Chain size designations e.g. 1/2 x 3/32 : how to measure?

How is the second number, e.g. 3/32, determined? Do you measure:

a) the thickness of the spacer between the inner links

or

b) the distance between the inner links

I have an older bike and don't know if the chain is 3/32 or 4/32 (i.e. 1/8). It could be either. The spacer itself is 3/32 thickness and the distance between the links is 4/32.

a) the thickness of the spacer between the inner links

b) the distance between the inner links

Those two measurements are the same thing. The roller (spacer) sits between the inner plates, so its thickness is the spacing between the plates.

If you look closely at the inner links the inner edge is chamfered, making it difficult to measure the actual distance between them - I guess that's why you are coming up with different measured widths. Measuring at the roller is more accurate.

So, I guess you have a 3/32 chain.

• Is it possible though for two chains with the same roller widths to have plates of different thicknesses, possibly making the thicker one incompatible with a given chain tool? Aug 22, 2020 at 18:39
• @K.Minkov 8, 9, 10, 11 speed etc chains do have progressively thinner plates to allow for closer sprocket spacing. All chain tools should accept all chain widths though. Aug 22, 2020 at 20:33
• Sorry I wasn't being specific. It's about a BMX chain that seems to be 1/8" (measured the distance between the inner links, it's just about 3.2 mm), but the links themselves are apparently too thick and just don't fit between the anvil and the cradle of the tool I have. Aug 23, 2020 at 15:54
• @K.Minkov sorry I misunderstood. It’s plausible a chain tool is only compatible with 3/32 chain and not 1/4 Aug 23, 2020 at 16:49

Based on https://www.sheldonbrown.com/gloss_ch.html, I would say you've a 3/32 chain (measured by the size of the rollers). It's a standard size, and in my impression the distance between the inner links can vary if the chain is worn, while the width of the rollers remains constant.

Chain size is specified by pitch and width. The pitch is the distance between rollers (1/2" on all modern bicycle chain). The width is nominally the width where the sprocket teeth fit in. Bicycle chain comes in four basic widths:

• 3/16" (.1875", 4.76 mm), used until the middle of the 20th century on many bicycles; this was inch-pitch chain.
• 1/8" (0.125", 3.18 mm) chain is used on most single-speed bicycles, and bicycles with internal gearing.
• 3/32" (0.094", 2.30 mm) chain is used on derailer equipped bicycles that have more than 3 cogs at the rear.

Even somewhat narrower chain, typically .090" or 2.29 mm between the inner plates, is used for derailer-equipped bicycles with 9 or more sprockets at the rear.

• I had looked at that Sheldon Brown page but the phrase "the width is nominally the width where the sprocket teeth fit in" didn't help me.
– Tim
Mar 6, 2019 at 13:08
• @ArgentiApparatus Indeed, the Park Tool paragraph was confusing. I've removed it, as it was more about compability of chains width of different speed drive trains. Mar 6, 2019 at 13:09
• @Tᴚoɯɐuo: if you would press the inner links towards each other, as would be for a new chain, they will be as narrow as the roller allows. The size is thus determined by the inner roller, not the links that might get worn over time. Mar 6, 2019 at 13:13
• That Sheldon Brown link seems to bea bit misleading. To me, it implies that 9+ speed bikes all use the same width chain, but 10-, 11-, and 12-speed chains are all narrower than 9-speed chains. The internal width between the inner plates may have been the same when the page was first posted, but even if that's still true (is it? Campy is up to 12-speed...), the outer width has gotten much narrower. Mar 6, 2019 at 14:27
• The space between my inner links, when they are pressed together with needle-nose pliers, is just over 3mm.
– Tim
Mar 6, 2019 at 15:15