SMO

SMO

Well Known Member
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Doing an inspection on a newly purchased 450 hour RV-4. Found cracks in the aluminum plate that attaches the air filter housing to the throttle body. I have drawn the cracks on the plate. This wasn't far from failing. Further, the bolts that held it onto the throttle body were loose (not wired or even lock washered). Will make a new plate out of 4130 and powdercoat it.

I understand that had this failed I would have suffered a serious loss of power, but more worrisome is the potential for fire if the fiberglass filter housing got pushed against the exhaust pipe(s).

Throttle%20Body%20Plate.jpg
 
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Mine had cracks and the hydroformed part it mates to was distorted in such a way the filter was not a good fit. I replaced both parts an had to lower the hydroformed part slightly to get a good fit on the air cleaner.

Important: Do not add any parts or shims or shim material to the inside of the box to snug up the filter fit if it is loose. Rebuild the box.

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20070228X00239&key=1

Replacing the part with steel is not a bad idea. Watch for cracks on the hydroformed part as well. There is so much vibration on that box these parts should be considered "wear items" that will require on condition replacement over time.
 
SMO said:
Doing an inspection on a newly purchased 450 hour RV-4. Found cracks in the aluminum plate that attaches the air filter housing to the throttle body. I have drawn the cracks on the plate. This wasn't far from failing. Further, the bolts that held it onto the throttle body were loose (not wired or even lock washered). Will make a new plate out of 4130 and powdercoat it.
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Don't powdercoat the replacement, that will make it harder to see cracks in the future.

Glad you found the cracked airbox plate, this is not an isolated issue. Make sure the airbox is not touching the cowl either at the chin scoop or on the bottom, you should have a gap large enough to accommodate engine shake. There should be some airseal material between the airbox and air intake.
 
N941WR said:
Sam,

What about if he has it powdercoated white or paints it white? That will really make those cracks stand out, would it not?
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My concern is the cracks might be present before they are large enough to fracture the powdercoat. Some powdercoating is quite thick and could conceal significant cracks. A steel plate could be more readily inspected if it was finished with a thin topcoat of some sort. The aluminum version needs no additional finish.
 
Primer?

Sam Buchanan said:
A steel plate could be more readily inspected if it was finished with a thin topcoat of some sort.
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Sam, for coating would you suggest an epoxy primer? Obviously some coating would be required for corrosion resistance.

I was also considering stainless but was advised that it would crack as easily as aluminum. I had thought it was significantly more ductile, but don't have much understanding of the properties of various types of stainless.
 
SMO said:
Sam, for coating would you suggest an epoxy primer? Obviously some coating would be required for corrosion resistance.

I was also considering stainless but was advised that it would crack as easily as aluminum. I had thought it was significantly more ductile, but don't have much understanding of the properties of various types of stainless.
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I don't have nearly the experience with the -4 airbox as I do with the -6 version, but I would spend my time finding out why the aluminum plate cracked. Something made it crack, and that something needs to be fixed. If the stresses are removed, the standard aluminum plate should be fine. If a steel plate is substituted, and the vibrations aren't minimized, the stress is just transferred somewhere else......like to the carb....not good.

The airbox on my -6 has 875 hrs with no cracks (yet?), but I made sure it doesn't touch the cowl, and the prop has been dynamically balanced down to a very low level.
 
fix

I had the same problem. Made some plate carbon and have not had a problem since. You can buy plate carbon or glass on the net or just make some. I have found that in many applications composites will be stronger than aluminum and tend to not crack. Fiberglass would work also. I think I sandwiched a layer of cotton t-shirt in between 2 layers of carbon fiber.

Jason
 
It took 450 hours to crack that far, I would not get too worried about it. Replace it and inspect it every 100 hours.
 
This is a very common issue. My friend had it on this -7 after about 100 hours. We fabbed a new one with 0.093 aluminum.... So far so good, but it has only been another 100 hours...
-Mike
RV-4 Flying
RV-10 FWF and wiring
 
I discovered very similar cracking on mine at the first annual (150 or so hours.) I made a new plate out of .090 al sheet, and I replaced the washers with large area washers to spread the load out more. Looking at your picture, the crack seems to follow the outline of the washers on the outer portion of the plate.

I think making it out of 4130 is overkill and will add weight. And powdercoat is not a great idea as it will hide cracks. Make it out of aluminum, don't prime it at all and replace it every five years, and you'll be fine.
 
Does this happen because the cowl/air box junction is too tight, causing the cowl to be stressing the airbox when the engine shakes at shut down?
 
Could be.....

......engine shake, Steve.

A friend placed putty on either side of the airbox at the closest area to the cowl. He then started the engine with the top cowl off and shut it down, watching it shake. He then looked at the putty and was happy with the clearance.

Regards,
 
Never liked the FAB...

I'm pretty sure the failure wouldn't have killed you, probably not even a little bit scared. Hey, what doesn't kill you makes you stronger!
I flew my RV4 for 1500 hours with no FAB at all. I used a home-made ram air system with a K&N filter on top.
The problem with the FAB is it sticks out into space with no support, the vibration inflight causes the frame to crack as yours did. The easy fix is to fabricate a support bracket that attaches to the forward bolt hole on top up to the lower engine area. I fixed my buddies RV4 FAB this way with no further issues. Works well, lasts a long time...

Rob Ray
RV4ormerly
F16 sadly formerly
HR2 currently
 
I'm convinced

Thanks for the input, I have changed my mind on using steel and will use a little thicker aluminum. I will also add a support as suggested by smokyray, and will inspect it every time the cowl comes off. The suggestion regarding getting the prop dynamically balanced seems very worthwhile as this is not the only part that could be helped by reduced vibration.
 
A very common situation on RVs. When mine cracked I made up a new mount using 6061T-6 which is MUCH less likely to crack.
 
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