It is a short straight connector that joins two pipe stubs of the same diameter - metal pipework, an air filter to an intake, two lengths of hard pipe, or a hard pipe to a component. Because it is flexible it also absorbs a little vibration and takes up minor misalignment, which a rigid connection cannot do.

The standard silicone coupler covers 13mm to 152mm internal diameter across 29 sizes, in 20 lengths from 50mm to 950mm, in blue, red and black. The fluorolined version for fuel and oil covers 6mm to 63mm in the same 20 lengths, in black. Order by internal diameter matched to the outside diameter of the pipe stubs.

Long enough that at least 25mm of pipe is engaged at each end on smaller bores, and proportionally more on larger ones, with the clip sitting over the supported section. That makes 50mm the practical minimum for a straightforward join. If you have room, a 75mm or 100mm coupler gives a more secure joint and more tolerance for slight misalignment.

Standard silicone to SAE J20 R3 for coolant, water and air - it is cheaper and does that job perfectly. Fluorolined to SAE J30 R6 for anything carrying fuel, diesel, oil or fuel vapour, including breather and catch-can lines that carry oil mist. Standard silicone swells and breaks down in contact with hydrocarbons, so this is not a corner to cut.

A small amount, yes - that flexibility is one of the reasons to use silicone rather than a rigid joint. But do not rely on a plain coupler to bridge a significant offset: it puts the joint under permanent stress and it will eventually leak. Where two pipes genuinely do not line up, or where one end moves relative to the other, use a hump hose instead.

Stainless worm drive clips for coolant and low-pressure air; T-bolt clamps for intercooler, turbo and any boosted joint. Position the clip over the section of hose that is supported by the pipe underneath, never overhanging the end. If the pipe stubs are unbeaded and the joint is pressurised, a T-bolt is strongly recommended.