A sudden knock at the bottom of the engine or a drop in thrust at high speeds often indicates a critical condition of the crank mechanism. crankshaft It's under tremendous pressure, converting the reciprocating motion of the pistons into the rotation of the propeller. In water conditions, where there's a high probability of getting abrasive or running on a poor mix, the wear of the knots is faster than in car engines.
Ignoring the primary signs of malfunction, such as metal ringing or vibration, leads to bullies in the cylinders and even a rod break, which turns a simple repair into a complete replacement of the cylinder block. Mercury, Yamaha or Tohatsu It requires not only special tools, but also a deep understanding of the working principles of two-stroke and four-stroke units, and in this article we will discuss the stages of diagnostics and replacement of parts that will return the outboard motor to factory power.
Before starting the dismantling, you need to carefully prepare the workplace and make sure that all the necessary seals are available. Tightness The crankcase is a key factor in the stable operation of the engine, and any assembly error can lead to air suction and piston burn. Crackshaft repair is an operation that requires surgical accuracy and purity, since even a microscopic grain of sand can destroy new bearings in minutes of operation.
Diagnosis of malfunctions of the crank-shaking mechanism
You can determine the need for interference with the work of the CSM even before the engine is completely disassembled, paying attention to the characteristic sound and visual signals. Deaf knock, which increases when warmed up, or a ringing metal clang at idle speeds indicate increased gaps in the engine. root bearings Experienced motorists often use a stethoscope or a long screwdriver, putting the ear to the handle to locate the source of the noise in a particular cylinder.
Visual inspection of exhaust gases also gives important information: the presence of blue smoke with the smell of burnt oil can indicate the grease from the crankcase into the combustion chamber through worn-out plumbingIf the engine is four-stroke, high oil consumption and oil-removable carbon monoxide are often associated with the laying of the rings, but sometimes the reason lies in the wear of the valves guiding the bushings, which indirectly affects the crankshaft due to the imbalance of gas flows.
⚠️ Warning: The operation of the engine with a clear knock in the crank mechanism is prohibited. Continued operation will lead to the check of the liners, cylinders bullying and expensive overhaul with the replacement of the unit.
For two-stroke engines, the critical parameter is the rod backlash, the check is done by wiggling the rod with the cylinder head removed, if you feel a tangible vertical or horizontal backlash, then bottom-bearing The rod needs to be replaced. It's also worth paying attention to the color of the piston: if it has uneven scorching or melting on one side, it may indicate a skewed rod or a violation of the geometry of the crankshaft.
Necessary tools and preparation for disassembly
Quality repair is impossible without a specialized set of tools, as standard keys are often not suitable for specific fasteners of outboard motors. The main tool is a dynamometer key, which allows you to observe the exact moments of tightening, which is critical for the development of the engine. aluminum-alloy You'll also need a bearing detacher, a set of probes to measure the gaps, and a clock-type indicator to check the shaft's beating.
Special attention should be paid to the cleanliness of the room and tools. Getting dirt inside the crankcase when replacing the crankshaft is unacceptable, so all work is recommended to be carried out on a clean, covered with rags surface. crankcase.
Prepare all the necessary supplies in advance: new oil oils, gaskets, rod bearings and, if necessary, the crankshaft itself assembled. The use of old seals is strictly not recommended, since they lose their properties when re-installed and do not provide the required tightness.
☑️ Preparations for repairs to the CSM
Decommissioning and Defaulting of Parts
The engine disassembly starts with the cylinder head being removed and the piston group released, and the pistons being removed, the rods are accessed where a thorough defect is required, first of all, the diameter of the seats under the bearings is measured, and the absence of production on the shaft necks is checked. liner-turn or changes in the color of the metal (blue run) indicate overheating and the need to replace the node.
If the engine is two-stroke, then after removing the rods, the halves of the crankcase carefully detach. At this point, it is important not to damage the glands and gaskets, although they are still planned to be changed. crankcase Often occur due to a hydraulic shock or foreign objects hit the screw.
For four-stroke engines, the process is complicated by the need to remove the oil pump and sometimes the camshaft. When a crankshaft is defective, the neck beat is checked with an indicator. The permissible beat values are usually less than 0.05 mm, but for high-speed outboard motors the requirements may be tougher. If the shaft is curvature, it can only be corrected on specialized equipment, but most often it is advisable to buy a new one. kneel.
Why can't you grind the crankshaft of the outboard motor?
Unlike car engines, many outboard crankshafts (especially two-stroke ones) have a hardened surface layer or are composite. Grinding removes this layer, drastically reducing resource, and it is extremely difficult to find repair-sized liners for outboard motors, while standard ones are always available.
Replacement of bearings and glands
The most time-consuming step is to press new bearings. Before installing new bearings, it is recommended to heat them in oil to a temperature of about 80-90 degrees, which will make it easier to land on the shaft. Carter, on the contrary, is often cooled to ensure a tight fit of the outer ring. It is important to observe the direction of installation: the closed side of the bearing is usually facing the outer ring. oil or lubrication.
Replacing the omentum requires special care. The working edge of the omentum should be clean and lubricated before installation. When installing, use a mandrel that relies on the metal clip of the omentum, not on the rubber part, so as not to damage the seal. Incorrectly installed plumbing It can wrap or tear, resulting in instant loss of compression or water entering the crankcase.
When assembling a shaky piston group, pay attention to the orientation of the pistons. At the bottom of the piston, there is usually an arrow indicating the direction to the exhaust side (see below).ExhaustIf you mix the sides, you'll disrupt the purging of the cylinders and the engine, and check the condition of the piston rings, they should move freely in the grooves and have the correct thermal clearance.
⚠️ Warning: When heating bearings, use only pure engine oil. Open flame heating is prohibited, as this changes the structure of the metal and reduces the life of the part.
Engine assembly and clearance control
The engine is assembled in reverse disassembly, but with strict rules for the timing of tightening of the bolts. The bolts of the mounting head of the block and the lids of the root bearings are tightened cross-cross in several stages. This ensures uniform fit of parts and prevents distortion. blockheadFor each type of motor, there are specifications for tightening moments that must be observed.
After connecting the halves of the crankcase (on two-stroke motors), you need to check the ease of rotation of the crankshaft. The shaft should be turned by hand evenly, without jamming or dips. If you feel resistance, the bearing may not fully sat down or skewed. Also checked the axial backlash of the crankshaft, which is regulated by the thickness of the adjusting washers.
New gaskets are mandatory, old gaskets cannot be used even if they look whole, and you can use an anaerobic sealant when you assemble them, but you need to apply them in a thin layer, avoiding getting into the oil ducts and combustion chamber. Excess sealant that gets inside can clog carburetor or oil pump jelly.
When assembling a two-stroke motor, lubricate the bearings and oils abundantly with two-stroke engines before the first start, which will prevent dry friction in the first seconds of operation until the mixture begins to flow into the crankcase.
Table of the moments of tightening of the main nodes
Compliance with threaded tightening moments is a guarantee of engine reliability. Below are the average values for popular models, but always check the service manual for your particular engine.
| Engine assembly | Diameter of thread | Moment of puffing (Nm) | Bolt material |
|---|---|---|---|
| Head of cylinder block | M8 | 25-30 | Steel |
| lid of the rod | M8 | 18-22 | Steel |
| Flywheel (nut) | M12-M14 | 40-60 | Steel |
| Ignition candle | M14 | 20-25 | Brass/Steel |
First start and run after repair
Once fully assembled and installed, the engine cannot be fully revved at once, the first run must be at low speeds to check for oil and fuel leaks, and to ensure that the ignition system is stable. Listen to the engine: any outside knocks should be an excuse for immediate stop and re-checking.
The process of running after the replacement of the crankshaft and the piston group is mandatory. For two-stroke engines, it is recommended to increase the oil content of the fuel mixture by 20-25% in the first hours of operation, this will ensure better lubrication of rubbing vapors that have not yet rubbed together.
During the break-in, monitor the engine temperature and oil pressure (if you have a sensor). A sharp rise in temperature may indicate a cooling system problem or a mixture that is too poor. piston-ring Take your place and provide the necessary compression.
The quality of the break-in directly affects the life of the repaired engine, neglecting the breaking in mode in the first 5-10 hours of operation negates all efforts to repair and replace parts.
Can you replace the sleeves without changing the sleeves?
Yes, if the necks of the crankshaft are not of the output, the bulls and the color of the run, and the beating of the shaft is within tolerance, it is permissible to replace the bearings alone. However, if the motor mileage is high, it is often more economical to replace the entire assembly in order to avoid re-disassembly after a short period of time.
Which sealant is best used for a crankcase?
For outboard motors, anaerobic gaskets (e.g., Loctite 515 or 574) are best suited, which harden without air and are resistant to gasoline and oil. Silicone sealants are not recommended, as they can dissolve in oil and clog channels.
Why does the engine smoke after repair?
In the first hours of operation, the smoke may be caused by burning out of the conservation lubricant or the residue of the sealant. If the smoke persists for a long time, it may be that piston rings (replaced with top and bottom places) or oil caps are poured in, and the reason may be too rich mixture when adjusting the carburetor.
Do I need to change the rods when replacing the crankshaft?
Most modern outboard motors only sell rods with crankshafts. If you have an old engine, where the rods are separate, they must be replaced if there is a backlash. Changing the rod bearing without replacing the rod itself (unless it is pressed) often makes no sense, since the seat also wears out.