I use 4.0 and I bought them from Hong Kong off of Ebay. I paid $.01 plus $6.99 shipping and they are just as good as the local ones I bought. They even came with a "free gift". 2 foot long pieces of red and 2 foot long pieces of black shrink tubing.Want to put bullet connectors on my motor connections to my esc so I can change motors without having to solder all the time. I'm using 12ga wire what size bullet connectors do I need to use 3.5mm or 4.0mm.