Contents: Service technology ↡ Fasteners ↡ Fastener dimensions ↡ Procedure and order of tightening…↡ Disassembling components ↡ Gasket surfaces ↡ Tips for removing hoses ↡
Dimensions/strength class markings for standard (SAE and USS) bolts
G — Strength class marking
L — Length (in inches)
T — Thread pitch (number of threads per inch)
D — Nominal diameter (in inches)
Dimensions/Grade Markings for Metric Bolts
P — Strength class
L — Length (in mm)
T — Thread pitch (distance between adjacent turns in mm)
D — Nominal diameter (in mm)
Marking of the strength class of bolts (top - standard /SAE/USS, bottom - metric)
Strength class marking of standard hex nuts
Strength class 5 |
Strength class 8 |
Metric Hex Nut Strength Class Markings
Strength class 9 |
Strength class 10 |
Service technology
There are several methods for performing vehicle maintenance and repair procedures, which the reader will find references to in the text of this guide. Following them will make the work of a home mechanic more efficient, will allow the best organization and quality execution of various technical procedures, and will be the key to thorough and complete execution of all work.
Fasteners
Fasteners are nuts, bolts, studs and screws used to connect two or more parts together. When working with fasteners, you must always remember some things. Almost any fastener uses one or another type of locking and fixing devices. These can be lock washers, lock nuts, lock flags or thread locking compound. All threaded fasteners used must be absolutely clean and straight, with undamaged threads and unrounded corners of the hexagonal heads onto which the wrench is put on. It is a rule to replace damaged nuts and bolts with new ones. Special self-locking nuts with nylon or fiber inserts cannot be reused, since they lose their locking properties when loosened and must always be replaced with new ones during assembly.
Rusted bolts and nuts should be treated with a special penetrating compound to facilitate unscrewing and to avoid damage before loosening. Many mechanics prefer to use turpentine for this purpose, which is conveniently applied from a special small canister with a long spout. After wetting the "stuck" fastener with the penetrating compound, before starting to loosen it, the compound should be allowed to thoroughly soak the oxidized contact layer for several minutes. Heavily rusted fasteners can be cut off with a chisel, sawed off with a hacksaw, or removed with a special nut splitter.
When a bolt head is sheared off or a stud is broken off on an assembly, the remaining threaded portion can be drilled out or extracted with a special tool. Most auto repair shops can undertake this, as well as other (for example, restoration of stripped threads in threaded holes), repair procedures.
When reassembling, flat and lock washers should always be installed in the same order and in the same way as before. Always replace damaged washers with new ones. Between the lock washer and the soft metal surface (for example, aluminum), thin sheet metal or plastic should always be fitted with flat washers.
Fastener dimensions
For many reasons, vehicle manufacturers are increasingly using metric fasteners. However, it is important to know the difference between the standard (also called American or SAE standard) and more universal in the metric system of measures, since, despite the external similarity, they are not interchangeable.
All bolts, both standard and metric, are classified by diameter, thread pitch, and length. For example, a standard S – 13x1 bolt is half an inch in diameter, 13 turns of thread per inch, and 1 inch long. Metric bolt M12-1. 75x25 has a diameter of 12 mm, thread pitch 1.75 mm (distance between adjacent turns) and a length of 25 mm. Both bolts are almost identical in appearance, but are not interchangeable.
In addition to the above features, both metric and standard bolts can be identified by examining the head. For starters, the distance between the flats of a metric bolt head is measured in mm, while a standard bolt is measured in inches (the same is true for nuts). As a result, a standard wrench is not suitable for use with metric fasteners, and vice versa. In addition, most standard bolts usually have radial notches on their heads that determine the maximum allowable tightening force of the bolt (degree of strength)The greater the number of notches, the higher the permissible force (cars typically use bolts with a strength rating of 0 to 5). The strength class of metric bolts is determined by a digital code. The code numbers are usually cast, as for standard ones, on the head of the bolt (automobiles typically use bolts of strength classes 8.8, 9.8, and 10.9).
Standard nuts can also be distinguished from metric nuts by strength class marks. To identify the strength of standard nuts, dot marks are used, stamped on one of the end surfaces of the nut, while metric nuts are marked using numbers again. The greater the number of dots, or the higher the value of the digital code, the higher the permissible tightening force of the nut.
The ends of metric studs are also marked according to their strength class. Large studs are marked with a digital code, while smaller ones are marked with a geometric figure.
It should be noted that a significant portion of fasteners, especially those of strength class 0 to 2, are not marked at all. In this case, the only way to distinguish standard fasteners from metric ones is to measure the thread pitch, or compare the thread with a uniquely identified one.
Standard fasteners are often referred to as SAE standard fasteners, as opposed to metric fasteners, but it should be remembered that only small fasteners fall under the SAE classification. Large fasteners with non-metric threads are American Standard Fasteners (USS).
Since the fasteners are of the same geometric size (both standard and metric) may have different strength classes; when replacing bolts, nuts and studs on a vehicle, attention should be paid to ensuring that the strength class of the new fasteners being installed matches the strength class of the old ones.
Procedure and order of tightening threaded connections
Tightening of most threaded connections should be carried out with forces determined by the requirements of the Specifications given at the beginning of each Chapter of this Manual (the tightening force of a fastener should be understood as the torque applied to it when tightening). Below, the tightening force will also be called the tightening torque of the fastener. Tightening with excessive force can lead to damage to the integrity of the fastener, while undertightening it leads to unreliability of the connection of the mating components. Bolts, screws and studs, depending on the material from which they are made and the diameter of the threaded part, usually have strictly defined permissible tightening torques, many of which, as mentioned above, are given in the Specifications at the beginning of each Chapter. Strictly adhere to the recommendations for tightening torques for the fasteners used on the vehicle. To tighten fasteners not mentioned in the Specifications, use the permissible torque chart below. The values given in the table are oriented towards fasteners of strength classes 2 and 3 (higher-grade fasteners allow for greater tightening force), in addition, it is implied that dry tightening is carried out (with ungreased threads) fasteners in steel or cast (not aluminum) detail.
Metric Thread Sizes
| M6 | 9 -12 Nm |
| M8 | 19 - 28 Nm |
| M10 | 38 - 54 Nm |
| M12 | 68 - 96 Nm |
| M14 | 109 - 154 Nm |
Pipe thread sizes
| 1/8 | 7 -10 Nm |
| 1/4 | 17 - 24 Nm |
| 3/8 | 30 - 44 Nm |
| 1/2 | 34 - 47 Nm |
American Standard Thread Sizes
| 1/4 - 20 | 9 - 12 Nm |
| 5/16 - 18 | 17 - 24 Nm |
| 5/16 - 24 | 19 - 27 Nm |
| 3/8 - 16 | 30 - 43 Nm |
| 3/8 - 24 | 37 - 51 Nm |
| 7/16 - 24 | 55 - 74 Nm |
| 7/16 - 20 | 55 - 81 Nm |
| 1/2 - 13 | 75 - 108 Nm |
A fastener located around the perimeter of a part (such as cylinder head bolts, oil pan bolts and various covers), to avoid deformation of the part, must be loosened and tightened in a strictly defined order. The order of tightening and loosening such fasteners is given in the relevant Chapters of the Manual. If a special order is not specified, then to avoid distortion of the component, the following procedure should be followed. At the first stage, all bolts or nuts should be tightened by hand. Then, each of them in turn should be tightened another full turn, and the transition from one bolt / nut to another should be carried out in a diagonal order (criss-cross). Then, returning to the first bolt/nut, repeat the procedure in the same order, tightening the fasteners another half turn. Continue to act in the same manner, tightening each bolt/nut this time by a quarter of a turn at a time until they are all tightened to the required force. When loosening the fasteners, you should also follow the described procedure, but acting in reverse order.
Disassembling components
Disassembly of all components must be carried out in such a manner that during assembly each part can be installed in its original place and in the correct way. Remember the characteristic features of the appearance, if necessary, make a landing marking of parts, the installation of which in place can be carried out in an ambiguous way (such elements include, for example, a grooved thrust washer on the shaft). It is a good idea to lay out the removed parts on a clean work surface in the order in which they were removed. It may also be helpful to make simple schematic drawings or take step-by-step photographs of the component to be removed.
When removing fasteners, try to mark their original position on the assembly. Often, immediately installing fasteners and washers in their original place after removing the corresponding part helps to avoid confusion during assembly. If this is not possible, all fasteners should be placed in a specially prepared box divided into sections and marked accordingly, or simply in separate marked boxes. This procedure is especially useful when working with components consisting of many small parts, such as an alternator, valve mechanism, instrument panel or decorative trim elements.
When disconnecting electrical contacts and connectors, pay attention to marking the wires or harnesses using insulating tape with a digital or letter code applied to it.
Gasket surfaces
On all vehicles, gaskets are used to seal the junction of the mating surfaces of two or more parts and serve to prevent oil and fluid leaks and maintain increased pressure or vacuum inside the assembly.
Often such gaskets are coated with a liquid or paste-like sealing compound before installation. Often, under the influence of time, temperature or pressure, the mating surfaces "stick" to each other so strongly that separating the parts becomes a difficult task. In many cases, dismantling such assemblies is helped by tapping them from the outside along the perimeter of the joint with a soft-faced hammer. You can also use a regular hammer for this purpose, striking through a wooden or plastic spacer. Do not tap cast housings and fragile components. If such difficulties arise, always first check whether all fasteners have been removed.
Avoid using a screwdriver or pry bar between mating surfaces to separate the parts, as the sealing surfaces can easily be damaged, which can later cause leaks. If it is impossible to avoid levering the "stuck" assembly elements, use the handle of an old broom for this purpose, but remember that all splinters must be carefully removed from the mating surfaces and from inside the assembly.
After separating the parts, their mating surfaces should be thoroughly cleaned, scraping off any traces of the old gasket material. Hardened fragments of the old gasket can be softened in advance using a rust converter or special chemical composition, and then removed from the mating surface with a scraper. In this case, a piece of copper tube with a flattened and sharpened end can be used as a scraper. It is recommended to use a copper tube for this purpose, since copper is usually softer than the materials used in the car, which reduces the risk of damaging the mating surface. Some gaskets can be easily removed with a copper brush, however, regardless of the method used, the mating surfaces must be completely clean and smooth. If for any reason the mating surface is scratched, fill the scratch with gasket sealant before assembling the components. In most cases, a non-hardening gasket should be used (or semi-hardening) sealant.
Tips for removing hoses
Caution! If your vehicle is equipped with an air conditioning system, do not disconnect any hoses from the air conditioning components under any circumstances until the system has been discharged by a Mercedes-Benz dealer or an air conditioning specialist at a service workshop.
The precautions to be taken when removing hoses are very similar to those for removing gaskets. Avoid damaging the surfaces of the fittings and pipes onto which the hoses are placed, as this may cause leaks. This applies especially to the procedure for removing radiator hoses. Various chemical reactions cause the rubber of the hoses to "stick" to the mating surfaces of the fittings and pipes. To remove a hose, first loosen the clamp securing it to the fitting. Then, using pliers with a sliding joint, grasp the hose near the clamp and begin to rotate it on the fitting/connecting pipe to the right and left. Continue this procedure until the hose is completely free, then remove the hose from the fitting. A small amount of silicone or other grease will facilitate the procedure if it can be introduced into the gap between the fitting and the hose. To make the installation of the hose easier, lubricate the inner surface of the hose and the outer surface of the fitting.
As a last resort, or in case of a clear need to replace the hose with a new one, the end of the hose put on the nipple for removal can be cut with a knife and then separated from the surface of the nipple. In this case, try not to damage the metal of the nipple/connecting pipe with the knife.
If the hose clamp is damaged, replace it with a new one. Twist-type clamps tend to loosen over time, so regardless of their condition, it is best to replace them with screw-type clamps when necessary.