I found that i have a sending unit for an oil pressure gauge on the side of my head and i wana take it off and put the bolt that goes there back on, cuz i think its leaking small amounts of oil. Does any one know what size the bolt for that is?

 

Its not a bolt its a plug. Its got around an S4 head (which unfortunately they don't sell) and its very small. What you could do is take the sending unit out, wrap the threads with some teflon tape and put it back in. That should fix the problem. I'm not even sure if you can buy that plug. It only has 2-3 threads.