We had these counterbores custom made for us by one of the top tooling manufacturers here in the USA.
These are a solid pilot TiAln coated solid micro grain carbide counterbore. They are .195 X .109 and are built to work with our 4-40 socket head and short socket head screws.
These are tighter than a standard #4 counterbore and will leave a .005″ clearance around our .185″ screw heads for a clean precision fit and look…
TIP: DO NOT clamp down the part you are counterboring. These do not like to flex . Hold your part free and you will get awesome results. We have counterbored hundreds of holes and are still using the very first one we ever bought!!!