The TR6 and probably the 4 have caps with no edges (shoulder), you drive them in and they bottom out on the inside edge. It is pretty tough to wedge a screw driver between the crack (without bending the cap).I can get mine off but instead of screwdrivers, chisels, and hammers-it would be nice just to grab the cap with a pair of channel locks--the way most other none british cars are.