On most CAT/Perkins engine platforms the serial number tag is on the fuel filter/oil filter side of the engine which is the right side when standing looking at the radiator. Once and a while I have found them in other locations on the engine so you might have to look around a bit.

• CAT Engines: Generally, the “arrangement” number will get us what we need to located your parts.
  Example: MXS81323
• Perkins Engines: Generally, the first seven letters/numbers will get us what we need to locate your parts.
  Example: YK36548*U753636G