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The use of magnetic drain plugs (to catch ferro-magnetic metal particles) is well established.

Is it better to attach a magnet to the oil-filter?

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No. The point of a magnetic drain plug is to check whether there are pieces of metal floating around your engine. It's not so much to keep these pieces from circulating as it is an indicator of the condition of your engine.

By the time you see an amount of metal deposits on the drain plug, you know that your engine is experiencing excessive wear and may need to be rebuilt/overhauled.

Putting a magnet on an oil filter doesn't make sense because you won't be able to quickly inspect it for the presence of metal shavings.

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    It's funny that my truck ('06 Chevy Silverado L33) comes with a magnetic tipped oil pan plug, but the main bearing faces in these engines are made of aluminum. Knowing the bearings are the first thing to wear out (or should be) leave me to wonder why. I guess the cam and lifters might wear, but these are of roller design, so don't show appreciable wear. Great fun. – Pᴀᴜʟsᴛᴇʀ2♦ Jan 9 '15 at 20:40
  • OK, if the point of a magnetic drain plug is to check whether there are pieces of metal floating around your engine, would it be a good idea to ALSO be good to have a magnet on the filter to capture small ferro-magnetic particles, even the very fine ones that would get through the filter. Will it work if stuck on the outside or does the filter cannister screw up the magnetic field effect inside? – Warren Hudson Jan 10 '15 at 9:50
  • Sometimes there are steel shards around the thread of the filter from the machining. The shards could possibly even perforate the medium. I wipe around the thread with an oil-damped tissue. Don't use your finger as the thread is sharp. – Warren Hudson Jan 11 '15 at 1:15
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    It won't hurt to put a magnet around the filter, but that might cause less deposits on the drain plug which would hide crucial evidence. – Captain Kenpachi Jan 12 '15 at 7:08
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Borrowed from another forum:

  1. Some magnets are designed for and put on the end of the filter. Inside the filter near that area is the bypass valve, which is mechanical (a spring) and magnetic (steel). If your magnet is strong enough, it will alter the pressure at which the bypass opens (not good).

  2. If you put a magnet on the side of the filter, you may start accumulating small pieces of metal (that's the point). However, you are also magnetizing those pieces, causing them to stick together in a clump. If enough pressure is created, the filter bypass opens and a chunk of magnetic goop could be pulled from the magnet and thrown into the engine. Not only is this piece large now, but it is magnetic, so it can attach itself to parts of the motor and do more damage than it otherwise would (as microscopic, non-magnetic, homogeneously distributed metal).

  3. If you put a magnet on the pan, you can likewise create a ball of magnetic goop. Now you take the magnet off to drain the oil. There is a magnetic pile of goop inside that you expect to drain out with the oil. What if it stays stuck to the pan (is the pan magnetic?)?

  4. (not very concerned about this, but it popped in my brain). A very high-powered magnet near a sensor could affect the sensor readings (or cause it to fail). Given how close it would have to be to the sensor, though, I doubt this would really be an issue.

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    If you've borrowed this from another forum, you should add a reference or link as to where you got it from, otherwise it's plagiarism. – Nick C♦ Feb 6 '18 at 9:46
  • Thanks for those thoughts Kyle. My magnets will remain on the filter to keep the cluster of (now-magnetised) particles in place until I have removed the filter. I will not attach magnets to the new filter. A magnetic drain-plug is still the only good idea for removing ferros. Maybe a centrifugal separator that could be cleaned out would be be the ultimate as water and all sorts of solid particles other than colloidal would be collected on a continuous basis. Such devices do exist but have not gained acceptance for whatever reason. With the trend to thinner oils it would work better nowadays? N – Warren Hudson Feb 7 '18 at 9:03
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Warren asked in his comment above: "Why then do the vehicle makers not supply a magnet to re-use every time you change the filter?" The simple answer to that is PROFITS! The faster an engine wears out the sooner they can sell you another one. If a parts fails you have to buy a replacement part. So, does lack of a magnet benefit them or you? (that is a rhetorical question by the way) Most of the time you need to use your common sense to extend the life of your vehicle investment; magnets to trap metal in the oil is one such common sense tactic.

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Standard practice on aircraft engines is to cut open the oil filter (on engines with filters, some just have screens) and look for glint, and other signs of particles.

No reason you can not do that on your car filter. I do, and I also do it with my tractor and equipment filters. The tool for cutting them open looks like a big pipecutter, and does not make (many) particles when cutting the can open.

Some engines like the ones on my VWs use filter cartridges which can be inspected immediately upon removal, as they are not in a can.

To address your question, a magnet on the outside of the filter will only provide a weak magnetic field on the inside of a steel can. Therefore, it is better to use a magnetic plug, as larger chips will settle to a drain plug and are more effectively removed that way.

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You are all missing the point. Its the metal that you cant see that you are after here. If you could see it.....it would be too late already. by trapping the micron size particles that go right thru a normal filter you lessen the wear on your engine simply because the oil is just that much cleaner. (any cleaner at all is better). JR

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  • From the comments, and summarising, it seems it is best to have a magnetic drain plug to get an indication of engine condition AND magnets on the oil filter to prevent engine wear from fine particles.. Why then do the vehicle makers not supply a magnet to re-use every time you change the filter? – Warren Hudson Aug 10 '16 at 2:01

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