High mileage does not always indicate that a rebuild is necessary, just as low mileage does not always indicate that a rebuild can be avoided. The frequency of maintenance is probably the most important factor. An engine that receives regular oil and filter changes and regular maintenance will likely provide many thousands of kilometers of service without any problems. On the contrary, an engine that is not serviced will require rebuilding very quickly.
Excessive oil consumption is an indication that the piston rings, valve stem seals and/or valve guides need attention. Make sure the valve stem seals are in good condition before assuming the rings and/or guides are in poor condition. Perform a compression test on the cylinders to determine the required work.
Remove the oil pressure sensor and check the oil pressure using a measuring instrument inserted in its place. Compare results with specifications. If the pressure is extremely low, it is likely that the bearings or oil pump are badly worn.
Lack of power, rough running, knocking or metallic sounds from the engine, loud valve noise, and excessive fuel consumption may also indicate the need for an engine rebuild, especially if they are present at the same time. If a complete setup and adjustment does not improve the situation, then the only way to solve the problem is to overhaul the engine.
(The publication is reprinted from this resource: Nissanbook.ru)
Engine repair involves bringing the internal engine parts to a new condition. During repairs, piston rings are replaced and cylinder bores are ground. When performing cylinder surface treatment in a specialized auto repair shop, it will be necessary to install larger pistons. The main and connecting rod bearings are usually replaced with new ones, and if necessary, the crankshaft can be ground to restore the journals. Usually, it is necessary to resort to repairing the valves, since their condition usually leaves much to be desired. When repairing an engine, its external components, such as the starter or generator, can also be repaired. The end result should be a good as new engine, providing many kilometers of trouble-free driving.
Caution: Cooling system components such as hoses, drive belts, thermostat and water pump must be replaced when performing major engine repairs. The radiator should be carefully checked to ensure that it is not clogged or leaking. We also recommend overhauling the oil pump.
Before you begin rebuilding your engine, please read the entire process description carefully to familiarize yourself with the skills required to perform the job. Rebuilding an engine may not be that difficult if you follow all the instructions carefully, use the necessary tools and strictly adhere to the control dimensions; however, this may take quite a long time. Plan to keep your vehicle out of use for at least two weeks, especially if you need to have certain parts repaired at a specialist workshop. Check the availability of the necessary spare parts and ensure that you have all the necessary tools and equipment ready in advance. Most of the work can be done using common tools, but specialized precision measuring tools are also required to determine whether certain parts need to be replaced. Often, a specialist workshop can take the necessary measurements and also advise on the need for replacement and/or repair.
Caution: Always wait until the engine has been completely disassembled and all components, especially the engine block, have been inspected before deciding which work should be taken to a specialist workshop. Since cylinder condition is a major factor in deciding whether to rebuild an old piston set or purchase a new one, never purchase parts or machine components until the cylinders and block have been thoroughly inspected.
In conclusion, it must be said that in order to ensure maximum service life and a minimum number of problems on the engine after repair, it is necessary to carry out assembly very carefully and in conditions of complete cleanliness.