Metric Bolts

Metric bolts are sized according to diameter, distance or spacing (in millimeters) between threads and length.

For example a size of 12x1.75x30 translates to a bolt that is 12mm in diameter, has a distance of 1.75 between the threads and is 30mm long.

Metric bolt sizes can also be listed as M6x1x25. This bolt is 6mm in diameter, has a distance of 1mm between threads and is 25mm in length.


Grades

The grade or strength of a bolt should be ink stamped on the head. It is not always there, so do not be overly concerned if you have bolts without a stamp. The higher the number, the stronger the bolt.

Threads

A metric bolt has metric threads. Period.

Metric Nuts

Metric nuts have no identifying marks. When you purchase them try to keep them either in the original package or separated by size so you do not run into a problem later on.

Metric nut sizes are diameter x thread spacing. A 6x1x35 bolt needs a 6x1 nut.

When you see nuts listed as Ny-lok or nylon threaded it means that they have a nylon-plastic substance in the threads that keep them from loosening due to heating and cooling cycles or vibration.


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SAE vs Metric Bolt Sizes

As I explained at the top of the page, metric bolt sizes are stated as diameter x distance between threads x length.

American or SAE bolt sizes are expressed as diameter x thread pitch x length. Thread pitch is the number of threads per inch.

An example of a SAE bolt size is 1/2x13x1. This bolt is one half inch in diameter, has 13 threads per inch and is one inch long.

SAE bolts will not thread perfectly into a metric nut or metric threaded hole.

SAE Grades

SAE bolt grades are identified by lines cast into the bolt head. For example 3 lines identifies a grade 5 bolt, 5 lines is a grade 7 and 6 lines is a grade 8.

The higher the grade, the stronger the bolt. A grade 5 bolt has a minimum tensile strength of 92 psi. The grade 8 strength is 130.

SAE bolts can also be graded with an ASTM scale. Bolts graded according to ASTM usually have an alphanumeric code on the head (BC or A 325). The are widely used in automotive applications.

Threads

SAE or American bolts come in two thread styles: fine and coarse.

If a box of American bolts says they are SAE, they are fine thread bolts. If the box says USS, the bolts are coarse thread.

SAE Nuts

American or SAE nuts have no identifying marks. When you purchase them try to keep them either in the original package or separated by size so you do not run into a problem later on when you try to thread one on a metric bolt.

SAE nut sizes are diameter x thread pitch. Sometimes the packaging will indicate fine or coarse threads. A 1/4x13x1 bolt needs a 1/4x13 nut.


Screw vs Bolt

A screw is not a bolt. Screws are pointed on the end, bolts are not. Please do not use screws on your motorcycle, dirt bike or quad, ever. I do not care that they were handy or sort of fit. Not only it is a stupid thing to do but, because they are sharply pointed, you can inadvertently cause damage to your machine.



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