Hi. What is the difference between a HG40 and HG50 and SRAM chain? I am putting a new chain on my cheap mountain bike (which does not do a lot of work) and am a bit baffled by the choices. It is a 12 or so year old bike with a Altus seven-on-the-rear group-set. Please do not do too much typing on my behalf but I cannot seem to find the answer anywhere on the internet, including the shimano web-site.