Generally yes — an HDD unit mounted on a trailer that is towed by a separate vehicle is non-self-propelled and would fall under 72344 rather than 72337, provided no more specific heading applies. The key is that the boring unit itself does not advance under its own power. Importers should document the towing/carrier arrangement in the technical description on the commercial invoice.

A complete boring or sinking unit — even if skid-mounted and stationary — is classified as machinery. Individual components such as drill bits, kelly bars, or swivel assemblies imported separately are typically classified as parts of machinery under a different heading. The completeness of the assembly at the time of importation is the operative criterion, and GRI 2(a) governs incomplete or unassembled machines.