There are standard bars and oversized bars. Additionally, there are steel bars (standard or oversized) and aluminum bars (standard inner diameter, oversized outer diameter). The type of bars you use will be determined by the type of compression you want to use and vice versa. Standard bars work with threaded compression, IHC, ICS, and small SCS (or fullsized SCS with shim). Oversized bars work with fullsized SCS and HIC. Aluminum bars work with ICS and fullsized SCS.