The practical distinction is between an M20 thread size and the Allen head dimensions. M20 describes the nominal thread diameter, not the wrench size. A buyer who asks only for “M20 Allen bolts” may receive the correct thread but the wrong head style, socket drive, strength grade, or finish for the application.
If the bolt sits in a counterbore or a recessed machine part, the outside head diameter and head height matter as much as the 17 mm Allen key size. If the bolt is exposed, clearance around the tool may be the limiting factor. State the standard or provide a drawing when the assembly cannot tolerate dimensional variation.
Start by identifying the head style. A standard socket head cap screw gives strong tool engagement and a tall cylindrical head. A low-head socket screw saves vertical space but usually changes the drive depth and tightening capacity. A countersunk socket screw sits flush but needs a matching countersink angle and does not use the same head diameter as a cap screw.
Match the bolt to the assembly, not just the thread. For an M20 fastener, check thread pitch, total length, threaded length, strength class, surface finish, and the material being clamped. Coatings can affect corrosion resistance and assembly behavior, while different strength grades may require different tightening practices.
A common mistake is checking only the Allen key size and missing the counterbore. The tool may fit the socket, but the head may not seat if the counterbore is too small or too shallow. For replacement work, measure the existing bolt head diameter, head height, socket size, and under-head length before confirming the order.
For a clear inquiry to Hebei Wotu Fastener Manufacturing Co., Ltd., send the required standard if known, the M20 pitch and length, the head style, strength grade, finish, and any drawing or clearance limit. If you are replacing an existing bolt, include photos or measured head dimensions so the socket and seating area can be checked together.
