Pressure on workshops
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GASKET failures almost invariably disrupt the operating schedules, so once /he fault has been found, pressure is put on the workshop to get the vehicle back on the road again as quickly as possible. This pressure must be resisted, since a rush in the later stages can lead to an unexpectedly early failure.
Job rushing can lead to trouble at all stages; to start with, the block and head faces must be quite clean, since small metallic particles can prevent the gasket sealing properly. Then, it is necessary to ensure all studs are screwed firmly into the cylinder block.
When the nuts are removed, it is all too easy to just loosen a stud which can loosen further in service, or can even strip the thread. So the first job, after cleaning all the surfaces, is to check that all the studs are tight with the aid of a stud extractor.
If there are a few threads showing above the surface, then the stud is not inserted properly, and it must be removed, and the reason for the problem sought out. Usually, the trouble will be caused by debris at the bottom of the tapped hole, and this should be removed and the threads cleaned up with a plug tap.
Incidentally, it is a dangerous practice to squirt oil into a tapped hole before inserting the stud, since the stud pressurises the oil, and it can exert enough pressurQ to cause the block to crack at the bottom of the hole. It is, of course, acceptable to apply a few drops of oil to the threads of the stud.
Some engineers advocate the use of a smear of grease on the gasket before it is installed, on the basis that this allows the gasket to settle in its correct position without creasing at all. With modern gaskets it is not . really necessary to lubricate and in some cases, the manufacturers advise against the use of oil.
It is best to follow the manufacturer's recommendaations in this matter, but if you do coat the gasket with oil or grease, make sure that you apply an even coat.
The gasket should not be creased at all when it is fitted, and so must be eased gently over the studs. Then, it is a case of fitting the head, and tightening the nuts in the correct sequence. Workshop manuals almost invariably include a tightening pattern, but the general principle is to work from the middle outwards. It's not a bad plan to tighten the nuts in two stages, but it is absolutely essential to use a torque wrench.
The torque settings on the larger diesels are pretty high, and you just can't judge these accurately by feel. In any case, if the gasket is to seal effectively, it must be held down by a uniform pressure — and that means tightening the nuts to the correct torque setting.
Lastly, with many gaskets it is necessary to retorque the nuts after the engine has run. Some engineers like the engine to be run up for a few minutes and then the torque settings to be checked, but it is a good practice to let the engine run for rather longer than this if the gasket is to be compressed fully before the nuts are retorqued — and the object of the exercise is to take up any compression of the gasket.
In any event, this is an important exercise with copper-asbestos gaskets, and must not be skipped — whatever awkward and time-consuming ancillary jobs it involves.