What type of fit is required for parts that need some force to assemble?

The required type of fit for parts that need some force to assemble is an interference fit. An interference fit is characterized by a slight overlap between the parts being joined, which means that when they are assembled, one part must be physically forced into the other. This tight fit creates a strong bond between the two parts, ensuring that they remain securely in place during operation. It is commonly used in applications where high load-bearing capacity and resistance to movement or separation are necessary.

In contrast, clearance fits allow for sufficient space between parts, providing ease of assembly and allowing for free movement, making them unsuitable when force is necessary to fit the parts together. Loose fits also provide greater space, which would lead to a lack of clamping effect. A transition fit offers a compromise between clearance and interference fits, where the parts may assemble with either a slight interference or clearance depending on tolerance variations. However, only an interference fit specifically requires force to assemble the components, making it the correct answer in this context.