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Air filter retaining bracket sucked up into carb. How to properly mount air filter on O-360 carbureted?

nickw9815

nickw9815

Well Known Member
Hey -

I have an O-360 carbureted with an airbox. The air filter was mounted with retaining brackets on the inside of the filter. They were riveted down, and a rivet failed and the bracket was sucked up into the carb. Thank fully the flow guides in the carbon caught it, but for obvious reasons I want to ensure this never happens again.

I forgot to take a photo of the air box set up, so I tried to illustrate it. The retaining brackets were riveted into the fiber glass air box. We could add a metal plate to the back and make it much stronger

1733634435481.png

IMG_6472.jpg
 
This type of failure rarely happens in just a few hours ( it actually usually shows signs many many hours before hand)
This may sound a bit harsh but the important factor here is thorough detailed condition inspections, having the knowledge of what signs to look for, and never deferring corrections until later unless you know for sure there is no risk in doing so.
That is one of the catch 22 factors of the very liberal rules we have with experimental aircraft…. Other than E-LSA aircraft, we can do condition inspections with no formal training on how to properly do them.

I have taken part in maintaining a lot of different RV’s and have never seen a FAB have issues just suddenly pop up.
Having said all that, I don’t understand how anyone could assemble the fab with the retainer clips so that they were positioned to the inside of the filter.

Edit- after zooming in and looking closer at your photo, I can see that at least some of your issue is that the fab was never built per plans. The clip is not the right shape and it wasn’t mounted as described in the plans. Each clip is supposed to be associated with a nut plate on the main top plate of the fab. It appears that someone made some clips and riveted them with a single rivet on the inboard side of the filter.
I would suggest you get a hold of a copy of the latest fab, build instructions and modify yours to match them.
Also, if your air filter really does fit the way you show with the red line in the diagram, you don’t have the proper air filter. A new filter will just barely fit within the opening in the top plate and is meant to be held within that space by the clips around the outside perimeter.
 
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Vans has an improved air box kit that addresses some of the lessons learned over the years, on mine I have a metal plate glassed in the bottom after I noticed some rubbing of the fiberglass by the filter. Also as Scott noted the filter is actually a tight fit in the air box but the filters can shrink over time (some threads on VAF about the issue) so that is also worth checking as you resolve the problem.
Figs
 
Do the new air boxed still have the internal brackets? I saw on the other thread that some people removed the internal brackets and just ran a flat metal plate on the bottom.IMG_6494.jpgIMG_6493.jpg
 
nickw9815 said:
Do the new air boxed still have the internal brackets? I saw on the other thread that some people removed the internal brackets and just ran a flat metal plate on the bottom.View attachment 76003View attachment 76002
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No, because the build instructions have never had a detail for installing brackets of any kind on the inboard side of the filter. Legacy FAB or the new version.
Any FAB that has them, it was a design change made by the builder without considering the potential consequences.
It looks like your filter was due for replacement a long time ago.
A new one will be just barely smaller than the oval shaped opening in the top plate.
If everything is built correctly (and the filter isn’t badly aged beyond when it should have been replaced), clips on the bottom are not needed.
 
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Thanks for the help everyone. Sounds like my best path forward is to:
1. Replace air filter, and remove all remaining internal brackets. Should I go ahead and bond on a small metal plate to prevent wear? What thickness is recommended?
 
One comment looking at the photos is that maybe the wrong filter is installed as the brackets look like they were matched to the filter but the filter is way smaller than the air box opening. As Scott noted the correct filter is a tight fit in that oval opening and against the top and bottom of the air box.
Figs
 
nickw9815 said:
That is an interesting point. I suppose I should check the plans and not just order the same PN again.

I see that it is sold out on Vans, is there another store that sells them? They are not on spruce
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You can buy the (automotive) filters on Amazon. Tempest also sells a filter of the same size - ACS has them
 
The top plate cut out measures 8.75" x 6.5". Our air filter measures 6.75" x 7.5"... Looks like a new air filter is 7.75" x 7.75", so ours has shrunk and ovaled.

On the top plate, we have 4 external clips to hold the air filter in place at 7.75" spacing so these should work very well to prevent movement once I remove the bottom brakcets and bond on a thin metal plate.
 
For more info and light reading there is a revived older thread currently running called "My air filter wore thru my air box". Interestingly enough it shows some installations with the same riveted clips inside the filter as your installation.
Figs
 
nickw9815 said:
The top plate cut out measures 8.75" x 6.5". Our air filter measures 6.75" x 7.5"... Looks like a new air filter is 7.75" x 7.75", so ours has shrunk and ovaled.

On the top plate, we have 4 external clips to hold the air filter in place at 7.75" spacing so these should work very well to prevent movement once I remove the bottom brakcets and bond on a thin metal plate.
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The specified filter is round, but they are flexible and is easily changed to an oval shape that matches the dimension of the oval opening in the top plate.
A properly built fab will have a retaining clip located at each of the six nut plate locations in the top plate.
 
supik said:
Paul, do you have any personal experience with the Tempest variant? Do they shrink as well..?
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I have about a year on a couple of the Tempest filters - not enough time to know if they are shrinking or not, but I check them when I have the FAB’s off. Check back in a couple years……
 
Okay, thanks to the help of the VAF and builder support I came up with a new solution:
1. I replaced the old air filter which had shrunk about 1" in diameter. Yikes
2. I removed all internal brackets on the filter. I then bonded in a stainless steel plate on the bottom since my air box had signs of wear. This plate is 1/32" thick and pushes the filter up enough that it will be compressed and not go anywhere when assembled. I will also drill a small hole in the center to let excess fuel drain (not shown in the photos).

IMG_6539.jpg
 
nickw9815 said:
Okay, thanks to the help of the VAF and builder support I came up with a new solution:
1. I replaced the old air filter which had shrunk about 1" in diameter. Yikes
2. I removed all internal brackets on the filter. I then bonded in a stainless steel plate on the bottom since my air box had signs of wear. This plate is 1/32" thick and pushes the filter up enough that it will be compressed and not go anywhere when assembled. I will also drill a small hole in the center to let excess fuel drain (not shown in the photos).

View attachment 76175
Click to expand...
The drain hole should be at the lowest point inside of the filter profile when the airplane is sitting on the ground, which on most all RVs is right at the very back, but just inside the filter.
There is supposed to be a second drain hole at the very back of the airbox outside of the filter to allow any water to drain out that may enter during flight, sitting outside during heavy rain, washing the airplane, etc.
 
@rvbuilder2002 - I see. The original drain holes are where the red dots are. Are you saying it is better to have drain holes where the blue dots are? (same location for the one outside the air filter). This does make sense since it is at the lowest point....

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