Richard is certainly correct, and exposing the outer edge of the plug will make things easier. Gentle heat from a hot air gun is also worth adding to the scheme, to soften any locking compound, varnish or jointing used to aid sealing. This is a used crank, so who knows the previous history.
To state the obvious, first check that the crank runs true and that the journals are not on their final grind or are within stated parameters for wear. It is a lot of work only to find the crank is screwed.
If the drill down the middle is the eventual method of extraction, a TORX bit hammered in works for me. If all else fails it's drill out as big as you dare and collapse the remains of the thread innwards, as outlined above. New, sharp drills offer the best chance of success, don't struggle with those old blued up blunt buggers!
That semi-circular slot in the plug is worse than useless. Best original type plugs for removal have a straight slot across the middle, but you still need a straight bladed socket type screwdriver bit, a lot of leverage plus downward pressure on the slot and a bad attitude get them out. For the later big journal crank, replacement hex socket plugs work fine. These are thicker and larger diameter than the small journal plug, so a hex offers more scope for a positive drive if they ever need to be removed at a later date.
Swarfy.
