To Set a Custom Balloon as the Default Balloon Style (AutoCAD Mechanical Toolset)

At the Command prompt, enter AMOPTIONS.
Select the AM:Standards tab.
In the Standard Elements list, double-click Balloon. The BOM Settings dialog box is displayed.
In the Balloon tab, in the Default style section, select Custom. The drop-down list next to Custom becomes available.
If the block to use as the custom balloon is in the current drawing, select the name of the block in the drop-down list.
If the block is in another file:
1. In the drop-down list, select Browse.
2. Open the file that contains the blocks. The names of the blocks appear in the drop-down list.
3. In the drop-down list, select the name of the block.
In the Default style section, click . The Balloon Settings dialog box is displayed.
In the Leader connection section, select the position of the leader connection.
Click OK until all dialog boxes close.