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Rebuilding my brake system and one of my calipers looks like it has a castle bolt on it. I know we have castle nuts on the tie rods and other components, but I have never seen or heard of a bolt like this.
I believe the head pattern of 6 lines means it's grade 8 and not truly a castellated fastener. An old Paragon bolt catalog shows a similar bolt head (below) as a 3/8"-24tpi x 1" but yours appears to be the caliper bolt to secure the two halves so it's got to be longer than 1".
Before I had impact guns, I would mount the caliper back on the car and use a six point socket with a breaker bar for leverage. Spray it liberally with Kroil or Liquid Wrench for a few days. Good luck. Jerry
Castle bolt?
