Hi. My header tank has 3 outlets.1 at 18mm,1 at 10mm and an overflow at 8mm so do I get hoses of equal diamiter or do they stretch slightly, and therefore get next size down eg 8mm hose for 10mm outlet? Cheers

They will stretch a little, and obviously the tighter they are the less chance of them ever popping off.

I'd get the same size, or slightly smaller if i couldnt get the right size.

10 for 10, 8 for 8 etc etc

they dont stretch much

If you put 8mm pipe on a 10mm outlet / inlet that will restrict water flow to what should be 10mm

Yeah but it's 10mm o/d outlet and 8mm I/d hose And the ally is at least 1mm thick all the way round so I would say flow I/d would be almost the same

Just get it 10 for 10 unless you cant get that specific size

Go size for size or a couple mm smaller if joining mismatched sizes. Like you say as long as you don't go so small that the is becomes a restriction.

If the alloy pipe has a barb then just use the 'non barbed' diameter. If however the alloy pipe if plain then going slightly (1mm or so) smaller will probably improve the strength of the join, but use decent clips like Mikalor.

The ID of the Silicon hoses should be the same as the OD of the pipe. Youll need a barb or bead to clamp the hose over, otherwise they slide off occasionally. IVe spent a lot of time on this lately, and they do stretch by up to a millimetre or so, but they dont like it. If they are warm they become more malleable, so keep a bowl of boiling water to hand when your fitting them.