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FAB Design Question

DavePlaneGrips

Well Known Member
Okay, I have a question about the FAB design. It seems one of the RVs use a design where a plate seals against the bottom of the filter, instead of the filter sealing against the bottom of the inside of the FAB shroud. To me, that seems like a better, easier, design. What's the benefit of having the filter seal against the bottom of the inside of the fab? from what I've read, this only seems to cause problems. I'd just like to understand why sealing against the shroud is the "better" newer design.

The only thing I can think of is for the ability to provision an alternate air door that bypasses the filter, but if I don't intend to have this feature I think I might go with a sealing plate.
 
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Okay, I have a question about the FAB design. It seems one of the RVs use a design where a plate seals against the bottom of the filter, instead of the filter sealing against the bottom of the inside of the FAB shroud. To me, that seems like a better, easier, design. What's the benefit of having the filter seal against the bottom of the inside of the fab? from what I've read, this only seems to cause problems. I'd just like to understand why sealing against the shroud is the "better" newer design.

The only thing I can think of is for the ability to provision an alternate air door that bypasses the filter, but if I don't intend to have this feature I think I might go with a sealing plate.

Our O-320 powered 6A had what you describe. There were long bolts hanging from the upper plate through a lower plate making a filter sandwich. It was a bit of a pain to get them all lined up but doable. It is the O-360 version where the filter rides on the bottom of the airbox. I think it is using a larger filter that completely surrounds the bottom of the carb. The O-320 version partially interferes with the accelerator pump on the bottom of the carb requiring a hack job on the filter. I prefer the O-360 version from a maintenance perspective. It's easier to get on and off and you don't have to worry about trying to seal up that pesky leak around the accel pump. Folks who have had wear or shrinkage problems usually cure it with a piece of aluminum bonded to the bottom of the FAB, a fairly easy fix. I don't know if there's a problem fitting the O-360 box to an O-320. I haven't tried that.

Ed Holyoke
 
Folks who have had wear or shrinkage problems usually cure it with a piece of aluminum bonded to the bottom of the FAB, a fairly easy fix.

Like many, I complacently thought I could just keep cleaning & re-oiling the thing forever but also wondering why it didn't seem to fit right any more -- then a few months back I watched an EAA webinar with Vic Syracuse who pointed out that the high heat environment under our engines isn't what they were designed for, and because of that they eventually shrink and lose their sealing capability and should be replaced. D'oh! So I finally did that after 20 years and 1500 hrs.

That said I like the idea of a plate on the bottom to get a better seal. On mine it's still just the fiberglass.
 
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