The Cessna 120-140 Association Forum

Isolated a leak to the oil screen housing. Just enough to allow a few drips per minute that splatter the firewall while flying and cause me annoyance.

Disassembled, made sure the screen is actually removed (spin-on STC), new AN900-28 crush washer, seem facing housing, reassembled, leak persists. Strip of flannel in the photos is there to capture the oil drips during a ground run. Checked, re-torqued, and at this point I can't crush 'er any better.

I notice that the AN900-28 is difficult to get centered. I never paid much attention figuring the crush takes care of the slop. However, the leak originates from the portion of the crush washer that is most outboard of centered.

Anyone have a strong feeling about how important it is to center that crush washer as you torque it? I have a new oil screen/nut on order as the one in my airplane is utterly mangled from decades of abuse with the wrong tools. This might solve it without needing to clutter the form but I'm curious for your opinions here.

No solution from me here just now, but I've got to tell you that your photography is absolutely superb.

I always center the washer but can't give feedback on whether or not uncentered will leak. I tried complying with instructions to only tighten it a certain number of degrees (depends on threads/inch of your particular screen) since apparently these washers don't need to be fully crushed to seal. Can't remember how many degrees as my chart is in the hangar but it leaked worse. Ended up going back to tightening it with the good-n-tight method. I marked the housing and screen with a sharpie once I got it tight enough for no leaks. That way on subsequent oil changes I could tighten it the right amount the first time.

Josh

Thanks John - I had to seriously downgrade the images to meet the file size limitation.

Thanks Josh. I agree. 16-20 tpi it's 135deg of rotation, 24-28 tpi is 180deg. I just went with good-n-tight. I've tried centering but by the time friction and crush begins to take over it seems to walk out of center.

Hopefully, a stupid question, but, are you sure you're using the right crush washer? Also, are both the housing and the "nut" flat?

Since not everyone is having this problem, there must be something unique about your situation.

Thanks John. I am confident it is AN900-28. I've tried 2. A third one will go on when my new screen nut arrives tomorrow.

Surfaces are flat as far as I can confirm by straight edge and eye.

However, seeing as the screen nut is so mangled on the flats there's a decent chance I'm not seeing some imperfections. Will update soon.

I put the split side of the gasket against copper on the screen. Seems to work better and doesn’t wear out the aluminum.

AN900-28 == MS35769-48 Both have 1-3/4" ID.

6298 wrote: ↑Wed Apr 03, 2024 7:34 am I put the split side of the gasket against copper on the screen. Seems to work better and doesn’t wear out the aluminum.

I found the same as Randy.
Josh

I had what I thought was a leak at the screen housing. Turns out it oil that was blowing back from another leak forward of the screen and housing.