raygreenwood
Samba Member
Joined: November 24, 2008
Posts: 21513
Location: Oklahoma City
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 Posted: Sat Mar 13, 2021 9:13 pm Post subject: In-tank fuel strainer alternatives (sock filter) |
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Gas tank strainer replacement alternatives:
Sorry this got so long. Yes….I could have just said…”this is what one could use for a replacement for the in tank strainer”….but I did not know for sure what the specs were.
This started out as a simple thought on the side while working on the gas tank outlet plumbing gaskets. I thought… “I should buy a new external strainer like I used years ago”.
Then as I started shopping for what I had already used ….it became clear that the advertised specs/capability had changed and were all over the place! Back in the day….I simply bought what was commonly used on a Mercedes 240D or 300D and thought …good enough!
Over the years, in lots of threads, owners were replacing their in-tank fuel strainers…also called a “sock-filter”…..either because they are clogged with silt/varnish from sitting still for years or others like me that have had to remove the sock filter and tank tube plumbing because the filter got clogged with silt from crappy gas or driving many miles.
To be clear…the factory sock filter works perfectly. It’s not noticeably restrictive and it filters silt, sand and rocks to keep the big stuff out of the main fuel injection filter. It really IS just a strainer…a pre-filter and not a filter. I have no problem with its FUNCTION.
BUT…it has a huge design flaw.
If you have a lot of crap in your local gas…it gets plugged up. It can cause you to stall, lose power or burn up a fuel pump over time. But that’s not its flaw.
Its “flaw” is that its INSIDE the tank!
I went through my log book on my 412 from when I was daily driving it 50k miles a year. On three occasions…literally hundreds of miles from home…it caused me to grind to a halt.
Our sock filter is not in the same class as modern car gas tank pre-filters which are huge mesh bags with maybe 100-200 square inches of filter space.
They lay flat on the tank bottom in many cases and get silt washed clean as you drive. They can go for decades without clogging.
And…because of that huge surface area…the modern car in-tank “pre-filters”…are much finer than our small ones can be. They are typically 30-50 microns (ours are in the range of 100-150 microns)
Example:
2002 Toyota Celica…31 micron
And if you think worrying about the sock filter …or finding something better than stock filter…. is trivial in the big scheme of things….ok…fine for you! Maybe you do not drive your car very often or very far and will rarely NEVER be able to get home to drain the tank into something and clean the filter out. If this is you.....great!….then just ignore all of this! 
But if you drive a lot…maybe a daily driver…or go on long trips….you can be many miles from home if the sock filter clogs up. Also…if you have not cleaned your gas tank out and its 45 years old…you are also at risk. If any of that applies to you….then this might be of interest.
Not only can it strand you until you sort it out, it can burn up the pump over time and/or it can run you lean enough to cause engine damage due to pressure drop and lean spikes.
And…we no longer live in a world where if you needed to.....you could get away with looking around in a shifty manner while you dump 5-10 gallons of fuel out on the grass or the side of the road (because you don’t have any containers with you) to get the damn thing out of the tank to clean it.
And….even if I wanted to just replace it with a stock factory in-tank strainer unit….you can’t actually …easily….find one for a 411/412. And even if you could…it’s still inside the damn tank!
So, for years I have been recommending and posting part numbers for the same external inline fuel strainers I used way back when to replace my sock filter after I removed it …all of which worked fine at that time….and which were mainly the Hengst and Meyle cheap 100 micron rated filters. About $1.89 each back then.
But when I started shopping on-line a couple of weeks ago, and I saw the listed micron ratings all over the place, I started worrying about my suggestions to other people…because:
A. I had never really checked the micron rating of what I used compared to factory (even though they worked fine). I would not really care…except that in researching those same disposable inline filters now…you see the same EXACT filters with a wide range of brand names and an equally wide range of micron claims. Who is actually making them for whom to what spec?
B. They are really small filters both in volume and in nipple ID. While they never caused ME any running issues…..I have no idea if they might have actually caused the pump to run hot because the volume through put may be restrictive. I just did not check that back then.
C. While they worked fine with a D-jet pump…which has pretty good suction….I do not have direct evidence that they are a good recommendation for an L-jet pump or Digifant 2 pump…with less suction.
So, I figured it was time to find out for sure before making another recommendation!
So, the stock part in question: VW part # 411 201 225
As usual there are a couple of key questions to be answered:
1. What IS the micron rating of the existing factory sock filter? It works fine whatever its micron rating is.
2. What micron mesh SHOULD a fuel tank….pre-filter….be? Professional opinions?
So, for a professional opinion…here is something from Aeroquip. Pretty knowledgeable filter company. I have also read the same from K&N and Baldwin in the past. They state that 100µm strainer is what you are looking for….nothing finer. Remember, this is only a STRAINER…a PRE_FILTER….designed ONLY to protect the pump and the main filter that filters down to much finer micron levels.
https://aeromotiveinc.com/fuel-filtration/
Make note though ….depending on what you read and where in Aeroquip’s literature….Aeroquip (in that article) is stating that the 100 micron pre-filter should have no less than 60 square inches of filtration space to not cause a load on the pump…..and…that’s not really necessary for our application…or else the stock model would never have worked.
Understand that virtually everything Aeroquip makes is designed to support upwards to 600 hp minimum….with huge pumps. It’s a different application ….but it’s the right KIND of part.
Also understand that in their smallest filters, they have an element that is about 3” long and about 1” diameter…but is super high surface area pleated specialty filtration mesh. A 3” x 1” filter of that type easily has about 50-60 square inches equivalent flow and filtration area.
More on this TYPE of mesh later.
While their pre-filters are excellent, cleanable and have stainless mesh….they are also bigger. The smallest model in Aeroquip, summit and others like it line up about 3.5” x 1.25” in diameter…without inlet/outlet fittings. On a 411/412…you may have a harder time finding a place to put that. But if you can swing it….$80 to $135 for their product is money well spent.
Filter mesh types:
Some may be bored by this level of detail…if so…skip past this…but I am going to talk about filtration mesh TYPES and sizes. I am doing this because when looking for affordable, small, READILY AVAILABLE fuel pre-filters, a lot of the claims of filter mesh size and micron sizes varied so wildly for the same products that they had to be either BS, or unknown meaning they are guessing or are based on the wrong test method by whoever wrote the marketing for the product.
Mesh numbers:
With filtration mesh when the spec says “#100 mesh” (NOT 100 micron mesh) or mesh number “100”…in US standard filtration mesh that means 100 OPENINGS per inch.
When they use the term “mesh size” (instead of mesh number) it means how big is the opening in the mesh. That is usually in microns…but can be in Mils or millimeters.
And…some of the cheap disposable filters…do not actually use dedicated “filtration mesh”. They are using plain nylon or polyester mesh….which is just fine….but uses different measurement numbers.
So…they may actually be correct that their pre-filter was tested to exclude 100um and larger rocks….but the actual mesh number and mesh size bear no relation to that rating.
The ratings get fuzzy depending on what mesh type they are using….and whether they mean “mesh number”, “mesh size” (mesh opening size) or if they mean actual tested filtration level in microns.
If you know anything about or work with other types of mesh…these numbers will not seem to compute because the filter mesh guys rarely talk about the actual thread diameter of the filtration mesh….which they call “strand size” …..as part of their mesh ratings even though it comes into play.
Since filtration mesh is rarely sourced or made based around its tensile strength, they don’t care about strand or filament/thread diameter as much. They are simply making the “strand size” smaller or larger per “mesh number” (number of threads per inch)….to manipulate openings in the mesh (mesh size) larger or smaller for more or less flow.
Here is a basic wire mesh chart for plain weave mesh like the factory used, for reference.
So, the basic 1 over by 1 under screen weave you see in the brass factory mesh (called a plain weave) may not be actual filtration mesh but since it’s just a single pass strainer…it’s perfectly fine.
Others (Like Aeroquip, K&N and many others)…use actual filtration media weaves. These are very sophisticated mesh weaves that can filter to an exacting micron level but still have very high flow through rates.
More on that in a minute.
Important takeaway at this point in time:
We need to understand exactly what is being stated…when a filter manufacture says “X microns”. When a filter mesh is said to filter to 100um….this is not instantly and exactly saying that particles of 100um will NOT go through the mesh and that all smaller particles WILL go through the mesh. It’s not like the particles you are filtering out of the fuel are perfectly square little 99.99 micron square blocks or spheres.
It is ONLY saying that particles LESS THAN 100um are CAPABLE of going through the mesh….theoretically.
In reality…unless the particle shape, size and flow behavior in the moving liquid…. is PERFECT….for the most part even particles/debris that have a largest cross section…. at some point…. that are slightly smaller than 100um…will probably not get through the mesh.
The crud that is in gasoline or any other liquid will have random shapes, lengths, thicknesses, texture etc. Think like cereal flakes. Even grains of sand are nowhere near uniform.
From studies I have seen in my industry using “real world” particles and not some uniformly engineered lab particle….on a 100um mesh opening….better than 95% of particles from about 70-80um on up are also filtered out and not just 100um and up. So….100um = 0.0039”. A pretty big particle actually in relation to gaps in a roller pump. By comparison….a 75um particle= 0.0029”. But these rocks should never reach the pump.
This is only a PRE-FILTER. It only purpose is to keep damaging sizes of rocks out of the REAL filter which should filter down to 10um which = 0.00039”.
And, the same flow dynamics apply with the main filter as well. You probably will not see many particles larger than 7-8 microns getting through a 10um RATED filter.
So, again….. this all started….. while I was working on the fuel tank inlet gaskets. I was measuring the inlet tube assembly and noticed there was still a scrap of filter mesh on my gas tank tube cluster, so I decided to measure it.
My stock strainer scrap….
The stock brass screen:
At about 200X
These below are within a few microns of the correct opening measurement at calibration and about 365X
The wire diameter is about 95µm (microns) actual
The opening size is about 161µm X 158µm. Mesh can either be engineered to be asymmetrical like this…but usually with plain mesh like this its just stretched in one direction.
So….does this mean that it is a ~160-ish µm filter?
Mmmmm…...probably a little finer at about 125µm due to the reasons listed above
But it is right at 87-92 openings per inch. By filter mesh standards that puts it right between a #80 and a #100 on the chart in number of openings and micron size of openings. That means its just a basic industrial mesh and not a true filtration mesh….which is still just fine.
So….can we get that exact mesh in an available inline filter? Or can we live with something a little finer like the common 100um?
So…the filter I used way back when was a Hengst or Meyle diesel pre-filter for a Mercedes W123 (240D or 300D)
The Hengst H102wk
The Meyle 014 047 0026
If you read enough listings for these two filters, most sources say they are 50 micron “nominal”. Some say 100 microns for the same filter which is why I call BS on a lot of the numbers.
It MATTERS…because a pre-strainer filter with internal volume and nipple openings that small…50 microns is way too fine….if it’s REALLY 50 microns….unless the filter is HUGE in surface area like a modern “bag” style pre-filter.
So…I bought an easy to get equivalent to the Hengst H102wk and Meyle 0026. The NAPA Gold or Wix version…the FIL 3007 for $5.75
And…it’s made in the Russian Federation! And virtually all of these I have seen whether Hengst, NAPA/WIX, Meyle or Baldwin….all seem to be made in Russia lately. Probably the same plant…comrade!
And of course, I cut the damn thing open!
Its filter mesh:
By the way if you want to do the math on the picture above for fun….take the micron measurement listed. Divide 25400 (1 inch in microns) by that number….then multiple by the number of openings the measurement spans and you have the basic mesh opening count or mesh #.
The mesh openings:
So, the basics are about 114 openings per inch, with 132µm X 135µm openings and a thread diameter of ~98µm.
So, it’s about 20 openings per inch finer mesh than the stock strainer….with ~30µm X 30µm smaller openings. So, is this a 135µm filter mesh? More probably about 100 to 110µm for the reasons already stated.
Because I am reading a single microscope sample for my math…and dealing in microns…there is always some error rate….but it’s probably close enough to 120 openings per inch to be either a #120 filtration mesh from the chart above….or it could be a plain nylon “non-filtration mesh” of 100um. Close enough either way.
So, the Hengst, Meyle and Wix are a simple and competent filter/strainer…but small in internal volume. I don’t see the volume of the filter as a big issue….but….
The nipple on each end is 7.99mm (8mm) OD …yes, that’s good. The nipple ID is 5.08mm. That may be a little small for pumps that do not prime well like L-jet.
On the stock gas tank tube unit, the OD is about the same at 7.97mm but the ID is 6.6mm…which I think is significantly larger flow capability compared to a filter of this small of a volume.
If the volume combined with the nipple ID is an issue…I think this filter below would be a better flow option if you want to remain with the smaller ID nipple of the disposable ones.:
Same thing as the WIS FIL 3007 just 2.5X the size. About $3 more so in the $7-9 range. You can also find this same filter in Baldwin brand.
So, we are looking for a 125µm to150µm-ish filter mesh strainer….with about 90-120 openings per inch right?
The problem with that is ….what I have been getting at.
The LISTED micron ratings of many of these filters (when they are listed at all) is 50µm. The FIL 3007 from Wix/NAPA…lists 50µm if you look hard enough.
I think…..this is because some of these filter companies are listing a micron rating….possibly derived by a method…. that is more useful for a pleated, non-woven oil filter media rather than a strainer mesh. I think that’s part of the issue.
The other part is that due to mesh dynamics and filter design…inlet liquid flow patterns….and the REAL world variation of particle shapes we already discussed…..the filtration capability of these mesh do not absolutely corelate to the mesh opening size….and marketing people do not know this.
When we are looking at JUST the mesh opening size….it’s called “absolute” filtration. But …oil filters and pleated filters also use a testing method called the “Beta number”….where they plan…like for motor oil…that the oil will make multiple passes through the filter. And…as the filter fills up and wets out…it becomes more efficient at trapping the maximum smallest particle rating on each pass through the filter and it’s used as an efficiency percentage rating for EACH pass.
A fuel pre-filter or strainer is a single pass filter. If the manufacturers of these filters are somehow applying Beta filtration number testing….it just becomes BS.
Example of multi-pass Beta efficiency number chart for testing pleated oil filters:
And really….any filter that is using that type of testing is looking at an average micron size and overall EFFICIENCY number and not absolute, single pass “rock size” micron rating like you need on a “strainer”.
We are only really interested in “absolute” filtration micron size.
Back to the fact that a particle of real-world crud will not be a perfect sphere. It may be oblong, three dimensional, flat etc. It may be soft and flexible. You may have a particle that is 100 microns long X 30 microns wide X 15 microns thick… that is trying to get through a two-dimensional 80-micron X 80-micron hole…and unless the flow allows it to “present” itself exactly right to the mesh opening…that 100um micron dimension will never allow it to go through…though theoretically it could.
So by absolute rating….a 150 x 150 micron hole…can actually filter out particles to about 115 to 120um….for the most part…depending on shape. A hybrid rating would say…”absolute filtration of 150um”…and….80% filtration of 120um and maybe 50% filtration to 100um etc…..and it all gets averaged together….which is kind of true…mostly.
But marketing…can stretch a number to half of that or less….if the filter mesh holder is oriented correctly to the inlet and outlet flow openings.
I believe (from what I can see) that most of these filters I am showing are really 100um….absolute filtering range….with a real world 75um particle bleed through….and that is being stretched by the marketing departments to make their stuff look better….by saying 50um.
The two main options....so far.... in my opinion:
First…a definition of what I am looking for:
Cheap, inline, disposable (or refillable or cleanable), simple mesh strainers with a mesh number of say …between #80 and #120 with #100 being ideal in the center…to filter out particles to a real-world micron level of about 75 microns and up.
***A bonus to me is that the filter body should be transparent or translucent. When and if the engine starts acting up, I want to quickly be able to know IF it’s the strainer being clogged or whether I should concentrate on other things.
1. In my opinion….a disposable pre-filter needs to be readily available in a wide variety of parts chains and in wide enough use that its not likely to be obsoleted or pushed out of production any time soon. It does not have to be $2 cheap….but cheap enough (say less than $10) so I have no issues of keeping a spare in the glove box and immediately buying another once I install the spare.
2. Or…..it needs to be a pre-filter/strainer of those same specs that can be disassembled and cleaned easily or has refill mesh inserts.
There are probably MORE cleanable strainers in this second category than there are disposable strainer versions.
If you go shopping on Summit racing and input “100 micron pre-filter” ….it brings back roughly 700 choices. Huge range of brands, connections, sizes, volumes etc.
In the disposable family, as long as the mesh size and hose connections are right and you feel you can fit it under the car safely….any of these and hundreds of others like them will work just fine.
In the cleanable/replaceable element family, the range is endless. Many of them (mesh insert wise) will be just as simple as the disposables. Others can use very complex and precise dedicated filter mesh and billet/machined/o-ringed cannisters.
As long as the filter element can be cleaned or replaced along with the gaskets or seals required and you can fit them where you need to put them under the 411/412….any of these will work. This is just a sampling.
So….from all of the above, I think I already stated what I had in mind for what I can use for disposable filters.
To start looking into the “cleanable” strainers….I picked up one that is common, damn cheap, cleanable, “see through” and/or had replaceable elements just to review it.
I bought a K&N 81-0300. It was $5.99 at the K&N store. It was so cheap I bought a pack of three replacement filters with new O-rings (K&N 81-0301) along with it for $3.99. Shipping was one day and $5. This design looks cheap and crappy from a distance and in the package….and kind of is in minor ways….but I was also surprised that it has a reasonably good build quality, materials and very functional!
There are many brands of this same type of glass/plastic “see-through” classic car pre-filter. Mr Gasket (Holley) is also a popular one. There are versions with different connection sizes, versions with multiple changeable connection sizes (see that family picture above), versions where the outer tube is plastic and versions where it is glass.
NOTE: with regard to what the outer tube is made of….don’t get a plastic bodied version of this filter. It will be Lexan and there are too many long-term fuel and chemical issues with these types of “cast” or molded plastic. The plastic of the disposable filters is a totally different animal than these.
The glass on these is thick enough and tough enough and for where you will be using them on a 411/412….the chances of rocks hitting it are about “0”…and for where it will need to be located. If you think you should be worried about striking something and breaking it….then you probably just tore your suspension off anyway. Its just not an issue IMHO.
And….the billet machined aluminum cleanable/replaceable filters listed in that group picture above…are the same EXACT concept as these…just with a billet aluminum housing with O-rings. And….while those are not see-through…they are fully functional and probably impossible to break.
So here is the K&N filter:
….and the filter refill pack with three filters and three sets of seals.
And the instruction sheet:
The filter
The slight twist to the filter element is because it squirms when you tighten the nut and gasket. You can just rotate the filter back straight.
The inlet and outlet nipples. Plenty of flow. The ID is right at .0207” or 5.25mm…about the same as the plastic filter….but it seems to look bigger.
It takes very little to tighten or open. Maye 2-3 foot lbs. at the most. But it is secure
The filter tube and seal in the loose cap/nut end
The tube is glass and its right at 0.098” thick. Pretty thick.
The entire unit is aluminum. The knurled nut is aluminum. The center spine is aluminum. The end caps are aluminum.
Just in case its not obvious, the fuel comes into the center tube from the right (green arrow). The yellow square is where the tube is blocked because its only drilled part way through from each end. The fuel exits the small holes on the outside of the filter cartridge (red square lines), goes through the filter and back into the center tube through the holes inside of the filter cartridge (green lines) and then exits to the fuel line.
The holes are right at 0.100”. I think they are plenty for volume. I may enlarge them to 0.125” just to keep suction effort down. But I will test it first. It may have “0” issues.
The white seal washer MUST be either white Neoprene (possibly Santoprene) or White Viton. Those are the only two rubber types that hold up immersed in fuel and I assume K&N is not stupid.
Just a shot of the filter. This is dedicated filtration mesh. Same “type” as the high end stuff like Aeroquip. More on that coming up.
The cartridge refill and seal set. This is from the Mr. Gasket (Holley) pictures online. This is the exact same filter and set. I simply assume that either K&N makes it or Holley does for both companies.
OK…..here is the mesh in the K&N filter. This is a true filtration mesh. Its pretty much the same style you see in Aeroquip, Fuel lab, Summit…and many of the expensive billet filters….though most of them are stainless…which is nice. So…though the K&N and Holley/Mr. Gasket glass “classic” filters may seem cheap….they are not low grade/technology.
This are the microscope shots of the K&N glass filter mesh.
This is called a “Reverse plain Dutch weave filter mesh”. It looks like a tight impenetrable mesh…but actually has big gaps between the fibers…they are just at an angle that it is hard to access by particles in the liquid flow. There are many other filter weaves that are much more exotic.
My calculations show that by one companies chart (see below) the K&N filter mesh is very close to an RPD “90-S”. I read 172 threads X 53 threads….which has a nominal geometric pore or hole size of 90um. Pretty close to 100m.
Here is a computer model of this mesh type to help you wrap your head around its structure:
Some final recommendations:
Disposable and cheap:
Wix 33972
https://www.oreillyauto.com/detail/b/wix-5291/filt...72/4833158
O’reilly says its 50um….not buying that. Its closer to 100um. You can get these at NAPA or many places.
Cheap/cleanable with cheap replacement element:
K&N 81-0300 (I am sure there are many more similar to this out there)
https://www.knfilters.com/81-0300-replacement-fuel-filter
This one is right below 100um at 90um…but will have a high flow rate.
Mr. Gasket/Holley 9706
https://www.amazon.com/Mr-Gasket-9706-Universal-Cl...&psc=1
Its easy to see this is the same filter as the K&N. It just has interchangeable nipple sizes.
moderately expensive to Expensive/billet cleanable with high performance element:
There are literally thousands of these and each are probably equal in quality. I am only listing this one because its PLENTY good for our use and is smaller enough to fit and has female connectors so you can put normal barbs on it instead of AN fittings.
Aeromotive 12316
https://aeromotiveinc.com/product/ss-series-100-micron-fuel-filter/
This one has female 3/8” NPT connections on each end. You will screw in a male barb on each end that fits your fuel lines. If for some reason you want AN fittings…Aeromotive has this same filter with almost any size AN fitting. Its is 100um stainless element and is 3.5” x 1.25” without fittings.
The replacement element:
https://aeromotiveinc.com/product/100-micron-element-for-38-npt-filters/
These come in Red, Black and silver.
So….as you buy and use any Pre-filters….post your results to this thread please.
Sorry for the very long post for such a simple item.
Ray |
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Lars S
Samba Member
Joined: October 04, 2007
Posts: 783
Location: Sweden
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| Posted: Sun Mar 14, 2021 10:02 am Post subject: Re: In-tank fuel strainer alternatives (sock filter) |
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Ray, wery much thank you for another massive writeup, you are the man!!!
/Lars S
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Porsche 914 -72, Bahia Red daily driver 
VW411 2-d -70, White, sold 
VW412 4-d, -73, Gold Metallic, daily driver
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BMW R69S -69, White, sold
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