Booleans suck, thats why its happening:) Though not many programs have perfectly working booleans:) so don't feel too bad. My suggestions on things to check would be the size of the subtraction object..make sure its a bit larger then the actually size of the whole/area you want...too close to the edge(internal and external) of the object you are subtracting from and it could cause some deformations. Also did you subtract a cylinder from a cylinder to create the large hole? The more boolean operations you perform the greater the risk of problems. Brian