Boost pipe dia
 

what should the internal dia be for boost pipes, is it 3 or 4mm. Does it matter which size it is?

i think it's 3mm. it can make a difference what size you use because of how it bleeds off the boost. what car/my is it for?

I have a UK MY99, i have replaced all from actuator/turbo/solenoid with 3mm stuff from the local garden centre, i plan to replace with silicon hose soon, as this was just a quick fix as the old stuff was poo big time

be careful with the silicone stuff, i maybe corrected but i think they don't like vacuum. apparently fuel pipe is good.