Question,. Actually two questions....1 what is the micron rating of the spin on secondary fuel filter on the OM617? 2. Does anyone know of a spin on fuel filter with a water/sediment bowl on the bottom that will fit the stock spin on fuel filter adapter? If so could you provide the part number. Many thanks and apologies if this has been asked before.

Stock filters are ~10 microns. You can use a CAT 1R-0750 or Baldwin BF1217 to get down to 2 microns with water separation, but you'll need an adaptor for the threads as well as a bracket to shift the fuel filter over.

I personally have installed the CAT 1R-0750

https://www.peachparts.com/shopforum/diesel-discussion/408560-better-fuel-filters.html

https://www.peachparts.com/shopforum/diesel-discussion/399682-larger-fuel-filter-w-water-separator-adapter.html

Lower Micron Fuel Filter Mod

2 Micron fuel filter CAT

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1984 300D "Elsa" odo reset 6/2011 147k
1983 300TD "Mitzi" ~268k OM603 powered
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It is a nominal 10 microns. Oddly what that means is that it filters out fifty percent of the 10-micron particles on the first pass though the filter.

There is another type of filter rating called a bata rating, but I don't know the figure on that.

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84 300D, 82 Volvo 244Gl Diesel

The percentage portion of a nominal efficiency rating is specified in the rating. It is not arbitrarily nor automatically 50%.

A nominal rating of "10 microns at 95%" is a different rating than "10 microns at 99%."

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Is that nominal or absolute?

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I guess my memory is failing. However: Nominal rating

"The nominal rating refers to a filter capable of cutting off a nominated minimum percentage by weight of solid particles of a specific contaminant (usually again glass beads) greater than a stated micron size, normally expressed in micrometers (mm). I.e. 90% of 10 micron.
It also represents a nominal efficiency figure, or more correctly, a degree of filtration."

But still 10 percent of that size particles pass through the filter.

https://www.lenntech.com/library/fine/absolute/absolute-nominal-filters.htm#:~:text=The%20nominal%20rating%20refers%20to,I.e.%2090%25%20of%2010%20micron.

This is the other popular filter rating: Beta Ratio

https://www.lenntech.com/library/fine/beta/beta-ratio.htm

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84 300D, 82 Volvo 244Gl Diesel

I am going to use the below as an example.

https://www.wixfilters.com/Lookup/FilterBySize.aspx?catid=4&styleid=10

At the bottom you will see filter 33959 and 33216. The claim is they both fit but the dimensions are slightly different.
I am going to pick the 33959 as it has both a nominal micron rating and a beta ratio rating. Also

Beta Ratio: 2/20/75=7/14/18
Burst Pressure-PSI: 355
Max Flow Rate: 180 GPH
Nominal Micron Rating: 10

If you pick the companies filters you want to use and go to their site, some of the supply the micron rating they used.

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84 300D, 82 Volvo 244Gl Diesel

The filter issue is interesting.
Take a chain link fence and ask yourself if the fence can filter out a handful of beebees?

I mean the mesh is really big compared to a beebee. Yet if you throw the beebees at the chain link fence not all of the beebees are going to make it through. So, the fence did filter out some of the beebee sized objects. In the case of the fence some of the beebees bounce back but in the case of filter media the get trapped in the media.

So, a 10 nominal micron filter does also filter out some smaller particles.

Also, you're the fuel filter system on my Mercedes 300D works differently compared to the air filter. The air filter only has one pass to trap particles and the air goes inside of the engine.

In the case of your fuel. Assuming there is nothing growing inside of your fuel tank in general the fuel comes into the tank with X number of particles in it. Some don't remain in suspension and go to the bottom of the thank. What is different here is that most of the fuel that goes through the fuel filter is returned to the fuel tank.

What you end up with is that each time the fuel passes through the filter and goes back to the tank it filters out more particles and over time. So as far as your fuel is concerned what is left in the tank is having less and less particles in it as you are driving till you put more fuel in.

Back to chain link fence and beebees. You throw the beebees at the fence. You gather up the ones that made it to the other side and again throw them through the fence and keep repeating that.
Each time you do that the number of beebees you pick up on the other side of the fence is less and less. As a system that is sort of how your fuel filtration system works.

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84 300D, 82 Volvo 244Gl Diesel