Oil Filter Inspection

[pa38112]

I am sure I am the only one to make this mistake, but I thought I should share:

For 20 years I have assumed that oil entered the center of the oil filter and flowed to the outside. This is because of the perforated cage in the middle. I figured large debris would get stopped there before entering the paper element. As such, when I cut open my filter I only inspect the inside surface.
Today I decided to look at the other side, and what do you know, for the first time in 20 years of inspecting filters I found some carbon deposits. Not sure how I could be so dumb, but I am!!!

[kaa]

I wonder if there is an engineering reason for this, or it's just a convention? The only possible explanation I have is that the filter media is supported by the tube on the inside and can better resist oil pressure this way.

Luckily (hm) my engine always generates enough carbon that it's always very easy to tell which side of the filter paper is which.

[JonJay]

Your filter inspections should include removal of the paper element, washing out of the pleats, both sides, and use of a magnet to gather any ferrous metal shards. Don?t expect to see metal with the naked eye.
Often, an engine making metal, may show only as a fine deposit, like a powder. The volume of that powder sets the acceptable limits, at least according to Lycoming.
Serious issues may show up as chards, but also may just show up as a bunch of fine sand like metal particles.

Also, 100 hour inspection of the suction screen should be done. This may collect large particles that never make it to the filter.

[BMC_Dave]

I vaguely recall reverse osmosis filters being the same. Leads me to believe there's a reason for this, though I don't see why.

[BillL]

Originally Posted by kaa

The only possible explanation I have is that the filter media is supported by the tube on the inside and can better resist oil pressure this way.

This. Else it will explode like balloon. Then push all the little pieces of the paper into the oil passages, blocking oil flow, lubrication, and well . . . you get the rest.