Hoses in General

[GarageBound]

Looking for guidance on the hoses & clamps.
Vehicles: 1991 318i w/ ~145K miles & 2004 330i w/ 199K miles both always in SoCal.

Questions:
1. Your preferred approach? BMW pre-sized or auto parts store hose in bulk?
I am planning on replacing most hoses in both cars soon.
2. Secondly how are metric sizes measured. ex. a 3.5mm x 1.8, is this inner and outer diameter?
3. can you provide sizing conversion for below to metric?
5/8” Coolant Hose
• 5/16” Fuel Hose (Make sure it is fuel INJECTION hose, not fuel hose for a carb)
• 19/32” Vacuum Hose

Thanks,

[Vincenze]

1. Coolant hoses are all different.
You can save some money by buying Rein, which is a Continental brand.

https://www.fcpeuro.com/BMW-parts/318is/Cooling-Hoses/?year=1991&m=20&e=175&t=6&b=9&d=63&v=

2. There are 2 specific fuel hoses around the fuel filter.
You will need a few meters of a 8X13MM hose. It must be a fuel injection hose.

8mm is 5/16" https://www.gates.com/us/en/fluid-po...219-05652.html

[cory58]

If you plan to keep the car for a while, buy good fuel hose, not the crap at the local auto parts chain.
Vincenze is correct - there are several fuel hoses with bends that cannot be replicated with bulk hose.
When metric hose lists two numbers, the smaller is ID and the larger is OD.
Google "convert mm to inches." A calculator will pop up.

[cscracker]

Also, although 8mm hose is close enough to 5/16" hose to be interchangeable, I have found that 6mm hose is different enough from 1/4" hose that it's worth buying metric hose. You can make it work, but it's not fun.
For coolant/water hoses, just buy the OEM parts. It's not worth trying to piece something together when you don't have to. Put your VIN in RealOEM to find the right ones.
For fuel, most of the time generic fuel injection hose is fine, just make sure you buy good quality. Sometimes there's tight bends in molded parts that make this less than ideal, but it's usually fine.
For vacuum lines, any decent quality vacuum line is fine.