Hoses come in metric and SAE sizes

Question:

Regarding the March question from Bud Elliot about vacuum problems with his 1971 Porsche, I have had problems using auto parts store vacuum lines on foreign cars. It seems that the diameter is sized in SAE measurement and many foreign car fittings need a vacuum hose with a diameter sized for a metric fit. I have had the vacuum lines seem to fit OK and be just slightly loose, causing big vacuum leak problems.

Answer:

You are quite correct that hose, like nuts and bolts, is available in both metric and SAE sizing. Using incorrectly sized hoses can lead to vacuum leaks but at the same time, using hose that is slightly too small to avoid vacuum leaks can cause other problems. It is therefore important to use the correct size hoses and if the fittings in your car are sized for metric hose then you should replace them with equally sized metric hose.