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Engine guides and mods

Oil Filter Magnets

As soon as you mention magnets and cars, people think about fuel magnets and stop listening but this has nothing to do with fuel and is a cheap but valid mod that was missed out during manufacture.

Modification Details




Millions of cars are produced each year, many of them have a magnetic sump plug.
This small magnet catches any ferrous metal floating around in the oil.

During a service, the drain plug is removed and cleaned, any metal swarf sitting
near the hole is drained out. The problem is that the smart doesn't have a sump plug.


During a smart service, the oil is sucked out but there is always about 0.2 of a litre left
in the bottom of the sump. The bottom of the sump is where most of the metal ends up.


The idea came to me when I read that a low percentage of normal canister oil filters have
magnets built in. I then decided it wouldn't really be that hard to add them yourself.


Getting The Magnets

The easiest way to view a choice of magnets is to look on Ebay.
Do a search for Neodymium which is a very strong type of rare earth magnet.
Because they are being applied to a curved surface you don't want to buy the biggest
mofo you can get your hands on. I went for 25mm x 5mm x 5mm bar magnets.

Adding Magnets

Firstly you need to have fitted the oil filter adaptor, the standard smart filter is plastic.
Here is the oil filter adapter in place with a normal spin on filter attached.



I took 8 of the 10 magnets that I bought and applied them evenly spaced around the
edge of the filter, I pushed them up against the lip for a little added sticking power.




I don't believe they will move but if you want you can secure them with a zip tie,
jubilee clip or self amalgamating tape. The magnets are reusable which means
that when you remove the old filter they are easy removed and place on the new one.

What's The Science Then?

Although the engine is mainly made of aluminium, many of the moving parts are made of ferrous metals. As the moving parts wear they create miniscule particles that float around in the oil. Generally these move into the oil filter and stay there but too much can block the filter making the bypass open meaning there is no filtering occuring.

Placing magnets on the outside of the canister allows even microscopic ferrous
metal particles to be trapped, well below the 10 microns that normal filters work to.


Due to the flow direction of the oil...



...the dirty oil passes into the canister right near the magnets. The magnetic field
easily passes through the case of the filter and attracts passing ferrous metals.
The added advantage of this is that the magnets catch the metal before it
reaches filter meaning the filter won't block anywhere near as quickly.

When the car is serviced, the oil filter is removed and disposed of which contains all of
the ferrous metal particles. The magnets can be removed and placed on the new filter.


Using Magnets On The Standard Filter

Although I haven't tried it, there is no reason it wouldn't work exactly the same. The problem is that the
thickness of the original filter housing will reduce the strength of the magnetic pull inside the housing.


The magnets would have to be secured with self amalgamating tape to stop them just falling off.
Self amalagamating tape seals itself and becomes water proof.


During a service, the oil filter cover should be removed, the magnets taken off and the cover cleaned to flush out
any built up deposits. The new filter can then be replaced
 and the magnets can be reapplied and stuck down.

Filter Plus

Thanks to Tim Leathy for the link to this product

The Filter Plus is a magnetic cap that attaches to the end of a canister filter.


The device costs about £4.95 plus £1.50 for postage and packing. Don't bother buying it.

Why Not?

There are two main designs that make the oil filter magnets effective:

They sit in the path of the flowing oil.
They are placed before the filtering medium.

The Filter Plus ignores both of these rules.

It is located right at the back of the filter so only a minority of the oil will flow passed it.
 It is placed after the filtering medium so the oil is already filtered when it passes over it.

One main point of the magnetic trap is it collects large molecules of metal before it 
gets filtered out, this increases the filtering and flow of the filter by keeping it clear.

Oh, and it's slightly more expensive and you get less magnet for your money.


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