Hollow screws for pipe OD 4 - 5 mm are provided with 2 or 3 holes at manufacturer's option (see c3 + d5).
Hollow screws for pipe OD 6 mm and larger are supplied in the standard design at manufacturer's option with 3 holes (see c3 + d5) or 4 holes (see c2 + d4).
Hollow screws with longer threads (e.g. like used in aluminum), are available on request as special design (see b1 + L1).