What type of joint is least commonly used for connecting clay pipe to metal pipe?

The least commonly used joint for connecting clay pipe to metal pipe is the molded rubber coupling joint. This type of joint is designed primarily for flexible connections and is typically used with pipes that have similar materials. While it can accommodate some degree of movement and misalignment, it is less suitable for dissimilar materials such as clay and metal, mainly due to potential differences in expansion, contraction, and structural integrity under varying conditions.

In plumbing practice, more rigid and secure methods are preferred for joining different materials. Cement joints provide a solid and permanent bond that is often suitable for materials like clay, while flanged joints offer a mechanical connection that is advantageous because they can be easily separated for maintenance. Threaded joints can also be effective in making secure connections where appropriate, especially with metal components. Therefore, when connecting clay and metal pipes, options that ensure a stable and durable connection, like cement or flanged joints, are favored over molded rubber coupling joints.