Due to integrity failure, the spark plug cone or O-ring is unable to seal the cylinder tightly. Thread restoration can be done in several ways, including without removing the cylinder head from the engine.
Maintenance operations of the machine in most cases are carried out with unscrewing and tightening of threaded fasteners. If parts such as a stud, nut and bolt are damaged, they are replaced with new ones. But the repair becomes more feasible when the threads in the housing element are damaged.
Spark plug attachment parts are not, but they have a connecting thread that is damaged in the following cases:
- When exposed to dust and dirt. To prevent this, in engines with a well, it will be useful to unscrew the spark plug a few turns before removing it and use a brush or air blow to remove dirt.
- Working with a candle key that does not provide proper fixation and subsequently leads to skew.
- Over tightening, the exact value of which can be found in the instruction manual.
Thread restoration is carried out by various methods with their own nuances. For each case, the most optimal one is selected depending on the price, labor costs, technological equipment and design. Each type of repair involves dimensional tolerance and hole ratio.
In order to restore the thread in the block, it is rational to install an insert of a self-tapping or spiral type, with a fixing belt, as well as to fill the hole. The insert must not touch the combustion chamber. At the end of the installation, it is calibrated with a tap to get rid of damage caused during installation.
What is cold welding
It is a way to build and combine metal elements, without being affected by high temperatures. In this case, the surfaces are not welded, but glued together. The mass for cold welding begins to deform, then penetrates into the parts used for work. There is no mutual penetration of substances into each other, the appearance of interatomic bonds, but a tight connection occurs. At the same time, this method helps to restore the candle thread, recreate the lost elements and repair the damage.
Work in progress
Today you can find a large list of goods of foreign and Russian production in stores. It is worth noting two main types: plasticine-like and liquid. The latter have twocomponent, which is the main mass used in gluing, and a hardener. Plasticine products consist of a bar with one or more layers, before use, mixing is necessary. In order to restore the internal thread by cold welding, it is advisable to purchase a two-component product sold in two bottles.
Preliminary degreasing of all surfaces and anti-adhesive treatment both outside and inside are required, since the presence of plaque and small particles can worsen the result. Next, the components from two bottles are mixed on a plastic or wooden surface. The resulting mass is applied to the thread, which is screwed inward and left to dry completely. Subject to the temperature regime, thread restoration will be completed after the time specified in the instructions.
Repair with conventional welding
The cylinder head is removed from the engine and the damaged section of the spark plug is melted by welding. On a milling or boring machine, the recess under the candle is machined, and the mating surfaces are restored. A new hole is drilled at the set angle and the thread is cut. Head pressure testing is the final step, it is a leak test in a water bath with high air pressure.
It is worth noting that too much heat in the welding area can cause cracks. Therefore, metal welding is used in extreme cases,when other methods cannot be used.
Restoring the thread with an insert with a supporting flange
A block head is installed on a milling or boring machine. The stripped thread is drilled out and a new thread is cut to use the repair insert. It is made on a lathe, most often from bronze. Inside there must be a thread with the required dimension. To perform the function of heat dissipation, the insert must be in close contact with the walls; for this, the diameter of the external thread is made larger than the nominal one. It is wrapped in a head after applying Bakelite varnish. The part is secured by flaring the edge of the insert.
Manual restoration
Restoring stripped threads manually without removing the block head from the engine is not able to properly match the old and new hole, which can ruin the part. It is also difficult to prevent chips from entering and secure the insert tightly, avoiding unscrewing along with the candle.
Spiral insert
This method is mainly used on a removed block, but with a suitable well depth, location and size, engine disassembly can be avoided. Before starting the restoration of the internal thread, the location of the valves and the piston must be checked to provide room for the tool to be screwed in.
The hole for the candle is prepared with a combined type tap, which makes it possible to work without a metal-cutting machine. Atwhile the head is on the engine, grease is placed in the recesses to hold the chips.
The tap is centered and cuts the thread without distortion when screwing into the broken hole for the candle. The support flange of the insert allows for irregular plug placement, and this method is not suitable for heads that have a conical seal due to the small diameter of the wells.
Insert with fastening belt
After forming the part, the external and internal threads are cut in such a way that there is no combination of depressions and protrusions of the profile, which makes it possible to achieve the required strength of the element. With the help of an installation device, the insert is wrapped in candle wells with great depth and is quite easily removed from the outside. Heat-resistant adhesive will help in ensuring tightness. The two outer turns of the insert must be knurled for tight fixation after flaring with a special profile.
