My air filter wore thru my airbox

[Dean Pichon]

Thanks. I see the pic, but still don’t get it. If the filter is blocked on the outside at tabs at the top, in multiple places around the sides, it basically can’t move. Any force applied on the side would try to make it longer, but would be blocked from doing so. Similar result if force were applied on an “end”—it would be blocked from getting “wider” by the tabs on the top. The aluminum bottom plate I added to prevent wear through the fiberglass has no tabs, and I don’t think they are needed. If the filter has shrunk somuch that it can move enough to create a problem, it’s long past the condition where it should have been replaced.

The issue some have observed, myself included, is that these filters shrink/distort significantly with time. Others installed retainers to constrain the ID. When I added the aluminum floor to reduce wear on the airbox, I followed suit and installed retainers. It never occurred to me they would crack in just a few years (~4). In addition, I now limit filter use to 2 years. After it comes out at the 2nd Condition Inspection, I throw it away and put in a fresh one. This practice does appear to mitigate the shrinkage issue.

 

[tom_AZ]

The issue some have observed, myself included, is that these filters shrink/distort significantly with time. Others installed retainers to constrain the ID. When I added the aluminum floor to reduce wear on the airbox, I followed suit and installed retainers. It never occurred to me they would crack in just a few years (~4). In addition, I now limit filter use to 2 years. After it comes out at the 2nd Condition Inspection, I throw it away and put in a fresh one. This practice does appear to mitigate the shrinkage issue.

Got it. Trying to stop shrinkage with a retainer is a losing proposition. Something has to give. Yes, they shrink over time—mostly in circumference but somewhat in height as well. When the amount of shrinkage is sufficient to allow movement to be a problem, it’s past time to replace. I keep two on hand. One in the FAB, one on the shelf ready to go. I swap at each C.I. Each seems to get about 3 one year cycles in the FAB before they need to be replaced.

 

[DanH]

That looks nice. What did you use for fuel proof resin - vinyl ester?

Max GRE

 

[BruceW]

Be ONE with the shrinkage. Let it happen.
My acceptance includes the same as Dean, just replace every 2 years and fly on.

 

[sansoneservices]

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

How about this (don't laugh) .. low grade silicone adhesive at the wear point. Even if you silicone the filter down it releases easily with wd40 at your next filter change

 

[nickw9815]

I'll cross post my new airbox solution here. My old air box set up had internal brackets rivets to the bottom, and one came undone and was sucked into the carb. There were also wear marks on the fiberglass air box.

Here is my new set up:
1. I replaced the old air filter which had shrunk >1" in diameter.
2. I removed all internal brackets on the filter. I then bonded in a 17-4PH H900 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).