{"id":15817,"date":"2026-03-11T10:54:00","date_gmt":"2026-03-11T01:54:00","guid":{"rendered":"https:\/\/webstylix.xsrv.jp\/tsr\/?post_type=tips&p=15817"},"modified":"2026-06-28T10:50:00","modified_gmt":"2026-06-28T01:50:00","slug":"blog10","status":"publish","type":"tips","link":"https:\/\/tsrjp.com\/en\/tips\/blog10\/","title":{"rendered":"The Hidden Threat of Motorcycle Maintenance: “Chemical Cracking”"},"content":{"rendered":"\n

This time, the theme is “chemical cracking.”
Even in the motorcycle industry, few people know about this.<\/p>\n\n\n\n

It is a phenomenon where plastic cracks due to chemicals.
When lubricants penetrate areas under stress, the material becomes brittle and can crack.<\/p>\n\n\n\n

I have personally experienced cracks in side covers and screens because of this.
In every case, the cracks formed around the bolt holes.<\/p>\n\n\n\n

At first, I thought it was due to over-torque, but that actually wasn’t the case.
The root cause was the grease applied to make up for a poor fit.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Mechanism of Chemical Cracking<\/h3>\n\n\n\n

Why does a lubricant, which is supposed to reduce friction, end up destroying plastic? The reason lies in the molecular structure of the plastic itself.<\/p>\n\n\n\n

When you look at plastic under magnification, long, string-like molecules are intricately entangled to form its shape. When stress is applied to areas like bolt holes, hooks, or pins, these molecular strings become stretched taut.<\/p>\n\n\n\n

If a chemical like a lubricant penetrates that area, it seeps into the gaps between the molecules and unravels their entanglement. As a result, the material can no longer withstand the tension, and a sharp crack snaps open. This is the true nature of chemical cracking.<\/p>\n\n\n\n

Incompatible Chemicals and Vulnerable Plastics<\/h3>\n\n\n\n

Among chemicals, acetone and benzene are particularly dangerous.
Depending on the type, silicone can also cause issues.<\/p>\n\n\n\n

The common thread among these chemicals is their high potency, which means their side effects are equally strong.<\/p>\n\n\n\n

In addition, plastics vary in their chemical resistance. Those classified as amorphous plastics are the ones to watch out for! You need to be careful with polycarbonate, ABS resin, and acrylic.<\/p>\n\n\n\n

PC
Polycarbonate<\/span><\/td>		Alkalis, aromatics, esters, and certain types of silicone grease<\/td>Highly transparent and durable, but extremely sensitive to chemicals. It risks cracking even from the penetration of silicone oil.<\/td><\/tr>
PS
Polystyrene<\/td>		Oils and fats, gasoline, citrus peels (limonene), and silicone additives<\/td>Because their molecular structure is fragile, they can easily crack from exposure to everyday oils. Even silicone grease can be dangerous depending on the types of additives it contains.<\/td><\/tr>
ABS resin<\/td>Thinner, benzene, ethanol, kerosene, and oils and fats<\/td>Be careful of residual solvents during painting, bonding, or cleaning. Areas around stressed screw holes are particularly prone to cracking.<\/td><\/tr>
POM
Polyacetal<\/td>		Strong acids, strong alkalis, and oxidizing agents<\/td>Highly resistant to solvents, but incredibly vulnerable to acids. It degrades and deteriorates rapidly when exposed to acidic cleaners or bleaching agents.<\/td><\/tr>
PVC
Polyvinyl chloride<\/td>		Acetone, esters, and aromatic solvents<\/td>Easily dissolved by solvents. If left in contact with rubber or other resins for an extended period, components can migrate, causing a sticky phenomenon known as “color transfer.”<\/td><\/tr>
PP \/ PE
Polyethylene<\/td>		Oxidizing acids , strong solvents (at high temperatures)<\/td>Boasts top-class chemical resistance among resins. While virtually invincible at room temperature, it has the characteristic of being difficult to bond or paint on its surface.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Parts cleaner is bad news too!<\/strong><\/h3>\n\n\n\n

Even though we have been talking about lubricants up to this point, we also need to be careful with degreasing cleaners! <\/p>\n\n\n\n

Parts cleaners contain acetone and aromatic solvents, which were listed in the table earlier. The ingredients vary depending on the type, but since metal cleaners have a high degreasing effect, they are also highly aggressive.<\/p>\n\n\n\n

In addition, some detergents contain acetone or benzene. However, when washing a car, you rinse it off with water at the end, so I don’t think you need to be that strict about it.<\/p>\n\n\n\n

High-risk areas on motorcycles<\/h3>\n\n\n\n

So, which specific parts should we be concerned about?<\/p>\n\n\n\n

Cowl bolt holes<\/span><\/div>
\n

In the case of ABS resin, as shown in this episode, applying grease to prevent seizing is dangerous.<\/p>\n<\/div><\/div>\n\n\n\n

Windshield<\/span><\/div>
\n

Many are made of polycarbonate or acrylic. <\/p>\n\n\n\n

If you use a strong solvent to remove bug stains, it can shatter starting from the base where it is bolted down.<\/p>\n<\/div><\/div>\n\n\n\n

\u30d8\u30eb\u30e1\u30c3\u30c8\u306e\u30b7\u30fc\u30eb\u30c9<\/span><\/div>
\n

It is often made of polycarbonate.<\/p>\n\n\n\n

Therefore, excessive use of strong water repellents or cleaners can cause cracks in the moving parts.<\/p>\n<\/div><\/div>\n\n\n\n

There are very few people who can identify the exact type of plastic just by looking at it.
So, it\u2019s important to be careful with the chemicals you use.<\/p>\n\n\n\n

If oil or a strong solvent gets onto materials that are prone to chemical cracking, such as ABS or polycarbonate, degrease them with a neutral detergent and rinse thoroughly with water.

Also, if you want to prevent screws from seizing, you should carefully choose a silicone grease (be sure to check the ingredients) or a thread-locking agent that is safe for plastics.<\/p>\n\n\n\n

Finally,<\/h3>\n\n\n\n

Even products labeled simply as \u201csilicone grease\u201d or \u201cparts cleaner\u201d come in many different types.
Their ingredients vary depending on the manufacturer.<\/p>\n\n\n\n

That said, choosing chemicals while constantly referring to compatibility charts can be a hassle.
So, when using them on plastics, the easiest approach is to choose products labeled \u201clow aggressiveness\u201d or \u201csafe for plastics.\u201d<\/p>\n\n\n\n

Chemical cracking can be seen as the downside of careful, detail-oriented work.
But at the same time, it can also come down to just a small lack of consideration.<\/p>\n\n\n\n

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