Air filter cutout questions

[Ed_Wischmeyer]

For the annual on my RV-9A, it was time to replace the air filter, and that means cutting away some of the material in the filter to clear the carburetor accelerator pump. I've been practicing on the old filter and have a rotary file on the drill press that seems to work well, but I don't know the dimensions...

* Diameter of the cut? (appx 3/4")
* Thickness remaining of the filter? (appx 1/4" or so. The old filter wasn't done very well, and there's RTV slobbered everywhere)
* Height of the cut? (appx 3/4")

Hopefully somebody can give me an answer promptly and I can get this all done tonight, and the RTV will have cured so we can re-install tomorrow.

Thanks!

Ed

PS. Guess who left the manuals in the hangar...

[Sam Buchanan]

Ed, I don't have dimensions handy, but just wanted to make sure you know the K&N filter can be washed, oiled and reused multiple times. I used the original filter on my RV-6 for many years after cleaning and oiling each year. When I finally decided to replace it I used the 'fit-n-trim' method at the plane to determine the size of the cut. The RTV was applied in place and allowed to set over-night.

But it sounds like you are just tidying up things up a bit and replacing a poorly installed filter.

[lr172]

I'll add that the K&N filters don't need as much cleaning as most think. Believe it or not, they perform better once they are a little dirty. They are not like paper filters. The pore on the cotton filters are bigger and the dust mixed with the oil helps to tighten up the openings for better protection. They have pictures on their site, showing what is in need of cleaning and what is not. Generally, if the holes in the metal screen are still visible, you're good to go.

Larry

[Ed_Wischmeyer]

Yes, the original installation was shoddy and yes we have all the stuff to clean the filter.

Thsnks tho. Off to the hangar this morning to get the instructions.

[Vac]

Ed,

It depends on the configuration of your FAB; but I have two filters that I clean and swap at regular intervals without a cut-out for the accelerator pump. The technique I use is to warm the filter in the oven at the lowest setting (about 150 degrees F) for a few minutes. This makes the filter more pliable and I'm able to install it and make a good seal at the pump bulge. Eventually, the rubber on the filter takes on a bit of a set, which makes it easier to get on and off each time.

Also, if you are going to modify a filter that has already been oiled, recommend a thorough cleaning, dry and clean with an appropriate solvent to get the silicone to properly adhere. Anecdotally, I helped a friend modify a filter than had been previously oiled and it was difficult to get good adhesion--may be easier to start with a brand new filter.

Cheers,

Vac

[Ed_Wischmeyer]

Two sizes of rotary file on a drill press did the cutouts in a small fraction of the time spent thinking about it. Masking tape to contain the RTV that might squeeze out and we're good to go. Come back tomorrow to trim excess RTV and clean things up.