I am pretty sure it is M20. I ordered an oil filter adapter for oil temp/pressure from prosport and got the M20 size. The threading is perfect, but I have not been able to intall it yet.
The reason is when I tried it out I got 2 small leaks where the small bolts went into the plate, since there are 4 holes but I only have 2 sensors in there. I bought some kind of threadlock sealant, so hopefully when I try again it won't leak.
