Diesel Engines -Tools and working practices

A lot of work on an engine involves removing and replacing components, so it involves dealing with a wide variety of fastenings – literally, ‘getting down to the nuts and bolts’.

Bolts vary in length and diameter, but come in a number of standard sizes quoted in imperial or metric measurements. Metric measurements are now used almost universally on British, European and Japanese equipment, while imperial sizes are found on old British equipment and on almost anything intended primarily for the American market. This means that you are unlikely to come across a mixture of the two on one engine, but doesn’t mean to say that you won’t fi nd imperial fastenings on the engine and metric elsewhere on the boat – or vice versa.

Even if you find two bolts of exactly the same diameter, they may not be interchangeable because there are a variety of different ‘standard’ screw threads which differ in crosssection as well as in the number of threads per inch (Fig 32). It’s important to make sure that you match the right nut and bolt together and that you screw bolts or studs back into the holes they came out of, because although some odd combinations are compatible, the vast majority are not. Unless a nut or bolt is clearly in poor condition, it should turn smoothly and easily until it reaches the final tightening-up stage: if it starts easily but suddenly becomes stiff for no obvious reason, or if it feels unusually floppy, it’s a pretty safe bet that it’s the wrong one for the job.

Screw thread sizes

If the differences between screw threads seem subtle, the differences between their heads certainly are not: it is obvious that you can’t use a spanner to undo a bolt with a domed and slotted head intended for a screwdriver! It’s surprising, though, how often DIY mechanics fi nd themselves trying to work with spanners or screwdrivers that don’t quite fi t. It’s important to use the right tool for the job, because although a 13 mm spanner will just about cope with a 1/2 in hexagon head, it is slightly too large. The difference is only about a quarter of a millimetre, but that is enough to allow the nut to twist between the jaws. Instead of the load being spread across the flats of the nut, it’s then concentrated at the corners, so as soon as you try to apply any appreciable force to the spanner, the nut levers the jaws of the spanner apart, while the spanner rounds off the corners of the nut.

Spanner sizes, Screwdriver shapes

Similar comments apply to screwdrivers – probably the most misused tools of all. Not only do you need the right kind of screwdriver – flatbladed for slotted heads, and cross-point for cross-heads – but it should also be the right size for the job. A screwdriver that is too big won’t go into the slot at all, but one that is too small will tend to twist out of shape and damage the edges of the slot. A flat-bladed screwdriver should be ground to a fairly shallow taper, so that it doesn’t try to lever itself out of the slot, and have a sharpedged square tip so that it doesn’t hit the bottom of the slot before it makes contact with the sides.

The drive towards more compact engines has made engine manufacturers turn increasingly to fastenings with ‘unconventional’ heads. Allen screws, in which the head of the bolt has a hexagonal recess, are now so common that they hardly rank as unconventional. Dealing with these requires either a screwdriver equipped with a selection of appropriate ‘bits’ or a set of Allen keys – bent pieces of hexagonal hardened steel bar. You may need lots of them: Allen screws come in a variety of metric and imperial sizes, and a good fit between the tool and the fastening is even more important than for a spanner – not least because if you damage the head of an Allen screw it can be exceedingly difficult to remove.

Pipe fittings

Pipe fittings also come in a variety of shapes and sizes, designed to cope with pipes of different materials and different sizes, and with operating pressures ranging from partial vacuum to several thousand pounds per square inch. There are, however, only three main types that you are likely to come across.

Pipe clips are bands of thin metal designed to fit round flexible pipes. A bolt or screw joins the ends of the band so that it can be tightened to compress the pipe against the rigid spigot to which it is attached. They’re found in all sorts of relatively low-pressure applications, from marine toilets to engine cooling systems and exhaust pipes.

Pipe fittings

Removing a hose clip is a simple matter of unscrewing the clip until it is loose enough to slide along the hose and then pulling the hose off the spigot. In practice, a common problem is that the hose may have glued itself to the spigot, in which case it may come free if you massage the hose to loosen the bond, and then prise it off with a screwdriver. As a last resort, a flexible pipe can always be cut, but do make sure you have a replacement available before you do so.

Replacing a hose can be more difficult, because it may be such a tight fit on the spigot that you wonder why it needs a hose clip at all. Dipping the end of the hose in boiling water may help by softening it, and a smear of washingup liquid can be used to provide some gentle lubrication. Whatever method you use, it’s worth threading the pipe clip on to the hose first, so that you don’t have to unscrew it completely in order to fit it on to the pipe in situ.

Fitting the pipe on to the spigot is especially difficult if the spigot has a bulge or ridge around its end, but these things are not put there just to make life awkward: they are intended to provide extra security once the pipe is in place. They only work, however, if you make sure that the hose clip is on the right side of the ridge – nearest the root of the spigot so that the pipe would have to drag the pipe clip over the ridge in order to pull itself free.

Compression fittings are used on rigid pipes, or occasionally on flexible pipes with a rigid insert. There are two types, but both look like an unusually deep nut with the pipe sticking out of the middle.

One kind, used mainly on relatively lowpressure applications such as domestic plumbing and sometimes on the low-pressure side of the fuel system, uses a straight-ended pipe with a brass or plastic ring called an olive threaded on to it. The end of the pipe fits into a recess in the spigot, but the olive rests on top. Then, when the nut is tightened down on to the threaded spigot, the olive is compressed between the ‘nut’ and the spigot, to grip the pipe and form the seal.

The only cause for concern when working with this type of pipe fitting is that overtightening it can distort or split the olive.

High pressure pipes, such as injection pipes, use a development of this principle in which the pipe itself is shaped to form the olive. Making the joint in the first place requires special equipment, but once the pipe has been shaped it forms a secure and leak-proof joint that is as easy to do up or undo as a nut and bolt. Perhaps the biggest danger is the temptation to treat these kinds of joint as though they are flexible: they’re not, so don’t try to bend or move the pipe without slackening the unions that hold it in place.

The third kind of pipe union is called a banjo bolt – so-called because one part of it is supposed to look like a banjo. The ‘banjo’ is a metal fitting that has to be brazed or soldered on to the pipe, and which then forms a hollow channel right around the central bolt. The ‘bolt’ part looks exactly like a conventional bolt, except that its head is often rather larger and thinner than you might expect, and its shaft is a hollow. A hole in the side of the bolt allows liquid to flow from the pipe, around the circular channel formed by the banjo, and down through the hollow bolt. Brass, fibre or nylon washers form a seal around the upper and lower edges of the banjo to stop leaks: be careful not to lose them when undoing a banjo bolt, and make sure that they go back when you replace it.

Seals and gaskets

Other joints in an engine, such as those between the face plate and casing of a water pump or between the injectors and the cylinder head, are just as important as the joints in the ‘plumbing’ that surrounds it. It’s difficult to get a good metal-to-metal seal over a wide area, especially as the parts concerned may be expanding and contracting at different rates when the engine warms up and cools down. To overcome the problem, these kinds of joint usually include a gasket of more resilient material.

Some gaskets, especially cylinder head gaskets, are quite sophisticated components in their own right, including layers of different materials and inserts to withstand particular stresses or to help them stay in shape, but the majority of gaskets are relatively simple sheets of steel, copper, cork, rubber or paper.

Copper gaskets are used in small, highly stressed joints such as between an injector and its seat, and usually look much like an ordinary washer. Ideally, a copper gasket should only be used once, because it loses its resilience once it has been fitted. If necessary, though, it can be renovated by heating it over a gas ring until it turns blue, and then immediately dropping it into cold water.

At the other extreme are cork or rubber gaskets used in places such as the joint between the rocker box cover and the cylinder head. There are no great stresses involved here, but the rocker box cover is so thin that it would easily distort if it had to be bolted down very firmly to achieve a seal. The only snag with these is that the gasket almost invariably sticks to one or the other of the two components: if part of it sticks to one and the rest of it sticks to the other, you may need to peel it away very carefully with a thin knife to make sure it comes away in one piece.

Paper gaskets are less resilient than cork or rubber, but they work well and are so cheap that although it’s almost impossible to salvage one once it’s been used, there’s no excuse for not using a new one every time. If you can’t get a ready-made replacement, it’s easy to make your own from a sheet of gasket paper. In a real emergency, it’s even possible to use writing paper or a breakfast cereal packet as a stopgap!

The first step in the process of replacing a gasket is to remove all traces of the old gasket from the surfaces. You may need to use a scraper, but be careful not to scratch the machined surfaces – if you must use a screwdriver for the job, file the corners off first! Then, if you’re making a new paper gasket, smear one of the mating surfaces with a little oil, and press it down hard on to the gasket paper so as to leave a clear picture of its shape and any holes that are needed.

Once you’ve cut out the new gasket, use another smear of oil to stick it temporarily to one surface, and make future separation of the joint easier. Then reassemble the joint and tighten all the bolts evenly.


The standard advice about tools is ‘buy the best you can afford’. All too often that advice goes with a list of ‘essential’ tools and spares that would not disgrace a professional workshop, but would go a long way towards sinking a small cruising yacht!

There’s no doubt that high-quality, high-price tools such as those made by Britool, Gedore and Snap-On are virtually indestructible and a pleasure to use, but unfortunately they sink just as quickly as any others if they get dropped overboard or in the bilges, rust nearly as quickly once they’ve been exposed to salt water, and are exceptionally prone to being ‘borrowed’.

Very cheap tools, such as many of those imported from India and China, will let you believe you’ve got a full tool kit until you try unscrewing a tough nut in an awkward spot. That’s when you find out that the reason they are cheaper is because they don’t fit as well in the first place, and that they get worse once they start to bend under the strain.

Fortunately there are plenty of mid-range tools made by companies such as Stanley and Kamasa, which are nearly as good as the frontrunners but are a fraction of the price.


A set of open-ended spanners, of the right sizes – imperial (AF) or metric – is essential, and if you have a very old boat or engine, you may need the now-obsolete Whitworth sizes, too.

Ring spanners are better if you need to use much force or work in an awkward position, but can’t be used on pipe fittings or lock-nuts. As you need two spanners of the same size to undo a nut and bolt anyway, it makes sense to have one set of rings and one of open-ended.

Combination spanners have a ring on one end and open-ended jaws on the other, but as both ends are usually the same size, you will still need two sets!

Socket spanners make life much easier, and are the only way of getting at some of the less accessible fastenings on compact modern engines, but it’s debatable whether they are essential for basic maintenance. They’re nice to have, but no substitute for conventional spanners because there are some fastenings they can’t cope with. Whitworth socket spanners are very hard to come by, but a set of tubular box spanners is a reasonable (and economical) alternative.

Few professional mechanics would admit to using adjustable spanners on an engine, because they are inevitably less rigid than proper spanners, and more likely to damage the nut or bolt as a result. Every boat, however, has at least one fastening somewhere that is an odd size. When you find out which one it is, a good adjustable will get you out of trouble. Anything less than the best is a waste of space.

Much the same applies to mole-grips or visegrips. Few mechanics will admit to using them, but few would be without them!

The most useful pliers are the square-ended ‘general purpose’ type, about 6–8 in (15–20 cm) long. ‘Needle-nose’ pliers are less versatile, but are a cheap and worthwhile addition to a tool kit, especially if your boat has much electrical equipment on board.

You’ll inevitably need several screwdrivers, including a couple of cross-point screwdrivers and three or four flat-bladed ones, including a small ‘electrician’s’ and a long-shafted ‘heavy duty’. Handle shapes are a matter of personal choice, though the oval handles of ‘carpenter’s’ screwdrivers may allow you to exert more force than the round handles of ‘mechanic’s’ screwdrivers.

Allen keys (or a screwdriver with a wide selection of hexagonal bits) are becoming more and more important.

A hammer is a nasty thing to threaten an engine with, but a light ball-pein ‘engineer’s’ hammer is worth having, as is a soft-faced hammer with a weighted nylon head instead of a lump of hardened steel.

Finally, there are cutting tools: a small hacksaw and some spare blades; a craft knife or large scalpel, and perhaps a small, flat-bladed scraper.

Tricks of the trade

The possibility that you might take an engine to pieces and not be able to put it back together again is probably the mechanic’s worst nightmare. The best tip for avoiding it is to be scrupulously and relentlessly methodical: lay the bits of your engine down in the order they were removed, and keep the fastenings with the relevant part – don’t tip all the nuts and bolts into one box, because it may then take hours of trial and error to find the right one, and don’t complicate matters by taking things apart that you don’t have to.

It almost goes without saying that you should refer to the manual for any job that is not completely familiar to you, but if you do find yourself working without a manual, don’t be afraid to make notes or sketches of the order in which things came apart or what went where. Look carefully at any component before you remove it, and try to figure out what it does, which bolts hold it on, and which hold something inside it.

Seized fastenings make life difficult, but are a common feature of many boat engines. Before applying brute force, it’s important to bear in mind that conventional spanners are designed to apply the right amount of leverage for the fastenings they fit, and that if you lengthen a spanner to unscrew a bolt that is already weakened by corrosion, you may make it shear off completely.

The first stage is to make sure that you are working efficiently:

  • Try to give yourself as much room and light as you need by removing any covers or hatches that are in the way.
  • Keep your hands and tools clean so that you can get a good grip.
  • Pull on the end of a spanner: you are less likely to get hurt if the spanner slips when you are pulling than pushing, and the end gives you more leverage.
  • Use a ring spanner if possible, rather than an open-ended one.
  • Turn the nut rather than the bolt.

If that doesn’t do the trick, it’s worth trying to tighten the offending fastening to break the bond between the threads, and penetrating oil can work wonders so long as it is left alone for long enough to penetrate.

More drastic measures include lengthening the spanner with a length of pipe; and shock treatment by tapping the spanner with a hammer while applying steady pressure by hand.

For a really stubborn fastening, you may have to resort to more destructive methods such as using a cold chisel or a gadget called a nut splitter (like an oversized ring spanner, but with a blade which can be screwed in to break a seized nut) to split the nut, or a hacksaw to cut the side off it. Bolt heads that have rounded off can be carefully filed down to take the next size of spanner, or can have a slot hacksawed in. Most drastic of all, but often quickest, is to cut through the bolt completely.

Destroying a nut and bolt isn’t too frightening because it can always be replaced, but the idea of cutting or snapping a stud or bolt that has seized into a casting is more worrying. Even so, it’s not the end of the world.

Once the load has been removed from the broken stud, it may unscrew relatively easily, particularly if it has been well soaked with penetrating oil. This is where mole-grips come in handy. Alternatively, you could try cutting a screwdriver slot in the remains, or screwing two nuts down on to it. Once the second nut has been tightened down hard against the first, a spanner applied to the lower nut can be used to unscrew the stud.

If the stud has broken off flush with the casting, it should be possible to drill a hole down the centre of it in order to use a tool called a stud extractor. This is a tapered rod of hardened steel with a very coarse thread cut into it. The stud extractor’s thread is in the opposite direction to that of a conventional bolt, so as you screw the stud extractor into the stud, it first grips the side of the drilled hole, and then unscrews the stud.

If this doesn’t work, it may be possible to drill out the stud altogether, and then use a device called a tap to recut the thread in the hole. Taps, unfortunately, are quite expensive, so it’s hardly worth having a full set in hand to cope with the occasional mishap, and it may be worth referring the job to a professional.

The ultimate sanction is definitely a professional’s job because of the equipment required, but it’s worth knowing that it is possible to drill out the hole oversize, and put in a completely new thread known as a helicoil insert.


Adlard Coles Book of Diesel Engines

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