingly, gasket makers do not like them. Cooper Payen believes they should not be used because some can react with the surface coating present on many modern gaskets and this prevents the gasket from bedding down properly. Walker was not quite so adamant, though it said that jointing compounds should not be necessary unless the joint face is damaged or corroded, and even then they are not a complete cure, only a palliative.
It did agree, however, that a jointing compound has its uses in some applications such as sticking a gasket on to a sump or crankcase during positioning, or for attaching one side of a rocker box gasket to the head so that it does not delaminate and tear when you take the cover off to adjust the tappets after the engine has bedded down. Both companies say you should never use oil or grease on gaskets. It dries out allowing the bolts to loosen and the gasket to go walkabout.
However, Hunting Lubricants, which makes Hermetite, did not agree. It maintained that using the right type of jointing compound with a gasket helps it to seal and prevent leaks. Jointing compounds are made in three basic types: hard setting; semi-hard setting; and non-setting.
Narrow oilways
It says, semi-hard setting is the type that should be used with most joint faces on an engine except for the cylinder head. There it recommends a special hightemperature compound formulated specially for this purpose. There are two sorts, one for cast-iron heads and one for aluminium. However, Hunting did agree with the gasket makers that there is a danger of using too niuch compound so that it oozes out inside the joint and gets into the engine's oil supply where it can clog narrow oilways, cause oil starvation to bearings and clog up filters. To get over this problem, some compound manufacturers use aerosol sprays making it easier to get a thin even coating. With the number of seals and gaskets used on modern vehicles it is difficult to criticise fitters or managers for wanting a one-tube-seals-all solution, Unfortunat
ly, one does not exist. What is clear, however, is that joint cleanliness and the use of proper procedures is vital — whichever sealing method is employed.