i know what your talking about, these below.

they are commonly called trousers, but i'm pretty sure there is no gasket; its unusual for them to leak there; make sure you havent got a hairline crack or anything.

the bolts are notoriously difficult to undo, alot of people end up rounding the bolts off, you will need a socket that fits in the middle and on the outside of the bolt - be prepared to get angry with em! - if you do manage to get them off there is no reason why you cant use sealant, but as i said very weird for them to leak there, my money is on it being pitted through or cracked maybe