The normal procedure of maintenance is to work with a purpose, but many Sunday mechanics gain experience with the motive of "I don't know what it is, but I want to see what's inside". The best teaching tool for such motorcycle tinkering is an air-cooled, two-stroke, single-cylinder model. Although the two-stroke model itself is now rare, its simple construction, which allows you to remove the cylinder head to meet the pistons, is still very appealing.
Simple construction that allows you to face the cylinder and piston in just 5 minutes
On an air-cooled, single-cylinder, 2-stroke engine, the cylinder head can be removed by simply removing the four nuts; on a 4-stroke engine, the pistons are aligned with the upper dead center of compression, the camshaft chain tensioner is loosened, the camshaft sprocket is removed and the camshaft chain is shifted, and then the nuts are finally removed. It can be removed, so the hassle of getting there is already very different.
In today's world of motorcycles, where 4-stroke engines and injection have made their way up to 50cc moped scooters, the air-cooled 2-stroke is a fossil. With the exception of the popular out-of-print Kawasaki Triple Series and racer replica models such as the NSR250R, moped class 2-strokes are rarely seen in the marketplace.
However, it's a fact that many of the veterans who have been familiar with motorcycles since the Showa era are familiar with the two-stroke machines. Apart from their hobbyist and maniacal nature, two-stroke motorcycles used to be affordable and simple to use as a material for tinkering with motorcycles. The big appeal of the 2-stroke engine was that it could be made faster than a 4-stroke by simply replacing the carburetor and chamber and shaving a few ports. Also, because the engine didn't have a camshaft chain or intake and exhaust valves like a 4-stroke, it was extremely easy to disassemble and reassemble the engine, which was a big plus for young riders at the time who didn't have special tools or service manuals.
From sports models to scooters to family motorcycles, small-displacement motorcycles were often revitalized with simple maintenance, and riders who enjoyed tinkering with their motorcycles would take off the cylinder heads and cylinders and polish the pistons.
Whether the result was good or bad, the parts themselves were not extremely expensive, so even if I failed once, I was able to try twice or thrice to hone my skills in tinkering with the motorcycle. It may be misleading to say that it is assumed to break down, but the time when I was able to tinker with my motorcycle thinking that trouble is my sustenance may be a blessing, depending on how you look at it.
If you haven't removed the cylinder for a long period of time, the cylinder base gasket may have taken the place of glue and stuck the cylinder and crankcase together. Hit the cylinder fins, being careful not to break them, and if they still don't come off, hit the hammer that made contact with the cylinder with another hammer to impact it.
- Point 1 - Simple construction and easy disassembly and reassembly of the 2-stroke engine
- Point 2 - Other than popular out-of-print motorcycles, the number of two-stroke motorcycles is on the decline.
Two-stroke and four-stroke engine oil usage are completely different.
On an old two-stroke engine from the 1970s, it's not surprising that the sides perpendicular to the piston pins have more or less wear marks on them. Cast-iron cylinders and aluminum pistons have different expansion rates for heat, so they're prone to scratching when used like a moped scooter that opens the throttle all the way up, even when the engine is under warmed up.
When repairing the scratched area with sandpaper, smooth out the scratches by applying the paper at an angle to the vertical scratches. It is not to try to shave off all the scratches, but it is good if the bumps and gaps become gentle. I use No. 600 which is not able to shave the eye of the paper too deeply.
It's a metaphor that engine oil is as important as blood in a four-stroke engine, and the same is true of a two-stroke engine: the mixture is sucked into the crankcase, where it is subjected to primary compression and then flows through the scavenging ports into the cylinders to burn. This is the same on moped scooters and the former GP500 racers. The four-stroke, on the other hand, draws in air from the intake valve in the cylinder head and the mixture goes directly into the combustion chamber.
Because of this difference in the mechanism, the four-stroke engine oil only lubricates the crankshaft, pistons and cylinders, camshaft, intake and exhaust valve stems, transmission and clutch, and everything outside of the combustion chamber, whereas the two-stroke engine oil goes into the crankcase with the mixture to lubricate the crank bearings and it is pumped through the scavenging port into the combustion chamber to lubricate the pistons and cylinders, which eventually burn with the mixture and are exhausted with the exhaust gases.
In other words, while four-stroke oil stays in the engine, two-stroke oil is what burns and is consumed with the mixture. This may be an obvious fact for riders who have been riding two-stroke engines for a long time, but for younger riders, for whom two-stroke engines were already an endangered species when they first started riding motorcycles, it may be more surprising that the engine oil disappears. Two-stroke motorcycles manufactured before the early 1960s used an even older method of supplying engine oil, called mixed gasoline, which was a generation old. When filling the fuel tank, the engine oil was added at the same time to achieve the correct mixture ratio.
However, since the mid-1960s, when oil pumps that could automatically supply oil for two-stroke engines became popular, it has become possible to fill an oil tank separate from the gasoline tank with engine oil, so that the optimum amount of oil can be supplied to the air mixture or into the engine, depending on the engine's speed and load conditions.
The reason why 2-stroke motorcycles are generally more likely to seize up than 4-stroke motorcycles is partly because of the way the engine oil is supplied. First of all, if the oil tank is empty, there is no more oil to lubricate the inside of the engine, so it will seize up 100% of the time. There is also a risk of seizing if there is a problem with the discharge of the oil pump, which is essentially linked to the opening of the throttle valve.
If you run out of lube, you'll burn out, even on a 4-stroke, but the danger is different depending on whether the oil is in the crankcase or not (some 4-stroke models, like the Yamaha SR400, have an oil tank in a separate location from the engine).
- Point 1- There are engine oils for both 2-stroke and 4-stroke, but their roles are very different.
- Point 2 - 2-stroke oil needs to be replenished as it burns with the air mixture.
Most of the engine noise and loss of punching power is above the waist.
The joint gap of the piston ring is measured by pushing the ring horizontally through the notch in the cylinder skirt, where there is no port opening. The standard gap is between 0.15 and 0.30 mm, whereas the limit of use is 0.7 mm, but a gap of 1mm or more can be driven if you turn a blind eye to the lack of power feeling. In the 1980s, there were many cases of people buying a new scooter rather than repairing it because they were told that it was time for them to change their moped.
The 4-stop piston ring can rotate in the ring groove, but the 2-stop piston ring groove has a knock pin and the piston ring can't rotate, because if the 2-stop ring turns freely, it will get caught in the port opening. When inserting the piston into the cylinder, make sure the joint of the ring is not riding up on the knock pin and push it down with your fingers.
Before tightening the cylinder head nuts, press down on the kick pedal a few times in a tentative state to help the piston and cylinder fit together. Then tighten the four nuts diagonally several times.
Seizing in a two-stroke engine occurs mostly at the sliding surfaces of the cylinder and piston. When the oil supply is absolutely low or the oil tank is empty, the bearings on the crankshaft and connecting rods can seize up, and in this case, the damage is more severe.
Light rubbing of the cylinder and piston without making any noise is still a common occurrence on old-fashioned 2-strokes, and if the damage is not deep enough to catch a nail, it can be corrected by lightly rubbing it with sandpaper and then reusing it. In this case, hang the paper over the pistons by stroking them with a coarse grit, about 600 or so, so as not to deform them significantly.
Apart from seizure, wear of the piston rings is an inevitable part of the engine's aging process, and in a 2-stroke engine, both intake and exhaust gases are expelled through ports in the cylinder wall, and as the pistons move up and down, the piston rings pass through the port openings, and the piston rings are then released. The piston ring itself is tensioned against the cylinder to prevent energy leakage as the mixture burns, but that tension also presses against the port openings, which tends to increase the wear of the ring compared to a simple cylinder, a four-stroke cylinder.
Piston ring wear progresses at the contact surface with the cylinder, but as the outwardly tensioned ring wears, the gap at the joint, where part of the circle is cut, widens. This causes the pressure generated by the combustion of the mixture to escape through the widened joint instead of pushing the piston down, resulting in a loss of engine power.
Using a moped as an example, the standard clearance for new piston rings is 0.15 to 0.30 mm, with a usage limit of 0.7 mm. The gap between the piston and the piston ring is measured with a thickness gauge when the piston ring is removed from the piston and inserted into the cylinder. The amount of wear on the piston ring can vary depending on the mileage, but once the ring has worn to the limit of use, the engine will feel like it's just spinning, with little punch. In this case, replacing the piston ring with a new one will greatly improve the engine's feel and make it feel much better.
There is no doubt that the 2-stroke engine is still a great teaching tool for tinkering with motorcycles, as you can easily disassemble and reassemble the engine, which you can't easily experience on a 4-stroke vehicle, and simply replace the worn parts to restore performance. If you get a chance, get a 2-stroke vehicle and experience the fun of engine tinkering.
- Point 1 - Minor piston damage can be repaired with sandpaper.
- Increasing the gap between the joints due to piston ring wear will cause a decrease in the power feeling of the engine.
See HONDA Moto Index Page
See YAMAHA Moto Index Page
See Accessories for HONDA NSR250R
See Accessories for YAMAHA SR400
