Any small, even microscopic, metal particles that can pass through the paper element, will be attracted to the powerful magnetic field and stay trapped in the filter. When you change the filter you can move the magnet onto the new filter. Simples!!

Where do you get your small powerful magnet?
Carefully dismantle a redundant hard drive from a computer and you will find them operating the head arm. Carefully remove them and there you are!
Take necessary precautions as they are very strong and can cause problems with heart pacemakers, computers, mobile phones, electronics, TVs, credit cards and the like!


With all that said and done, I used these magnets to great effect after the rebuild of my Series II. I cut open the old filter after the run in oil change and found a small mound of metallic mush on the opposite side of the magnet! Not the kind of metalic grinding paste you want running around your engine after a rebuild or at anytime for that matter!
Certain types of magnets (Neodymium for instance) degrade at very high temperatures, but they still cling and do their job.
Suitable magnets are also available on Ebay.
I have also used this method on our two Euroboxes and have had curious comments from the servicing garage as to the modified filters!