As I recall, the head and back plate bolts are 2.5mm. Get a good, aluminum handle wrench. It'll set you back $6-$7 bucks, but it's a really good investment.
Another thing, again as I recall, the OS head gaskets for the .46 & .55AX are one time use. They crush down when installed and don't seal well on second use unless you get them EXACTLY in the same position. They are also pricey.