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FAB Fiberglass crack

uk_figs

uk_figs

Well Known Member
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Doing the condition inspection and after removing the K&M air filter I noticed that the bottom of the fiberglass section of the FAB is cracked at the rear in perfect alignment with K&M filter outline. Does not look worn through so I suspect vibration induced crack, 9 years and 460 hours. Is this something others have seen? As I will probably replace the lower fiberglass section is there something I should do to prevent this happening again?
Thanks
Figs
 
uk_figs said:
Doing the condition inspection and after removing the K&M air filter I noticed that the bottom of the fiberglass section of the FAB is cracked at the rear in perfect alignment with K&M filter outline. Does not look worn through so I suspect vibration induced crack, 9 years and 460 hours. Is this something others have seen? As I will probably replace the lower fiberglass section is there something I should do to prevent this happening again?
Thanks
Figs
Click to expand...

Is your filter in contact with the airbox? That might explain it.
 
Kyle
Yes, the filter sits on the airbox, it is the Vans "Filter Airbox- Vertical Draft" http://vansaircraft.com/cgi-bin/store.cgi?ident=1489264626-500-225&browse=engines&product=fab-vertical. Hopefully I can just get a new fiberglass section although I did think about either a) repairing the glass but it has oil on it so I am not sure how well the glass would bond, or b) putting in a metal plate insert.

Here is the crack: https://get.google.com/albumarchive/104601752855779734389/album/AF1QipPhMFVOmZe-t7N1n3_DdSQf06c97weJkd-P-aQM/AF1QipPHTdCR3pzq2jpXXhSL9DeFM50UJX6yZcruMXoC?source=pwa#6396348874481939554
Figs
 
Mine was wearing badly on the bottom of the FAB so I cut aluminum sheetto match the area of filter contact an held in place with several rivets. Seems to have stopped the problem from progresing
 
Dave,
I just had to modify my whole air box and snout to fit my James cowl with a carb engine. It was not the easiest, but it worked as far as I can tell - not flying yet!

In my research on how to make the modifications, I saw several pictures of exactly what you are talking about. The bottom just wears out and cracks. Putting a piece of aluminum inside and re-glassing the bottom would probably work fine.

I made my box about 3/4" deeper and got the next size KN filter to give a bit more airflow. At least that is my hope.

Kyle's comment gives me a question. Doesn't the air filter need to fit tight against the bottom to keep unfiltered air from going to the carb? That is why I upsized the new filter.

FAB%202_zpsh9oageuw.jpg
[/URL][/IMG]
 
Last edited:
rockwoodrv9 said:
Dave,
I just had to modify my whole air box and snout to fit my James cowl with a carb engine. It was not the easiest, but it worked as far as I can tell - not flying yet!

In my research on how to make the modifications, I saw several pictures of exactly what you are talking about. The bottom just wears out and cracks. Putting a piece of aluminum inside and re-glassing the bottom would probably work fine.

I made my box about 3/4" deeper and got the next size KN filter to give a bit more airflow. At least that is my hope.

Kyle's comment gives me a question. Doesn't the air filter need to fit tight against the bottom to keep unfiltered air from going to the carb? That is why I upsized the new filter.

FAB%202_zpsh9oageuw.jpg
[/URL][/IMG]
Click to expand...

No. The filter has a plate that goes on the bottom. It clears the fiberglass by about 1/4 inch or more. The air box does not seal the bottom of the filter element.
 
Thanks Warren. I will be back at the hangar Monday and get a few more shots.

That doesn't change anything about the OP just glassing the bottom back on does it?
 
Depends on aircraft model

Kyle has a 6, I have buddies that also have 6's their filters do not seal on the FAB. I have a 7A and mine does seal on the FAB and I believe another buddies 9A does also.
 
bhester said:
Kyle has a 6, I have buddies that also have 6's their filters do not seal on the FAB. I have a 7A and mine does seal on the FAB and I believe another buddies 9A does also.
Click to expand...

More proof that the -6 is the pinnacle of homebuilt aircraft development. ;-)
 
uk_figs said:
Doing the condition inspection and after removing the K&M air filter I noticed that the bottom of the fiberglass section of the FAB is cracked at the rear in perfect alignment with K&M filter outline. Does not look worn through so I suspect vibration induced crack, 9 years and 460 hours. Is this something others have seen? As I will probably replace the lower fiberglass section is there something I should do to prevent this happening again?
Thanks
Figs
Click to expand...

Seems common - clean it well with lacquer thinner, sand it, and lay a few layers of glass in there with epoxy. I did this somewhere around the same # of hours as you, and it has held for 1100+ more hours. No need to replace the whole housing. Mine is abrading a bit wherever the filter makes contact, so I may have to add another layer sometime soon. The filter gets tortured by the intake pulses, apparently. I do not think it is general engine vibrations which cause it.
 
Me too

Same here too.
Years ago I fixed my wearing Vans FAB bottom.
Tip: The fit is supposed to be snug or at least touching to seal air.
K&N sells a grease for sealing that can be used to help.
Plus, when repairing, you really don't want make the distance the filter sits in any smaller as it was rubbing through already.
I properly cleaned the worn grooves and applied flox to just fill the grooves.
Then added additional layers of cloth on the exterior of the FAB where size doesn't matter.
One thing to keep in mind while working in this area is adding small pieces that could possibly come apart or break and be ingested into the engine.
 
Thanks

Good feedback, I like the idea of a metal plate for the filter to sit on and I think there is room for this as the filter sits a little below the top plate on my assembly. I also think I will add a few layers of glass on the outside to strengthen the glass a bit. If the mod works for another 500 hours that is about 10 years of flying for me :D
Thanks
Figs
 
My FAB setup requires the filter to be in contact with both the upper and bottom surfaces in order to seal. I put two layers of clear packing tape on the F/G lower assembly to avoid F/G Wear. 150 hours at first condition inspection and it's holding up well. If the tape wears out, I'll pull it and re-apply new. It was intended to create a non-friction interface and be sacrificial.

Lalrry
 
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