Airbox issues

The other day, I had a customer tell me his air box loosened up and actually fell off his RV-7 engine. He was asking advice as to how to proceed with inspecting the engine after such an incident. After that discussion, I asked if he forget to safety the four bolts that hold the air box on. He said no, he safetied them per the builders instructions and bent the lock tabs to keep the bolts from turning. I asked him how they could possibly have loosened up come out if the lock tabs were bent. After looking at it, he realized that although he bent the tab up to lock the bolt there was nothing preventing the tab from turning, so in reality the bolts were not safetied and they eventually loosened up and fell out. After checking around, this seems to happen a fair amount of time. So I post this as a heads up about a possible problem. I think it might be a good idea to check this and replace those bolts and lock tabs with 4 drilled head bolts and actually safety wire them. Not being a builder, I haven't actually ever assembled this setup but I think I would go with the drilled bolts after having it all described to me.
My customer was extremely lucky and escaped this calamity with out any engine damage but ingesting even one of those bolts has the potential to cause some real problems.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts
are hopefully helpful to you. Please use the information provided responsibly and at you own risk."

Alternate method

Now take what I'm about to say with a grain of salt...I have horizontal induction and my airbox is nothing like the ones you vertical guys have. But it seems to me there's a better way to do this --

Why not rivet nutplates to the inside of the airbox, and then just install the bolts from the outside (top). Or go crazy and use drilled-head bolts -- you could then safety the bolt heads to each other, but the safety wire would be on the outside. The bolts won't loosen, the nutplates are captive (no risk of ingesting anything), and installation/removal is even easier.

I'm sure Van's would have thunk of this if it made sense, so I expect somebody to tell me why my idea stinks...

)_( Dan
RV-7 N714D
http://www.rvproject.com

different bolts

Dan and others: The top plate of the airbox is the problem. If it comes loose then the whole thing would fall off or even if only one bolt came loose it could be sucked into the carb. When I built my 9A the instructions had changed or someone posted a problem like the one described by Mahlon and suggested a fix. Can't remember which. The bolts mate the top plate to bottom of the carb and are hidden inside the airbox when it is mated to the top plate. So they go up through the top plate and into the 4 holes provided on the bottom of the carb. I used the drilled head and safety wire method. They are not regular AN bolts, threads are different. The airbox is then attached to the top plate with nutplates as suggested by Dan. Jack

Dan,
I'm not sure you understand the problem. The bolts that came loose screw into blind holes in the carb airhorn. I think the bolts you are talking about do go in from the top and into nutplates.
Mel...DAR

Roger that

Mel said:

Dan,
I'm not sure you understand the problem. The bolts that came loose screw into blind holes in the carb airhorn. I think the bolts you are talking about do go in from the top and into nutplates.
Mel...DAR

Click to expand...

Ah...roger that. I didn't realize the carb had blind holes like that, as opposed to a mounting flange (I'm an AFP fuel injection weenie with very little carb exposure).

)_( Dan
RV-7 N714D
http://www.rvproject.com

I come from the motorcyle racing world where everything will fall off unless you take actions to keep it on. If I want to keep something from falling off I safety wire it or at least put a dab of RTV on it. I have also found that Shoe Goo works very well at keeping things from rotating. It's real sticky and dries fast. Those little folded over washers have failed on a race bike of mine before, I won't trust them anymore. Basically if you want it to stay on, take a good look at it and figure out what would make it fall off. I recently got burned by some powder coating that I tightened a bolt against. The powder coated wore away and the bolt came lose. I should have caught that one before it happened. Won't happen to me again.

the ones mine came with were pan head slotted bolts. I went and bought some hexhead with flats and a spring washer. I hope some loctite on them will the job.

maybe

Mark: Looks like your set up will work. I'm not an A&P and far from an expert. However, don't forget, you probably will not see those bolts for awhile. They are the only 4 bolts that I can think of in the engine compartment that you can't see by just remove your cowling. Your engine vibrates constantly and more so on shut down. Drilled heads and safety wire 2 X 2 may give you an added measure of confidence. They are cheap and it's pretty easy to do with just a little practice.

AN500 Drilled Fillister head screws

I used these screws to hold the top airbox plate onto the bottom of my carb. They have drilled heads so they can be safety wired. 1/4-20 is the thread you need. Can be found on page 93 of the new Spruce catalog. If I'm not mistaken, this is only a problem on the 0-360 carb, the airbox for the -320 mounts different.
Good luck everyone!
Bill Waters

Use Lock Wire

Throw the stupid bent tabs away! They don't work. They were supplied by Van's, I believe, because a drilled head 1/4-20 doesn't exist.

The threads are indeed a 1/4-20. The bolts (grade 8, good enough) can be bought at Home Depot/Lowes/etc, and with a drill press it's fairly easy to poke in a hole for lock wire. If you can get them pre-drilled through Spruce or its ilk, it will save a bit of time, but read the description carefully as I found naught but AN 1/4-28s a year ago when I looked.

John Siebold

The drilled head Pilaster head screws that Bill mentioned will work.
Drilled head 1/4-20 course bolts are available.
P/N is
MS20074-04-04 is .594 long
ms20074-04-03 is .469 long
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts
are hopefully helpful to you. Please use the information provided
responsibly and at you own risk."