I am looking for advice in what to look for when buying a used M1 Garand op-rod. I am looking for a post war HRA and have found several but each discussed the piston diameter and I am unfamiliar on where to measure and what number is within specs. Are there any other areas too check when buying? Does the piston diameter of a Garand op rod equate to the tab thickness of a M14 op rod when it comes to the primary wear point? Thanks.