Every clamp works by shortening its circumference, which pulls a band tighter around whatever is inside it. That action assumes the thing inside is already one continuous surface. Put a small pipe inside a larger hose and the band closes until it meets the big pipe, leaving the small one untouched in the middle.
That is the difference between a joint that leaks and a joint that looks tight. The clamp reads as fully closed, the hose looks seated, and the only path for a leak is straight along the annular gap between the two diameters. No amount of tightening closes a step.
A clamp is sold with a range rather than a size, and the ends of that range are not equally useful. Near the fully open end the band is barely engaged and the fastener has little left to give; near the closed end the band has bottomed out and cannot follow the hose as it settles.
The usable size is the one where the clamp sits comfortably inside its range with the hose compressed a little. If a clamp has to be wound most of the way in just to touch the pipe, it is the wrong clamp, and it will lose what little tension it has once the silicone hose takes a set.
The correct way to bridge two diameters is a hose moulded with a step in its bore, so each end fits its own pipe properly and each clamp squeezes round metal on a matching diameter. The step does the adapting, and the clamps do only what clamps are for.
This matters most where the difference is large. A coupler asked to close a big gap sits conical, with its wall in tension on the small end and compressed on the large one, and the tensioned end is where it tears. A reducing hose has no such preload.
Where a single hose covers two different pipes without a moulded step, the joint needs a clamp on each pipe, each one fully on its own diameter and clear of the transition. The two bands should not overlap, and neither should sit on the conical section between them.
Space them so each band has a bead or a flare behind it where possible. A band sitting just behind a bead is being asked to hold the hose against sliding, which is a much easier job than holding it against pressure on a plain tube with nothing to register against.
Clamping force is only useful if the material under the band can resist it. A thin walled tube, or one already crushed from a previous clamp, will deform before the band reaches useful tension, and the joint will feel tight while the pipe has simply gone oval underneath.
Check the pipe after removal. A ring of bright marks with a flattened section between them means the previous clamp was overtightened onto a soft wall. That pipe needs its end brought back round, or a liner sleeve, before it can be clamped again and expected to seal.
Assemble the joint with no clamps and no lubricant, push the hose fully home on both pipes, and look at the gap between hose bore and pipe at each end. Any visible gap all the way round means the hose is too large for that pipe, regardless of what the clamp can do.
Then clamp it and try to rotate the hose by hand on each pipe separately. It should resist on both. If it turns easily on one side, that side is bridging a step, and the answer is a reducing silicone coupler rather than a wider clamp or a second band stacked beside the first.
