Tech Q&A: Magnetic Oil Drain Plugs

Q: Why don’t the manufacturers put magnetic drain plugs in motorcycle crankcases? They are in rear ends on shaft-drive bikes. I know rear ends don’t have filters but debris can still get in the crankcase between oil changes. One company does have them but not in a size to fit my Honda CB1100. Best I can tell it is a 13.5mm plug. – Ace Miller, Apollo Beach, Florida

A: You don’t see OEMs using magnetic drain plugs on the engine because they’re not necessary. On modern forced-lubrication motors the oil is pumped through a filter that removes particles as small as 25 microns. (For reference, a human hair is about 50 microns in diameter.) The filter scrubs the hard ferrous stuff a magnet would pick up as well as softer non-ferrous debris like aluminum, clutch-plate fibers, bits of carbon and other crud. That said, a magnet is effective at snagging all ferrous material regardless of size, but only if it’s in the right place. So rather than using a magnetic drain plug that just sits there at the bottom of your oil pan, you’d be better off using an oil-filter magnet like those sold by Motion Pro (motionpro.com). These thin, donut-shaped discs sit under your filter so they’re right in the flow of oil, ensuring they snatch up every errant particle. If you’d still prefer to put a magnetic drain plug in your CB1100, an M14x1.5 bolt will do the trick.