Index: app/models/issue.rb
===================================================================
--- app/models/issue.rb	(revision 14155)
+++ app/models/issue.rb	(working copy)
@@ -300,6 +300,10 @@
       end
       @custom_field_values = nil
       @workflow_rule_by_attribute = nil
+	  if tracker.disabled_core_fields.include?("estimated_hours")
+		self.estimated_hours = 0
+		recalculate_attributes_for(self.id)
+	  end
     end
     association(:tracker).writer(tracker)
     self.status ||= default_status