If the hole is obstructed, these counterbores drill from the reverse side instead. They're handy for adding counterbores to existing installations.

After drilling your through hole, put the shank (sold separately) in your drill chuck and insert it all the way through. Then reach into the confined space to stick the counterbore head onto the shank. Pull back on the drill to counterbore the hole.

If an obstruction blocks access for standard countersinks, these tools enter from the reverse side instead. They're often used to add countersunk holes to surfaces in preassembled parts. Put the shank (sold separately) in your drill chuck and insert it all the way through the hole. Then reach into the confined space to stick the countersink head onto the shank. Pull back on the drill to countersink the hole.

If an obstruction blocks access for standard countersinks, these tools enter from the reverse side instead. They're often used to add countersunk holes to surfaces in preassembled parts. Put the shank (sold separately) in your drill chuck and insert it all the way through the hole. Then reach into the confined space to stick the countersink head onto the shank. Pull back on the drill to countersink the hole.