2003 Suzuki LT-Z400 Factory Service Manual, Page 170Get this manual
CHASSIS
" Remove the front wheel, 10) Remove the brake disc,
Remove the caliperthe brake disc for cracks or damage and measure Ule thickness using the micrometer, If damages are found or if the thickness is less than the service replace the brake disc with new one
09900-20205: Micrometer Brake disc thickness Service Limit: 2.5 mm