Q: What size countersink tool should be used?
For this particular part number you would want a .140-.147" hole, a #28 or #27 drill would work.
Q: What is the material thickness of these countersunk washers?
Approximate material thicknesses: #6 = 0.016", #8 = 0.024", #10 = 0.035", 1/4 = 0.035".
