I bought a replacement chain for my BMX – obviously a 1/8” chain. I cut it down to the same number of links as the chain I was replacing. However, when I installed it, it was so long that to get any chain tension I had to have the axle right at the end of the dropout, almost falling out of the dropout. If I remove one link, then the chain is so short that the axle has to be right the way forward in the dropout, and it’s almost impossible to get the chain on the chainring & sprocket. With the old chain, the axle was in the centre of the dropout.

While the obvious solution to this is to get a half link chain, that’s not what I’m interested in – what I want to know is how two almost identical chains can give such a huge discrepancy. Any ideas?