The bottom of the outboard motor, known as lower unit, takes on enormous loads during operation. gearboxIt's a powerful protection for gears, shafts and gearshift systems, and many owners forget that it's constantly in an aggressive aquatic environment, being hit by underwater obstacles and corrosion.
Any crack or chip on the metal surface can be catastrophic, even to the point of jamming the engine. Understanding the design and operation of this unit allows you to notice the malfunction in time and prevent costly repairs. In this article, we will discuss in detail Construction of the lower part of the engine and its maintenance.
Ignoring the state of lower unit is the most common reason for outboard motors to fail in high season, and water trapped in the water through a microcrack mixes with lubricant, turning it into an emulsion that cannot protect the rubbing parts, so regular visual inspection and proper winter preparation are critical procedures for any water turbine.
Design features and materials of manufacture
Modern. gearbox It's a complex piece of engineering, usually made of aluminum alloy, and it's not a coincidence that aluminum is very corrosive when it comes into contact with salt water, and it's light enough, which is important for the overall weight of the outboard motor, but it also has a weak side, which is its low impact resistance compared to steel.
Inside the hollow space of the body are the main transmission elements, and there is a vertical shaft from the engine crankshaft and a horizontal propeller shaft, which is mounted on the propeller, and there is a gear system between them, which provides a gear ratio and reverseability, and the quality of the casting and treatment of the internal surfaces directly affects the resource of the entire unit.
Special attention should be paid to the sealing system. plumbing The design often provides special channels to bring water to the cooling system if the water pump (impeller) is located in the lower unit, although more often the pump is higher, in the lower unit section or on the cylinder block.
When buying a used motor, be sure to tap the key on the lower unit: a deaf sound may indicate cracks or poor-quality previous filling with epoxy.
Some manufacturers use magnesium alloys to reduce weight, but they require more care and protection from galvanic corrosion. The inner cavities of the case are often covered with special anti-corrosion compounds that can be washed out over time, requiring renovation during major repairs.
Typical lower unit Damage and Their Diagnosis
Operating a outboard motor in a real water body inevitably causes defects. The most common problem is mechanical damage caused by collision with rocks, snags or concrete piers. Even a small impact at high speed can leave a crack that will grow invisible to the eye over time.
Corrosion is also a serious enemy, especially for motors used in salt water. The galvanic pair "aluminum casing - steel screw - bronze sleeve" creates ideal conditions for electrochemical destruction of the metal, unless there is a serviceable zinc protection, the owner must regularly inspect the condition of the anodes and replace them if necessary.
Diagnosing hidden defects often requires dismantling the knot, and if you notice that the oil level in the gearbox is falling or, conversely, the oil has turned whitish (emulsion), this is a sure sign of leakage, and the presence of cracks can also indicate vibration when working at high speeds or extraneous noise when changing gears.
⚠️ Attention: Never ignore the appearance of a "mayonnaise" substance on the control probe of the gearbox, which means that the water has already entered and the corrosion of the bearings and gears is started, and delaying repair will require the entire assembly to be replaced.
To detect microcracks that are not visible on visual inspection, specialized workshops use the method of defects using penetrating fluids or ultrasonic control. At home, you can use the method of air pressing, dipping a closed gearbox into water and watching for bubbles.
Lubrication replacement process and leakproofness check
Regularly replacing the transmission oil is a basic maintenance procedure that every owner can perform, which allows you to assess the condition of the unit and extend the life of the gears. First, you need to install the motor vertically and twist both the control screws: the top and bottom.
First, the body produces waste lubricant, and the color and consistency of the lubricant tells you a lot about the health of the gearbox. If the oil is black and it contains metal shavings, it indicates severe wear on bearings or gears. The silver paste indicates normal wear and tear, and the white emulsion indicates water intake.
☑️ Checklist of oil change in gearbox
You need to pump the new oil from the bottom up, using a special syringe pump, which allows you to push all the air out of the cavity. Once the clean oil leaks out of the top hole, you need to quickly twist the bottom screw, then, without wasting time, remove the spout of the syringe and twist the top screw, breaking this sequence will lead to an air plug.
It is important to use only specialized oils for boat gearboxes that have appropriate tolerances for viscosity and anti-scoring properties (usually API classification GL-5).
Repair of cracks and restoration of geometry
Restoring integrity gearbox After serious damage, it is a task for professionals. The aluminum alloys from which it is made are not easy to weld at home due to high thermal conductivity and oxidation. The most common repairs are argon arc welding (TIG) using additive materials similar to the base metal.
Before welding, defective areas must be thoroughly cleaned, the edges of the crack cut at an angle and the part warmed up to relieve internal stresses. Incorrect welding technology can lead to deformation of the bearing seats, which will make further assembly of the unit impossible or lead to its rapid failure.
Can you seal the crack with epoxy?
Epoxy compounds (cold welding) are only suitable for temporary solutions or elimination of micropores that are not subject to mechanical stress. On a lower unit that experiences vibration and shocks, epoxy will not last long. It is an emergency measure to reach the shore or workshop, but not a full repair.
In some cases, when welding is not possible or economically feasible, the method of installing repair inserts or even completely replacing the damaged part of the body, if the design of the engine allows it, is used. However, most often, in case of serious damage, it is recommended to replace the whole lower unit, since the cost of quality repairs can be up to 70% of the price of a new part.
After welding and machining, it is mandatory to retest the leakproofness, and it is also recommended to cover the restored areas with special anti-corrosion compounds to level the electrochemical potential and protect the seam zone.
Bearings and oils: when you need a replacement
Inside the gearbox, bearings are installed that allow the shafts to rotate, depending on the quality of the lubricant and the lack of water, and when disassembling the gearbox to replace the ossils, it is always recommended to change the bearings, even if they seem to be working, since their resource is often comparable to the resource of the seals.
Replacing the ossels (cuff) is a procedure that requires care and special detachers. Old ossels are often attached to the shaft or body, and removing them requires care to avoid damaging the landing surfaces. New seals must be lubricated with lithium lubrication before installation so that they do not turn dry on the first run.
| Component | Symptoms of wear and tear | Consequences of neglect | Recommended interval |
|---|---|---|---|
| The ointment of the paddle shaft | Oil film on water, emulsion | Water intrusion, cog corrosion | Every 2 seasons or 300 motor hours |
| paddle-shaft bearing | Knock, backflip, hum | Set-up landing, jamming | With every replacement of the galls |
| The salad of the vertical shaft | Oil in the crankcase of the engine (rarely) | Mixing oils, piston bullies | In the overhaul of the gearbox |
| Stiff bearing | Backward vibration | Reversing gear failure | When the backlash comes along |
When installing new parts, it is important to observe the tightening moment of the fasteners, which is indicated in the manufacturer's manual. The tightening can lead to deformation of the body, and the tightening can lead to loss of tightness of the connection. All gaskets and sealing rings during assembly must be replaced with new ones.
Replacing the boulders without replacing the bearings is a temporary measure. If the boulder leaks, then the bearing has already started to let water through or vibrate, destroying the new cuff from the inside. Change the knot.
Corrosion protection and preservation for the winter
Durability gearbox It depends on proper preservation, and before long-term storage, you need to replace the oil, even if it has changed recently, because during the season it may accumulate products of wear and moisture, and the old oil is left only if the engine is sent to the service for diagnosis.
The outer surface of the lower unit must be cleaned of dirt, algae and old lubricant. After cleaning, the metal is treated with an anti-corrosion spray. Pay special attention to the places of the paint chips - they must be cleaned to metal, degreased and painted to prevent focal corrosion.
Keep the engine upright so that the oil residues cover all the interior parts. If the engine is removed from the boat, remember to remove the screw, clean the shaft of the winding fishing line or grass and lubricate it with a special consistency lubricant to prevent boiling.
⚠️ Attention: When washing the motor with a kerkerker, never direct a jet of high-pressure water directly to the ossels and seals. The jet pressure may exceed the operating pressure of the seals, and the water will be forced into the gearbox.
Zinc anodes that protect the motor from galvanic corrosion should always be clean and replaced if necessary. If the anode is completely dissolved, the aluminum itself will protect the body, which will lead to its rapid destruction. In fresh water, the anodes are consumed more slowly, but you need to check their condition regularly.
Frequently Asked Questions (FAQ)
How often should you change the oil in the gearbox of the outboard motor?
The minimum frequency is once a year, before or after the season ends. If you are actively using the engine (more than 100 hours per season), it is recommended to replace twice. The main indicator is the color of the oil: if it becomes milky, you need to change immediately.
Can I use solidol or litol to lubricate the gearbox?
Absolutely not. The gearbox It requires a liquid transmission oil that circulates freely and is sprayed with gears. It will not be possible for thick lubricants to penetrate all friction nodes, which will lead to overheating and bullying. Use only oils labeled "Gear Oil" and the viscosity class specified by the manufacturer (usually SAE 90 or 140).
Why does the water get into the gearbox if the glands are intact?
Water can penetrate through microcracks in the body, leaky gaskets under the screws or through a sapoon (if it is clogged or defective), and condensate is a common cause, which is formed during sharp temperature changes if the motor is stored in an unheated room. Sometimes water is sucked through the glands when the engine is overheated when there is a sharp cooling.
What to do if the screw is turned on blanks?
This means that the veneer has been cut off (on old engines) or the slats have been turned (on modern ones). The screw must be removed and the propeller shaft and the screw hub examined. If the shaft slats are damaged, the shaft or the whole lower unit will need to be replaced. If the screw hub is damaged, the screw or its hub must be replaced.
Do I need to lubricate the splints of the propeller before installing the screw?
Yes, you must. Use a special lubricant for slit joints (often included with the motor or sold separately) that prevents the screw from sticking to the shaft and protects against corrosion. Conventional solidol is not suitable for this purpose, because it can be washed out and can not withstand high loads.