You can reorder thread types, customize thread designation and thread type names, and also create custom thread types or clearance hole types.
You can change the order in which the thread types appear in the Hole and Thread dialog boxes so that the thread type you use most frequently appears first.
Enter a thread designation string for each thread size in the Custom Thread Designation column of the Thread Data Spreadsheet. The string is then available in thread and hole features and can be recovered in hole notes in a drawing.

With Inventor closed, open the Thread.xls file.
Click the tab of the worksheet you want to edit.
Locate the Custom Thread Designation column (column D in a Parallel worksheet or column B in a Taper worksheet) and edit the cells as appropriate.
Save the spreadsheet.
Start Inventor and view your changes.
You can change the thread type names so that they appear in the Hole and Thread feature dialog boxes as appropriate for the local language or to suit the conventions of you company.
Close Inventor, and open the Thread.xls file.
Copy an existing worksheet of the same type (parallel or taper).
Delete most or all of the data rows (row 4 and below), and add data rows as required.
Rename the worksheet name (thread type name) and custom thread type name (cell B1/C1).
Edit cells A1 and D1.
Save the file.
Use the same procedure to add a clearance type to the Clearance.xls file