Link to an excellent article at Finehomebuilding.com I have referred to several times during home projects. http://www.finehomebuilding.com/pdf/021190076.pdf
How to Fix a leaking valve cover on a zero turn mower. This is a common cause of oil leaks on riding mowers and very easily corrected with little time or money. Do it before it causes a catastrophic engine failure. $5 now can save a thousand later!