Ford 2003 Mustang Workshop Manual page 2890

gear.
Insert a feeler gauge and measure the clearance, while applying pressure and
rotating the blocking ring. The measurement should be the same around the entire
circumference. If the clearance is less than 0.38 mm (0.015 in), install new
blocking rings.
16. Using the special tools, install the countershaft rear bearing.
17. Using the special tools, install the countershaft front bearing.