valve clearance specs.,cat 426c 3054 perkins engine serial# 06XN03729

Hello, Everyone, I'm trying to track down the valve clearance specifications for my backhoe, I can not find anything online. I've had this machine since new and was doing some maintenance in the engine compartment, changing fan belts, radiator temp housing fixing broken muffler brackets and valve cover gasket,
Now I begin to think if I had ever adjusted the valves, and I don't think I ever have, so my question is after18-20 years is this normal...

valve clearance specs.,cat 426c 3054 perkins engine serial# 06XN03729

valve clearance specs.,cat 426c 3054 perkins engine serial# 06XN03729