Adding or removing chain links is almost the easiest thing a mechanic can do. It doesn't hurt the integrity of the chain in the slightest. If he is a mechanic he will put the right length in to start with but can change it. The easiest way to do it is he will just count the number of links in the old chain. If it is a new install with no old chain there are several ways to figure the length that are again simple to know how to do. They all work about the same. If you are doing a fixie without a horizontal drop out bike it may not be possible to do it. Roger