I think calling the cassette insert "small" may be jumping the gun until we know it broke. On the thought of keel threads, if they are tapped into cast iron that brings back memories of tapping cast iron. It is harder than titanium to form a thread in. As a material it is so hard and brittle and as a result the thread can be malformed. It is possible to form a quality thread in cast iron but it requires proper technique and equipment.