A machined and slightly tapered piece of pipe found in cast-iron sectional heating boilers is referred to as a:

The term "push nipple" refers to a fitting used in specific plumbing and heating applications, particularly in cast-iron sectional heating boilers. In this context, it serves as a connector that allows for thermal expansion and contraction of the boiler components without compromising the integrity of the system.

The machined and slightly tapered design of a push nipple aids in forming a tight seal when inserted between two sections of the boiler. This helps prevent leaks and ensures that the system can efficiently manage the high temperatures often encountered in these heating systems. The push nipple's function is essential in maintaining the boiler's operational efficiency and safety, as it accommodates movement while providing a reliable, secure connection.

In contrast, other fittings mentioned typically serve different functions: expansion joints are designed to absorb movement due to thermal expansion, flange fittings connect two pieces of pipes or equipment with bolts, and couplings are used to join two pipes together without accommodating specific movements or temperature changes. This distinction clarifies why "push nipple" is the correct term for the described fitting in this scenario.