http://forum.grasscity.com/attachmen...nformation.jpg
here is a chart for measuring your stem so you get the right fit. Just measure the stem that came with your ashcatcher and buy a diffuser of the same joint size and same length and youll be set son.
its really not rocket science, i dont know why people would tell you to get a reducing stem unless you see its absolutely nessicary. If you have a 18.8 joint roor, with 18.8 bowls. just get an 18.8 stem or diffuser so you dont have to buy 14.4 accessories after you buy a reducing adaptor like some advised