I am currently nearing the end of a build and went to my local hobby store to buy some couplers to finish the build. As everyone knows small parts, such as couplers, can be expensive. So whenever I get a chance I turn to Hobbyking both to save some money and buy in bulk. I have looked on their website at couplers and was confused by the numbers. I am looking for 4-40 couplers but all I found were threaded couplers that the size given was M2x20MM? I get that a 4-40 push rod is thicker than a 2-56 so what up with HK discription? Is the 4-40, 2-56 an imperial measurement and the M2x20MM metric?