|biff's on the right track. you require hi tensile grade 8 king pin bolts.
i suggest you visit your local caterpillar dealership taking one of your old kingpins as a sample.
most sk8 kingpins are 3/8 inch national fine thread.
they use those bolts all around bulldozers.
the partsman will easily be able to match your sample.
they are also very inexpensive.
mine have been hanging in there sice 1980, no probs.
goodluck and happy rolling, cheers, harry.