[Steeveedee]

The bolts are 3/8-16 UNC thread, that's why the head can take a 14mm, though the correct wrench size is 9/16". If you need to drill the bolts out, start with a left-hand twist drill bit, because sometimes when drilling the bit will catch and unscrew the bolt. If you use a right hand twist drill, it won't have that loosening effect. If you want to clean up the thread, a chasing tap is what you want, not a regular threading tap.