The process of repairing or overhauling a 150 cubic centimeter scooter power unit requires the master not only to have a special tool, but also to have a deep understanding of the thermodynamic processes occurring inside the crankcase. 157QMJThe vent, which is the de facto standard for most Chinese motor vehicles, has design features that can cause even a newly assembled assembly to fail quickly, and unlike the simpler 50cc models, gaps in the valve mechanism and the correct orientation of the oil pump are critical.
The quality of the assembly directly affects the life of the piston group and the reliability of the CVT. Many owners make the fatal mistake of relying solely on factory labels without checking the actual condition of the interface surfaces. Accuracy of fit The parts in this class of engines often leave much to be desired, so manual control of planes and gaps becomes a mandatory stage. Before starting work, you need to make sure that all gaskets are replaced with new ones, and the seats are cleaned of old sealant and soot.
In this article, we will discuss the key stages of assembly, focusing on the points that are most often overlooked in garage conditions. You will learn how to correctly set the timing phases and why the direction of installation of crankshaft glands is critical for oil pressure. 157QMJ It is capable of traveling tens of thousands of kilometers without opening, if all the technological maps of the manufacturer are complied with.
Crack-case preparation and crankshaft installation
The assembly should begin with a thorough revision of the half of the crankcase. 157QMJ, prone to deformation when overheated, so checking the plane of fitting of the cylinder and the generator cover is mandatory. Use a metal ruler and probe for control: the gap should not exceed 0.05 mm over the entire area. If micronervousness is detected, they are eliminated by lapping on glass using abrasive paste, but remove more than 0.1 mm of material forbidden.
Installation of the crankshaft requires special attention to the bearing seats. Before installation, warm the crankcase with a dryer to a temperature of about 80-90 degrees Celsius - this will allow the shaft to sit in place without hitting the hammer, which can damage fragile rolling paths. The orientation of oil-reflective washers on the cheeks of the crankshaft is strictly defined: the protruding part must look towards the bearings, providing lubrication and preventing oil from being thrown into the combustion chamber. Incorrect installation will lead to oil starvation or, conversely, to increased charm.
When assembling the halves of the crankcase, make sure that the guide bushings (pilots) are clean and have no slacks. The bolts of the screed case must be tightened cross-to-cross in several stages to avoid skewing. The moment of puffing for the M6 bolts is usually 10-12 Nm, but it is better to focus on the force specified in the manual for the specific engine manufacturer. After connecting the halves, check the ease of rotation of the shaft - it should be turned from hand without jamming and backlashes.
⚠️ Warning: When installing the crankshaft glands, lubricate the working edge with engine oil. Dry oal oil can overheat and lose leakage in a matter of seconds of operation at the first start.
☑️ Checking the crankcase before assembly
Installation of the piston group and CPG
Installing the cylinder and piston is the stage where the fate of the compression is decided. 157QMJ It has a specific shape of the bottom and the location of the beans, so it's important not to confuse the front and the back. EX (Exhaust), which should look towards the exhaust channel. Before mounting the rings, check their thermal clearance in the piston groove and in the cylinder itself. The gap in the ring lock should be within 0.2-0.4 mm for the upper compression ring.
Grease the cylinder mirror and piston finger with a lot of oil. When setting the rings, keep the order: first oil removable (composite or whole), then lower compression, and lastly the top. The ring locks should not match each other or be located above the piston beans; optimal distribution is in 120 degrees. Hong on the walls of the cylinder should be visible to the naked eye; if the cylinder is glossy, it must be honed again.
When installing the cylinder head, the state of the gasket of the HBC is critical. Use only high-quality gasket made of multilayer metal or high-quality paronite. The bolts of the head are tightened strictly according to the scheme "cross-cross" with a dynamometer key. The moment of tightening of the M8 bolts for the head 157QMJ is usually 20-25 Nm. Under-tight will lead to a gasket, and strain the thread in the aluminum crankcase.
The secret to the longevity of rings
Before installing new rings, be sure to remove the coar from the piston grooves. The remnants of the old coar can split a new thin ring when the engine is running, causing compression to fall.
Gas distribution mechanism (GRM) regulation
Assembly of the timing shaft on a 150 cc engine is often difficult due to the presence of a decompressor and a specific profile of the cams. First of all, set the camshaft so that the mark on the asterisk coincides with the edge of the plane of the cylinder head when the piston is in the BMT (top dead point) compression stroke. Make sure that the cams are distributed to the sides (extended), and the rod is not clamped.
The GRM chain must be installed without distortion, the guide circuit (tensioner) must be installed correctly, taking into account the direction of rotation of the shaft. 157QMJ Often an automatic circuit tensioner is used, which starts working after the engine starts, but the initial sampling of the slack must be done manually when assembled. Check the tension of the chain with a finger palpating in the middle part - it should not sag more than 5 mm.
Once the camshaft is installed, you need to manually turn the crankshaft two full turns and reconnect the marks, which confirms that the teeth are not jumping and the assembly is correct. If the marks do not match after the turn, the procedure for installing the asterisk must be repeated, and an error even on one tooth will lead to the wrong phases of gas distribution, loss of power and possible collision of the valves with the piston.
Adjustment of valve gaps
Valve adjustment is perhaps the most important step for quiet and smooth engine operation. For 157QMJ, the optimal clearances on the cold engine are: 0.05 mm for the intake valve and 0.10 mm for the exhaust valve. The exhaust valve needs a larger gap due to higher temperature expansions. You need to use a special flat probe, not probes for spark plugs, since the latter are more thick and can give the wrong result.
The adjustment process is done on a fully cooled engine. Relax the counternut of the adjustment bolt, insert the probe and rotate the adjustment screw until there is light resistance when the probe moves. The probe should walk freely, but with tangible friction throughout the plane. Once the screw position is fixed, tighten the counternut, holding the adjusting screw with the key so that the gap is not changed. After the tightening, be sure to check the gap with a probe.
Incorrectly exposed gaps have serious consequences: too small a gap in the inlet will lead to clapping into the carburetor and unstable idling, and at the outlet to burn the valve and saddle, too large a gap causes the characteristic tsokot (tractor sound) and reduces the opening time of the valve, which reduces the filling of the cylinder and power. Thermal clearance This is not a static parameter, it can change after the first 500 km of run, so it is recommended to repeat the test after running.
| Parameter | Inlet (mm) | Release (mm) | Tool. |
|---|---|---|---|
| Standard clearance | 0.05 | 0.10 | Flat probe |
| Permissible error | +/- 0.01 | +/- 0.01 | Micrometer |
| Engine condition | Cold. | Cold. | - |
| Time of verification | VMT compression | VMT compression | Indicator |
Use a threaded hold (anaerobic sealant) on the screws before the final tightening of the counternuts, which will prevent the vibration gap from spontaneously changing.
Assembly of lubrication system and oil pump
The 157QMJ engine is equipped with a trochoid oil pump that is integrated into the right-hand cover of the crankcase or installed separately depending on the modification. When assembled, pay attention to the condition of the gears of the pump and the body. The gap between the ends of the gears and the body should not exceed 0.15 mm. The wear of these parts leads to a drop in oil pressure and rapid failure of the crankshaft bearings.
The most important element is the reduction valve, which regulates the pressure in the system. Check the ease of travel of the valve plunger and the condition of the spring. Jamming the valve in the open position will cause the oil to constantly drain into the crankcase, without going to the rubbing vapors. In the closed position, this threatens to squeeze out the os and rupture the filter (if it is in the system).
When installing the generator cover (where the pump is often located), use a new gasket or a high-quality heat-resistant sealant. Bolts of the lid fastening evenly. After assembly and pouring the oil, you need to check the system for leaks and make sure that the oil pressure indicator (if designed) goes out immediately after start-up.
⚠️ Warning: Never start the engine without pumping the oil system first. Scroll the engine 5-10 seconds with the candles turned out so that the oil fills the channels and lubricates the bearings before pressure appears.
First launch and break-in
The final step is the first start of the assembled unit. Pour the manufacturer's recommended oil (usually 10W-40 for 157QMJ) and fuel. Check the fluid levels again. On the first start, the engine may not start on the first try due to no fuel in the float chamber or air in the system - it's normal. Don't spin the starter for longer than 5-7 seconds continuously, let it cool down.
After a successful start, let the engine work at idle speeds for 2-3 minutes. Listen carefully to extraneous noises: knocking, grinding or whistling is unacceptable. Check the oil pressure (if there is a sensor) and the absence of leaks from the gaskets. In the first minutes of operation, a slight haze from the exhaust pipe is possible - this burns out the remnants of conservation lubricant and process fluids.
The process of running a new or rebuilt 157QMJ engine should be in a gentle mode. the first 500 km is not recommended to open the throttle more than 50-60% and keep constant speeds for a long time. Variator. It should be paired with a gradually rubbing piston group, and the sharp accelerations and maximum speeds during the break-in will lead to local overheating and bullying, negating all the assembly work.
The quality of the 157QMJ engine build depends on 80% of the purity of the parts and accuracy of compliance with the puff moments, not on the cost of spare parts.
What is the point of tightening of the block head for 157QMJ?
The optimal timing of the bolts of the cylinder head for the 157QMJ engine is from 20 to 25 Nm (Newton meters). Puffing must be done in two stages: first preliminary puffing to 10 Nm cross, then the final to 25 Nm according to the same scheme. The use of a dynamometer key is mandatory, since the aluminum thread of the crankcase is easily broken when stretching.
Why is there no compression after assembly?
The lack of compression after assembly most often indicates improper installation of piston rings (locks are combined or the ring is broken), confused phases of the GRM (valves are open at the time of compression stroke) or damage to the gasket of the HBC. Also, the cause may be the absence of a gap in the valves, because of which they do not close until the end.
Do I need to clean the valves when assembly?
If you've changed a valve or a saddle, you have to pressurize it, but if the engine is only designed to replace the rings, and the valve has no coil or sink, you don't need laundry, just check the tightness with kerosene, pour it into the channels of the inverted head, so that the liquid doesn't leak to the candle hole.
What oil to fill in the engine 157QMJ?
For a 150cc engine, it is recommended to use semi-synthetic or synthetic motor oils with 10W-40 or 15W-50 viscosity (for hot climates). The oil volume when replaced is about 0.8 - 0.9 liters. It is important to use oils designed for four-stroke motor vehicles (JASO MA/MA2 standard), as they contain additives that protect the clutch in the oil bath (if it is structurally related to the engine, which is not the case in 157QMJ, but the standard is uniform).