High mileage is not a sufficient indicator of the need for a major overhaul, on the other hand, low mileage does not exclude a major overhaul. The most important indicator is probably the timeliness of routine engine maintenance. With timely oil and air filter changes, as well as with all other necessary maintenance work, the engine will serve reliably for many thousands of kilometers. On the contrary, insufficient maintenance can cause a sharp reduction in service life. Increased oil consumption indicates wear of piston rings or valve guides. It is necessary to make sure that leaks are not the cause of increased oil consumption, and only then draw a conclusion about the unsuitability of piston rings and guides. To determine the scope of upcoming work, measure the compression in the cylinders, or check the tightness of the combustion chamber, for which it is recommended to contact an experienced specialist.
If increased noise or knocking is heard while the engine is running, the probable cause may be wear of the connecting rod or main bearings.
Unscrew the standard sensor and measure the oil pressure with a pressure gauge, compare the obtained value with the one specified in the manual. If the pressure is low, the cause may be wear of the support bearings or the oil pump.
Loss of power, "dips" in engine operation, increased noise from the valve timing mechanism, increased fuel consumption indicate the need for a major overhaul, especially if all these factors occur simultaneously. If performing all adjustments does not lead to improvement, then the most appropriate means of elimination is repair.
Major repairs involve restoring engine parts to the condition specified in the technical data for a new engine. During a major repair, piston rings are replaced and cylinder walls are bored to the repair size (or honed).
After repairing the cylinders, new pistons will need to be installed. The connecting rod and main bearings are also subject to replacement; if necessary, the crankshaft should be ground to restore the clearances with the connecting rod and main bearings. As a rule, the valve mechanism does not change, since its condition at the time of repair is usually quite satisfactory. During a major overhaul of the engine, such units as the carburetor, ignition distributor, starter and generator are also repaired. As a result, the engine should have the qualities of an almost new unit and serve reliably for a long time.
Before starting a major engine overhaul, read the description of the relevant procedures to get an idea of the upcoming scope of work and the requirements for them. Major overhaul is not difficult, but it requires a lot of time. It will take approximately two weeks, especially if the repair and restoration of parts (grinding, boring) you will have to contact a specialized workshop. Check the availability of spare parts and take care of purchasing the necessary special tools and equipment in advance.
Almost all work can be done with a standard set of tools, although precise measuring instruments will be needed to check and determine the suitability of certain parts. Often, the condition of parts is checked in specialized workshops, where recommendations are received for replacing or restoring parts.
Warning: You should only contact a car service workshop after completely disassembling the engine and checking all parts, especially the cylinder block, in order to decide what maintenance and repair operations will be performed in the workshops.
Since the condition of the cylinder block is the determining factor in making a decision on its further repair or on purchasing a new repair cylinder block, spare parts should be purchased or machining operations should be performed only after a thorough inspection of its technical condition. Make it a rule that the true price of repair is time, then you will not have to pay for the installation of worn or repair parts.
In conclusion, we note that the assembly of any units should be carried out with the utmost care in a clean room to avoid further failures of the repaired engine.
The original publication in its entirety is posted on the website AUDIMANUAL