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My air filter wore thru my airbox

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Dean Pichon

Well Known Member
For the second time in almost 20 years the K&N air filter in my -4 has worn thru the fiberglass bottom. The first time this occurred, I replaced the airbox assembly and repaired the old one to keep as a spare. The spare will now be pressed into service while I repair this one.

Has anyone found a solution to keep this from happening? (I assume I'm not the only one experiencing this issue.)

Thanks,

Dean
 

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I didn't see the filter, but can you mold in a grove that the ring on the filter will fit, trapping it so it dosen't vibrate?
 
Many builders have had good success by incorporating an aluminium plate into the bottom of the FAB. That's what I did.... 520 hours and performing like a champ. Without the metal plate it is only a matter of time before the bottom of the composite FAB wears excessively. Vans should change their design....if they haven't already.
 
Yes - easy fix.

Add a piece of 0.032” aluminum to the bottom of the air box. Add four pieces of aluminum to help prevent the filter from going conical while you are at it.

Carl
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Very common issue.

Fix is put an aluminum plate in so it gets the wear instead of the fiberglass.
 
I reinforced the bottom of the -7 FAB with several layers of carbon fiber after it showed wear at about 150 hours. An additional 700 hours showed little to no wear. In the 10, I put in an aluminum plate.
 
I've repaired mine 3 times in 1700 hours. The first two times were by grinding down some thickness and laying new glass and epoxy in the bottom. The third time was about 2 years ago. I ground down some of the thickness again, but this time I used kevlar cloth. So far, so good, but I do not have many hours on this latest repair.

Aluminum is a good solution, but keep a keen eye on it for fatigue cracks. Loose pieces of aluminum would not be good there...
 
Thanks all. It sounds like an aluminum plate is the way to go. A couple of questions:

Is the geometry of the plate such that it can be installed in a completed airbox or does the "top" have to be removed to get it in place?

Once installed, is it bonded or riveted to the fiberglass?

Thanks,
 
Dean Pichon said:
Thanks all. It sounds like an aluminum plate is the way to go. A couple of questions:

Is the geometry of the plate such that it can be installed in a completed airbox or does the "top" have to be removed to get it in place?

Once installed, is it bonded or riveted to the fiberglass?

Thanks,
Click to expand...

I installed the plate as Carl suggested. It fits through the existing hole in the top. I used both rivets and PRC to bond it in. A few hundred hours and ok so far.
 
It shouldn't touch it

Built per plans, the filter should have an aluminum bottom plate, and the fiberglass FAB is a good 1/4 inch below it. The FAB has been over trimmed on may and is too close to the filter and lower plate.
 
O-360 engines = filter sits against fiberglass bottom of FAB.
O-320 engines = filter has an aluminum plate on bottom of filter. Space in-between filter bottom/plate and FAB fiberglass

On the 360 FAB's I have had filters rub thru the bottom of the FAB and repaired/reinforced them with kevlar.

I also have a DXF file to CNC a new top plate out of 1/8". Haven't had any problems with cracking top plates as I have with the stock thickness.

I do not like anything riveted to the inside of the filter....have seen too many things disappear into engines without a trace.
 
fixnflyguy said:
Built per plans, the filter should have an aluminum bottom plate, and the fiberglass FAB is a good 1/4 inch below it. The FAB has been over trimmed on may and is too close to the filter and lower plate.
Click to expand...

rocketbob said:
O-360 engines = filter sits against fiberglass bottom of FAB.
O-320 engines = filter has an aluminum plate on bottom of filter. Space in-between filter bottom/plate and FAB fiberglass
Click to expand...

Not really correct on either of the above posts. While this was true on early versions of the O-320, FWF plans post-2006 or -2007 used the same style as the O-360, where the filter just sits on the bottom of the FAB.
 
fixnflyguy said:
Built per plans, the filter should have an aluminum bottom plate, and the fiberglass FAB is a good 1/4 inch below it. The FAB has been over trimmed on may and is too close to the filter and lower plate.
Click to expand...

That’s the O-320 airbox you’re talking about. The others are different.
 
alpinelakespilot2000 said:
Not really correct on either of the above posts. While this was true on early versions of the O-320, FWF plans post-2006 or -2007 used the same style as the O-360, where the filter just sits on the bottom of the FAB.
Click to expand...

How is it not true if every RV I've worked on with an O-320 has the bottom plate?
 
This happened to my -8 not long after I bought it. It has an O-360 and the air filter sits on the fiberglass. For this time I just rebuilt it with fiberglass. I may consider Kevlar cloth if it happens again. I suspect that the K&N air filter shrinks over time and is able to move around between the tabs on top of the airbox causing the fiberglass to wear. I now plan to replace the air filter at each annual instead of washing and oiling it.

I know the filters shrink as it happened with the -6A I previously owned. We had two filters and switched them out at each annual. One year, the spare filter would not fit easily. My mechanic at the time spent a couple of hours working it into place.
 
rocketbob said:
How is it not true if every RV I've worked on with an O-320 has the bottom plate?
Click to expand...

Don’t know. I’m sure you’ve worjked on a bunch but maybe you’ve not worked on O-320s RVs built after mid 2000s? At least not mine. I don’t have my FWF plans handy to give you the drawing number but can assure you that my RV9 O320 does indeed sandwich the filter between the carb plate and the fiberglass bottom of the FAB. As noted above, earlier versions did use the 2 aluminum plate connected by long bolts method but Vans changed that for some reason sometime around 2007 or earlier. Just did a quick search—see thread below, particularly posts 3, 4, and 6 for confirmation that I’m not the only one...

https://vansairforce.net/community/showthread.php?t=182023&highlight=320+airbox
 
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Fuel tank sealer

I cleaned mine up and put a layer of fuel tank sealer only on the wear/sealing area. Been fine ever since.

FYI my filter shrunk over 1 inch in diameter in 14 years! Yes, it has a new one now and the old was still good, just smaller....
 
664781 said:
I cleaned mine up and put a layer of fuel tank sealer only on the wear/sealing area. Been fine ever since.

FYI my filter shrunk over 1 inch in diameter in 14 years! Yes, it has a new one now and the old was still good, just smaller....
Click to expand...

Good continued luck with your solution.
I would caution anyone else employing this method to vigorously clean up & properly prepare the wear/sealing area before applying sealant as if the rubbing motion breaks it loose, it is going to be sucked up through the engine.
Also, if there are grooves in the affected fiberglass area, it should be built back up to compensate for the thin parts.

I have found air filter grooved FAB units numerous times in the RVs I've serviced. Seems to show up most in planes where owners prime (or over prime) during start, excess fuel drains back & creates a damp grit with dust & dirt and wet sands the fiberglass through. I advise those owners to prime less, which seems to help in extending the time between airbox rebuilds. This is just one of the FAB maintenance concerns I regularily inspect for.
 
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Dean Pichon said:
Thanks all. It sounds like an aluminum plate is the way to go. A couple of questions:

Is the geometry of the plate such that it can be installed in a completed airbox or does the "top" have to be removed to get it in place?

Once installed, is it bonded or riveted to the fiberglass?

Thanks,
Click to expand...

You'll need to remove the aluminum top section to install the plate. I used a nice fat layer of ProSeal to bond the plate to the fiberglass airbox. Good luck!
 
My RV-6A was built per plans with the filter being compressed against the bottom of the airbox and top plate mounted to the updraft RSA fuel servo. Every year I’ve had to work on this airbox and remedy many issues over the past 11 years of flying. This last time it’s managed to rub through the bottom plate that I had installed in the air box. I’ve also had rivets pull out they were holding the alternate air plate to the outside bottom of the box. The filter is definitely shrunk both in diameter and in hight. The filter has definitely shrunk both in diameter and in hight. This results in a lot of fretting. I’m about to do a full rebuild this year with a new filter and two metal plates on the bottom of the FAB, one inside and one outside in bottom surface sandwiching the bottom, riveted and prosealed . Also, I thought I put four bolts through the bottom of the box to the top plate on the servo so that I can put proper pressure to hold the filter in place and also help keep the filter from shrinking in diameter. Has anyone else tried this or have a more permanent solution than the stock system? Marty
 
AlexPeterson said:
I've repaired mine 3 times in 1700 hours. The first two times were by grinding down some thickness and laying new glass and epoxy in the bottom. The third time was about 2 years ago. I ground down some of the thickness again, but this time I used kevlar cloth. So far, so good, but I do not have many hours on this latest repair.

Aluminum is a good solution, but keep a keen eye on it for fatigue cracks. Loose pieces of aluminum would not be good there...
Click to expand...

Yeah, I just put more glass on. Easy to do now. Will try Carbon or Kevlar next time.
 
Martyrv6a said:
My RV-6A was built per plans with the filter being compressed against the bottom of the airbox and top plate mounted to the updraft RSA fuel servo. Every year I’ve had to work on this airbox and remedy many issues over the past 11 years of flying. This last time it’s managed to rub through the bottom plate that I had installed in the air box. I’ve also had rivets pull out they were holding the alternate air plate to the outside bottom of the box. The filter is definitely shrunk both in diameter and in hight. The filter has definitely shrunk both in diameter and in hight. This results in a lot of fretting. I’m about to do a full rebuild this year with a new filter and two metal plates on the bottom of the FAB, one inside and one outside in bottom surface sandwiching the bottom, riveted and prosealed . Also, I thought I put four bolts through the bottom of the box to the top plate on the servo so that I can put proper pressure to hold the filter in place and also help keep the filter from shrinking in diameter. Has anyone else tried this or have a more permanent solution than the stock system? Marty
Click to expand...

I have seen many FAB air boxes with same issues you describe.
Those missing alt air plate rivets, do you wonder where they go? I once had a spark plug mysteriously short out (knocked flat), replaced it, than had to overhaul that cylinder shortly after. I fully suspect the lost rivets from my alt air plate (discovered when I pulled the cylinder) were the culprits to these failures.
My fix - I modified my FAB box with an aluminum plate (glued to the lower fiberglass floor with nutplates & AN507 screws to lock the alt air plate in place. I solved the filter wear, loose alt air plate, and errant pulled rivet problems in one shot and 500hrs later is still holding.

The 4 bolt arrangement you describe is very similar to what FAB 320 uses.

Also - I would be wary of using carbon fiber in that location, grinding carbon dust might not be a good thing being sucked through the engine.
 
I made a template in SolidWorks. It's for the FAB-320-1. If it's useful I could make a template for the for the FAB-360/540.

I put wings on it to keep the filter from swelling outward at the bottom.

PDF here
 

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If you rivet a aluminum plate into the bottom of the airbox, ensure that if the rivets/hardware should come loose they are either held in by the filter itself, or sit outboard of the filter.
We don't want any loose stuff being sucked up the carb / servo and thru the engine. Or just getting stuck in the butterfly...

And, my 0-320 has the filter element sitting on the bottom of the housing, built per the plans circa 2012.

I don't want to get started on my opinion of the bottom "alt air door". Suffice to say it lost rivets in under 100 hours.
 
I had a similar problem with one of mine - sorry I can’t find the pictures, but I bonded an aluminum “donut” to the area where the filter was making contact. I used a structural epoxy adhesive with a wide temperature range to bond the aluminum donut to the airbox. I drilled a few holes in the donut to act as epoxy rivets to keep it from trying to move around. The filter element pretty much clamps it in place, but the epoxy secures it. I haven’t done it yet to my current RV6 (O-360), but I will as soon as I see any wear on my airbox.
 
Ralph Inkster said:
I have found air filter grooved FAB units numerous times in the RVs I've serviced. Seems to show up most in planes where owners prime (or over prime) during start, excess fuel drains back & creates a damp grit with dust & dirt and wet sands the fiberglass through. I advise those owners to prime less, which seems to help in extending the time between airbox rebuilds. This is just one of the FAB maintenance concerns I regularily inspect for.
Click to expand...
Priming with actual primer... or the pump the throttle method?
I'd expect the pump the throttle method to drop more fuel into the 'box.
 
I had to reinforce my filter box back in 2014, it’s still holding up good.
Details on my site:

My 2014 Flying Adventures

This site follows the flying adventures of my Van's RV7A Experimental Airplane
www.newtech.com
 
There is nothing wrong with adding an aluminum plate to the bottom of the airbox if people choose that as a remedy, but the root cause is from using an air filter that has shrunk, and should have already been replaced.
If built per the instructions, the filter is under a slight compression between the top plate, and the bottom of the airbox, so it cannot move around and wear through the bottom.
If the filter shrinks excessively, it is obvious that this compression will no longer exist, and the filter is then rattling around in the airbox.
If it does that a long enough period of time, it is obvious that wear is likely to occur.
 
blaplante said:
Priming with actual primer... or the pump the throttle method?
I'd expect the pump the throttle method to drop more fuel into the 'box.
Click to expand...
In Lycomings, the electric primer (or manual, if you are so inclined) feeds directly into the intake manifold on usually two or more cylinders. The pump on the carburetor, if used without the engine cranking, will drip raw fuel into the airbox through the air filter. I would imagine, however, being fuel, it would vaporize quickly. Perhaps it's the grit picked up by the oil in the filter that would act as a grinding substance. Moral to the story: Don't use the throttle primer pump without the engine cranking. :rolleyes:

Another subject: Is this more common in certain models? The box on my -4 looks pretty much like it did the first day I put it on; a little dirtier, perhaps. 😊 I do have a small dab/line of silicon where it might get rubbed on but nothing going through like the above pictures.
 
blaplante said:
Priming with actual primer... or the pump the throttle method?
I'd expect the pump the throttle method to drop more fuel into the 'box.
Click to expand...
One owner’s procedure was to press the primer for the count of 4, than turn it over. Usually there was a 10 second pause between prime & turning engine over. A good portion of that prime fuel would be draining out & thru the carb.
Another owner was pumping the throttle accelerator pump priming, than also pausing before turning the engine over. Same result, primed fuel was flowing out the bottom of the carb.
Both scenarios results with an ugly grimy mess in the air cleaner.
 
Captain Avgas said:
Many builders have had good success by incorporating an aluminium plate into the bottom of the FAB
Click to expand...
Same here... my ride's approaching 20 years and 2,000 hours, and the filters dug some grooves over time. Used 1mm alu sheet and epoxied this "ring" in.
At the price those FABs do retail now (FAB-360/540, $656.25 😲), I sure hope being good for another time period.
 

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