In my case, this never happened with city bikes (mamachari), but with sports bikes, I sometimes want to replace the bolts.

For example, when they rust, or when I want to replace a bolt that I felt was too long from the start.

This is just my personal observation, but when you buy foreign parts, the bolts tend to be longer.

Perhaps there's no need to worry much about it, but as a mechanical designer, bolt length is a point of great concern for me.

I will tell you what to pay attention to when replacing bolts.

You might think you just need to replace it with an identical bolt, but please check the following points.

■Standards

Screws have a helical, wavy shape. The peaks and valleys (or valleys and valleys) have a certain length (pitch).

There are mainly inch and millimeter pitches, and if you buy the wrong one, even if the screw is of a similar thickness, it won't fit into the hole properly.

In most cases, buying a millimeter screw will be fine, but occasionally inch screws are used, so please be careful.

Millimeter screws are defined by JIS (Japanese Industrial Standards), so you can easily search for them if needed.

If you search, you might find "coarse" and "fine" pitches, but in most cases, it's "coarse" so you can forget about "fine" pitches.

When I went to a home center to buy screws, most of them were coarse millimeter screws.

If rare parts are used and millimeter or fine-pitch screws are utilized, I think buying them through online mail order is the easiest way to obtain them.

If you really don't know, there are screw shops in town, and if you bring it to them, they will investigate thoroughly and inform you.

■Head Shape

Screws have head shapes.

The one in the image is what is called a cap screw.

In most cases, it is called a "cap screw" and may be written as "CS", but in Japanese, it is called a "hex socket head screw".

As the name suggests, it has a hexagonal hole for an Allen wrench.

You cannot tighten or loosen it with a Phillips or flathead screwdriver.

Since it is used in places where you want to tighten it with proper torque, it is not recommended to replace it with a screw of a different head or hole shape.

■Length

Regarding length, I think it's better to replace overly long bolts with appropriate lengths.

Overly long bolts protrude and can catch on things.

They can also cause injuries.

The part that protrudes through the hole has no mechanical effect.

If the screw can be properly tightened, replace any excessively long screws with appropriately sized ones.

■Thickness

If limited to those used in bicycles, I think there are only M4 and M5 thicknesses.

M4 means the diameter of the thread crest (the thickest part of the screw) is 4mm.

M5 is 5mm, and so on.

There are no odd sizes like M4.2 or M4.5 for standard screws.

If you take the necessary screw to a home center and compare it with what's on sale, you should quickly know if they are the same.

■Material

Material is also very important.

Some people think that "stainless steel is the strongest because it doesn't rust," but that's not always the case.

If you need many, iron can sometimes be cheaper in terms of cost.

If limited to bicycles, you probably won't use many screws in the same place, so stainless steel is fine, but sometimes iron screws are better for iron parts.

■Plating

Plating may not be very familiar to the general public, but it has the effect of preventing rust.

The image should make it easy to understand.

From the left, stainless steel is usually not plated.

It is used in its raw state.

It has a slight luster, but it's a silver color that isn't very shiny.

The center is an iron screw, nickel-plated.

Iron rusts in its raw state, so in most cases, it is plated before use.

Shiny silver screws are nickel-plated.

Next is "chromate plating," which is a slightly bluish silver.

It's cheaper than nickel plating, and its rust prevention is inferior to nickel plating.

Finally, the one on the right, which has already rusted, is "blackening."

It has various names such as "triiron tetroxide film" or "parkerizing."

Its rust prevention is weak, to the point of "not being nonexistent," and it rusts quickly if it gets wet.

When talking about rust prevention, care must be taken when using iron screws on aluminum parts.

This is called "galvanic corrosion" or "electrolysis," but when different types of metals come into contact (detailed explanations omitted), they become more prone to rusting.

Some combinations are more prone to rust, and aluminum and iron are a combination that tends to rust easily.

Therefore, plating is used to prevent this.

Depending on the type of plating, the bare materials may come into contact when the screw is tightened, which can cause rust.

From my personal opinion, plating equal to or better than nickel plating would be preferable.

In the case of stainless steel, there is no plating, but it has the ability to form a thin film on its surface, making it resistant to rust.

However, when exposed to water, a phenomenon called "contact rust" can cause yellow rust to appear, so it's a good idea to wipe it dry after it gets wet.

As you can see, even for just replacing a screw, there are many points to consider.

If you're concerned, it might be a good idea to consult a bicycle shop.

External writer: Koichi Okuno