Ford 2003 Mustang Workshop Manual page 1688

Remove the valve spring retainer keys, the valve spring retainer, the valve spring and the valve
stem seal.
Installation
1. Using the special tools, install the new intake valve stem seals.
2. Using the special tools, install the new exhaust valve stem seals.
3. Install the Valve Spring Compressor Spacer between the valve spring coils to protect the intake
valve stem seal from damage.