the step 1 2 and 3's mean more than just bore size. heres the hks kits brief run down
Step 1, pistons rods and crank
Step 2, pistons with coatings rods and crank
Step 3, pistons with coatings and billet rods and crank
the tomei step 1 and step 2 are slightly different with the step 2 having the cooling channel pistons