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M6 vs. M8 Bolt Size

M6 vs. M8 Bolt Size

Introduction to M6 and M8 Bolt Sizes

Bolts are among the family of threaded fasteners, and they are usually paired with a threaded nut when in use. Bolts can be divided into various types based on their unique head shapes, materials, strengths, and so on. Bolts are used for joining two or more components together, and you will find them all over your mountain bike, holding several parts together.

About the ISO Metric Screw Thread

ISO metric screw threads are the most popular type of screw threads for general use. It was also among the first international standards agreed in 1947 when the International Organization for Standardization (ISO) was set up.

Letter M usually designates an ISO Metric screw thread (outer diameter of the screw in mm), followed by the maximum thread diameter in mm (represented by the value of the nominal diameter “D”), a hyphen sign, and then the pitch (“P”) in mm. An example of the metric ISO screw is M8-1.25.

If the pitch of the bolt is the coarse pitch that was listed in ISO 261 or ISO 262, you can omit it (e.g., M8.) The length of a machine bolt is denoted by x, and it is expressed in mm, e.g., M8x30 or M8-1.25×30. The tolerance classes that were defined in ISO 965-1 can be appended to these designations, for example, M500-6g.

Note: external threads are designated with lowercase letter g or h, while upper case letters G or H designates internal threads.

What do M6 and M8 mean for bolt sizes?

The M denotes metric. Metric threads are usually designated by diameter and thread spacing. M6 means 6 x 1 mm threading, and it is considered the second most common threading on bikes. It has an ISO dimension M6 x 1mm. You can use 5mm Allen wrenches or 9/10mm hexagonal wrenches to screw and unscrew the bolts and nuts.

M8 bolts mean 8 x 1.25mm threading, and you can find other pitches being used in the 8mm diameter. It has an ISO dimension of M8 x 1.25mm. You can use 6mm Allen wrenches or 13mm hexagonal wrenches to screw or unscrew the bolts and nuts.

What are M6 and M8 bolts used for on a bike

M6 bolts are common on bikes, and they are used for various functions. You will find M6 in brake caliper retaining bolts, derailleur mounting bolts, stem bolts, seat post bolts, anchor bolts for brake and shifter cables, seat post bolts, brake and shifter lever mounting, and lots more.

M8 bolts are not as common as M6 and M5 bolts on a bike, and you will find them on quill stem bolts and some seat post bolts.

Measuring of bolt size

Here is a video showing how to figure out the size of the bolt that you need (Credits to Handy Man):

The instruments that you can use for measuring bolt size are thread gauge or thread checker, and calipers. You can also use a metric ruler to work out the size. For instance, if the bolt size is M8, you can determine this by measuring the shaft diameter in mm to get the M8 value.

Note: The bolt size cannot be determined with the socket wrench that the bolt uses.

Final Comments

If you need to replace a missing bolt, finding out the bolt size you need is not too difficult. However, if you’re unsure of a particular bolt’s size, you could always take your bike to the nearest bike store and have it checked.

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