Hi, I'm new to the forum. Last fall I purchased a 1999 Simplicity Sovereign 18 hydro as an upgrade to our 32 year old Allis Chalmers 910, a Simplicity twin. Unfortunately the tractor isn't in the condition I was lead to believe and after less than 20 hours I'm having to replace the engine. The engine is a Kohler Command OHV , model CH 18 S. A cylinder sleave has moved/came loose. My repair guy indicates the engine was a new design at the time and didn't work out. There is no short block or other fix other than a new engine of different design and kit to fit the Sovereign. So beware of a Sovereign with the Kohler OHV 18 engine. Dan