Kiểm tra "xi lanh x piston" trong quá trình tháo động cơ RZ250R

The engine that was unloaded for the restoration is in fine condition and has no defects. ......?

Inspection of the upper waist of the water-cooled RZ-R engine of the second generation of the Yamaha RZ series

Unlike the first generation RZ250/350 series, the RZ250R/350R series has the exhaust device = YPVS function and has succeeded in dramatically increasing power. 29L is a popular name among fans, since the model model model of the 250 series was 29L.

The cylinder head is removed and the cylinder is pulled out to expose the piston. The piston becomes a single piece by removing the piston pin clip on one side and pushing the piston pin out. Having a sickness gauge allows for various inspections.

Piston is removed before checking skirt condition.

Let's stare at the skirt of the removed piston. Check for scratches or burrs that look as if they have been scraped off, as it is natural to expect fine scuff marks. Also, check for burrs in the scavenging holes of the piston skirt. If there are burrs, remove them with a scraper.

There were no noticeable scratches on the exhaust side skirt, which was good, but there were scratches that straddled the piston rings only on the exhaust side above the piston. I assume the scratches were made by sliding against the exhaust port. I would scrape up such scratches.

After removing the piston ring using the tip of the toe to avoid breaking it, prepare a piece of 600-grit waterproof paper and rub the piston crosswise with the waterproof paper folded in two, as if you were "sliding and stroking" it, rather than rubbing it hard. It is advisable to use a rust-preventive penetrating spray in combination with the paper.

Instead of rubbing out all scratches, just enough rubbing together to remove burrs at the edges of scratches is fine. Just a gentle, cross-like hit-and-run removal is all that is needed to take care of the cylinder and piston. They are more delicate than you can imagine, so don't rub too hard for no reason.

Cylinder and piston rings are combined to check condition.

The inner walls of the cylinder should not be cleaned, but rather the edges of each port hole should be checked without removing the cylinder. If the port edges are free of metal powder buildup, but there is the slightest trace, remove the buildup and any burrs from the cylinder with a file or precision rheater blade.

Fit the piston rings horizontally in the cylinder, taking into account the orientation of the piston ring incorporation. Let's check the gap between the rings with a chicness gauge. The standard value for new parts is 0.30 to 0.45 mm for both the top and second rings, and if it reaches 0.70 mm, it is beyond the limit of use.

Piston clearance can also be temporarily checked with a Thickness Gauge.

Inserting a single piston into a cleaned cylinder to check the situation. When removing and cleaning the exhaust device, use a chemical such as carb cleaner to remove dirt and do not use water resistant paper to remove adhered carbon stains!

After inserting the piston into the cylinder (be sure to align the orientation), you can temporarily check the piston clearance by inserting a sickness gauge into the center of the front and rear skirts. 0.060 mm or so for the RZ250R engine. The pin side clearance is wider than the skirt side.

Carbon deposits are removed by "dissolving with chemical application" and then wiping off.

Carbon buildup is deposited, but instead of soaking the car in kerosene or washing oil and then rubbing or polishing with sandpaper, spray the car with a spray-type carbon cleaner and wait a while for the deposited carbon to soften and loosen to make it easier to remove.

It is best to soak the carburetor for a while in carbon cleaner or liquid carburetor cleaner to dissolve the carbon before removing it, but if time is short, a soft brass brush can be used in combination with a soft brass brush to scrape off any accumulated carbon. Do not scratch the piston.

A piston that has been cleaned after removing carbon deposits and a piston that is still disassembled. Forcing the pistons to remove dirt and smooth out scratches on the piston skirts will increase piston clearance and cause mechanical noise.

The waist-high part of a 2-stroke engine is relatively easy to disassemble and reassemble due to its simple structure. Some users must have experienced disassembly and maintenance in the past out of curiosity; before the late 1980s, two-stroke engines were the norm for super sports models, small-displacement models, moped scooters, and other models from all manufacturers. There must have been many riders who had such experiences, for example, forgetting to add engine oil when they were running all the time, causing the engine to burn up, or being saved from a crash because they were running at low speeds. In particular, many veteran riders in their 50s or older have such episodes. Nevertheless, the recent boom in old cars has led to an increase in the number of young riders who like 2-stroke engine models. In any case, even a 2-stroke engine is still a precision machine, so a great deal of care and know-how is essential when disassembling and maintaining it.

If a seized engine needs to be disassembled and repaired, it may require some rough treatment, but if the engine has been running in good condition, such as the Yamaha RZ250R reported here, many say it is better to let it run as it is until it is needed. If you must disassemble and clean the engine, be sure to remove carbon buildup from the piston tops, combustion chamber, and exhaust valves, as well as any unburnt engine oil.

To clean such dirty parts, use a chemical cleaning solution called carbon cleaner or carb cleaner chemical. Keep in mind that dirt should not be removed by "scrubbing" but should be dissolved, broken down, and then "washed away.

Let me share an experience with you here. When we disassembled the cylinder of an air-cooled 2-stroke engine, the piston top was black and dirty with carbon deposits, and there were numerous scratches on the piston skirt. So, while soaking the pistons in washing oil, we roughly removed the carbon buildup with a wire brush, and then polished the piston tops with 240-320-grit water-resistant paper, which removed the carbon. With the momentum of the process, even the piston skirt was polished hard, so that scratches were no longer noticeable, and the piston regained its aluminum surface from carbon stains. The piston rings were assembled to the polished piston, and the engine waist was reassembled and restored. When the engine was started, the engine noise (piston slap sound) was clearly louder than before disassembly.

The 2-stroke engine can run under such conditions, but the unusually loud mechanical noise is a big problem. Such an experience made me realize that scrubbing off the carbon stains with sandpaper would cause a different kind of trouble.

When disassembling and cleaning an engine that had been running in good condition, like this RZ250R, we should be especially careful to remove the dirt carefully.

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2026.8.9

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