Be carefull! Many model engines, especially the older ones leak by the valves. When runiing the castor oil seals this and the leak is not relavent. If it is running fine and producing the power it should leave it alone. You may gind and grind and never get the leak stopped and only end up with a sunken valve face that can not be sealed by the castor oil. Please see Bill Robinson's comments in the Saito Club regarding this topic.