8409.91 - Suitable for use solely or principally with spark-ignition internal combustion piston engines (including rotary engines):

This HTS category, 8409.91, encompasses parts specifically designed and intended for use solely or principally with spark-ignition internal combustion piston engines, which includes rotary engines. These are the types of engines commonly found in gasoline-powered vehicles, motorcycles, and many small engines. The scope of this heading is limited to components that are integral to the functioning of such engines.

Distinguishing this category from its sibling, 8409.99, is crucial. While 8409.99 also covers parts for engines of heading 8407 or 8408, it is for "Other" categories not specified elsewhere. This means 8409.91 is reserved for parts falling under the "Other" subheading that are *exclusively* for spark-ignition engines, whereas 8409.99 might capture a broader range of engine parts if they don't fit the specific criteria of other subheadings.

The subcategories within 8409.91 further refine the classification. For instance, 8409.91.10 specifically addresses cast-iron parts that have undergone minimal processing, such as cleaning and basic machining for finishing. The subsequent "Other" subcategory is a catch-all for parts fitting the general description of 8409.91 but not meeting the specific criteria of the preceding subheadings, allowing for granular classification based on material, processing, and ultimate engine application.