Onsoku, I'm home so I checked for you. It's 5/16. The first time I took it off, I had to use a hollow metal pipe to loosen it since it was on pretty tight but I didn't tighten as much when I was done. Sounds like it's your first time removing this screw. Be careful if you're refilling or replace the CTSC oil. I made a huge mistake and overfilled couple of weeks ago and the oil came out all over the ground. I'm still not sure where it came out from. I suck all the fluid out and I filled it a little below the hash mark. I haven't seen any leaks since then.
Frankie