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My air filter wore thru my airbox

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,
 
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,

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.
 
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

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.
 
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.

That’s the O-320 airbox you’re talking about. The others are different.
 
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.

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.
 
How is it not true if every RV I've worked on with an O-320 has the bottom plate?

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....
 
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....

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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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,

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
 
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...

Yeah, I just put more glass on. Easy to do now. Will try Carbon or Kevlar next time.
 
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

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.
 
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