The Amazing Shrinking Air Filters

[rjcthree]

I had 25H TT on my filter before needing to do my condition inspection. I removed the filter which fit just fine, it did not snap off or anything odd. It sat for a couple hours, still fine. I washed it (K&N filter wash) , set it out to dry (breezy sunny day after a gentle air blow off). Was wrapping up and pulling stuff in to find one side of the filter had shrunk significantly, diametrically, over three hours of drying time. It looks like a trunchated cone now. I can't say if just letting it sit would have caused the same shrinkage, but I'm suspicious of the washing process. I'll go to blow-off/shop vac/re-oil for regular condition inspections now, and a periodic replacement (3 years? 5? Depends on degradation or deformation) rather than washing.

[az_gila]

Originally Posted by rjcthree

I had 25H TT on my filter before needing to do my condition inspection. I removed the filter which fit just fine, it did not snap off or anything odd. It sat for a couple hours, still fine. I washed it (K&N filter wash) , set it out to dry (breezy sunny day after a gentle air blow off). Was wrapping up and pulling stuff in to find one side of the filter had shrunk significantly, diametrically, over three hours of drying time. It looks like a trunchated cone now. I can't say if just letting it sit would have caused the same shrinkage, but I'm suspicious of the washing process. I'll go to blow-off/shop vac/re-oil for regular condition inspections now, and a periodic replacement (3 years? 5? Depends on degradation or deformation) rather than washing.

The certified K&N filters are sold as Challenger products. This is a link to their cleaning instructions. No mention of blow drying -

http://www.challengeraviation.com/Il...structions.pdf

The Challenger filter for my Tiger has the original K&N part number on it - 60's era Dodge slant 6 I believe - and an extra stamp of "FAA-PMA" on the rim.

[rvbuilder2002]

Originally Posted by rjcthree

set it out to dry (breezy sunny day after a gentle air blow off).

The K&N cleaning instructions say "After rinsing your filter, gently shake off the excess water and only allow filter to dry naturally. Do not apply oil until the air filter is completely dry."

I admit that "dry naturally" can have many different meanings (the sun and wind are natural), but my interpretation has always been to do nothing to artificially accelerate the drying process.
The filter media is cotton. It will shrink just like a 100% cotton shirt will.
If you do anything to accelerate the drying process, it will shrink that much more.
I always do the cleaning immediately after removal and then set it aside on my work cart to let it air dry over night. They can be reinstalled immediately after being oiled.

[JonJay]

I bought a new filter at the first CI. I then cleaned the old filter and let dry as Scott suggests, in the hangar in free air. When dry, after a couple days, I put it back in the box until next year. I then oil the spare and install it (if it is new it is pre-oiled). At the next CI, repeat.
My two filters have lasted 750 hours total and more than ten years with no shrinkage..

[rjcthree]

So the feedback is blow drying is somehow the cause of the shrinkage. I understand the cotton consideration. The shrinkage is diametrical, not in height, and only on one side. I've not been able make these elements come together with blow drying as a contributor. I'm suspect of the washing solution as a contributor for the same reason now too. A lunchtime Ishikawa diagram, maybe, get everything on paper. I'll report back after I deconstruct it, maybe the rubber returns when free of the cotton. I'm curious about the durometer, and if there's a difference top to bottom. If nothing else, it's an excuse to goof off in the materials lab at work.

[Robski]

I think I have the same issue with the K&N in my O-320 powered RV6.

I bought the plane as a flying beastie about 10 years ago with what I believe is probably its original filter.

Not having realised the saga of the shrinking air filters I have just been popping it off, cleaning and re-oiling it once a year. I always wondered about needing to loop some locking wire around the last bolt and give it a heave to get it to locate into the hole in the bottom plate. Thought that was just the way it was.
The cutaway is an odd feature so why not this?

"Bottom plate" I hear some of you say?
My RV was finished in 1994 so has lots of early features - floor mounted pedals etc. - and one of these 'features' seems to be that the FAB sits around the filter but doesn't actually retain it. The filter is clamped between the top plate and a bottom aluminium plate by 4 long AN3 bolts. The fibreglass plenum (with hot air door etc) is then fitted around it and attached to the top plate.
This may, of course, be a modification the original builder added, but it does mean that one can see the filter is (very!) snugly located on the carb inlet before covering it all up with the plenum box.

A friend gave me a secondhand RV6 filter a while ago, and mine is appreciably smaller than that one. As the 'new' one was in a box I was able see the K&N part number was E-3260 - and then found it on the Van's site anyway - and have ordered a new one through Amazon. £21. About $28.

We shall see what size that turns out to be!

[Robski]

Quick update.

The 20 year old K&N in my RV-6 was 18 1/2" round. I think that has shrunk around 2 inches from the 20+" of the new one!

I now think the bottom plate on the K&N in my FAB is meant to be there as it is exactly the right length for the rib around the new K&N to go around it.

[ReidVaitor]

500 hours and K&N telling me that I didn't need to clean it if i could clearly see through the filter, which I could. So never cleaned, but sure enough it shrunk enough that I can not use it anymore. I called K&N, based on this feedback from forum and they mentioned the filter is not warrantied due to not being used " as designed" as the E-3450 is not meant to be used as a aircraft filter. I bought a new one from Aircraft Spruce, which fits correctly.
Pascal
RV-10

[Dustyone]

Just finishing up my 4th Annual inspection and I went to put back my air filter that I had washed and put aside.



The filter had shrunk in height and length, so much so that I had to put a .063 spacer in the bottom of the FAB on top of the Stainless wear plate that was already fitted just so it would seal.

This is the 2nd filter in 4 years ,the first one shrunk the same way at the 2 year mark.

I do fly through a lot of rain.

Time to look at a total redesign of this area .

[Pilot135pd]

Originally Posted by JonJay

I bought a new filter at the first CI. I then cleaned the old filter and let dry as Scott suggests, in the hangar in free air. When dry, after a couple days, I put it back in the box until next year. I then oil the spare and install it (if it is new it is pre-oiled). At the next CI, repeat.
My two filters have lasted 750 hours total and more than ten years with no shrinkage..

I think the takeaway is to do this and if it shrinks take it back to the autoparts store and tell them it was in your car for the warranty.