I was out riding this past weekend with some buddies, and my 04 450 was running great until we were doing some uphill runs on a deep sand hill. The motor was under a lot of strain, and I soon began to smell coolant. The bottom 1/4 of my right rad was wet and dripping coolant on my exhaust. I pulled the rad off and had it tested at 20 lbs pressure with no problem. I just noticed a round indent on the front of my cylinder just above the exhaust looks like something (like some kind of plug) was there, but now is not. Can anyone identify what was there and what I need to replace it with? I cannot find it in my manual. Thanks in advance.