172 carb heat box

ScottK

ScottK

Pre-takeoff checklist
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ScottK
Our club is flying a 1968 172I with the O-320 in it. We had it in the shop to fix an alternator and during the work, they found that the carb heat box was bad. The description of the problem I got was this: Air box for carb heat and air intake was broken at carb heat inlet. Also, there was evidence of looseness and wear where box hooks to carb.

We are being told that this will require some sort of rebuild and that only one place in the country does it. Now I've found carb heat boxes on Aircraft Spruce and some other sites for the O-320. What would need to be rebuilt? Is there some sort of mounting system that could be worn or broken?

I haven't seen the problem in person, so I'm only going off of what I've been told to this point. And I realize it's difficult to diagnose a problem from a two sentence description, but any insight would be greatly appreciated.

Thanks.
 
Mine was badly cracked when I bought my plane (182). I had it welded, put the stiffening kit on it, and installed new bearings.

I was told it couldn't be welded. That you can't weld that type of aluminum. It'll never work. It'll soon crack again.

Ten years later it's fine.

The guy who welded it for me practices on beer cans...seriously.
 
yep, this is the perfect candidate for an owner-produced part

I don' think I've owned a plane that I haven't made a new airbox for
 
ScottK said:
Our club is flying a 1968 172I with the O-320 in it. We had it in the shop to fix an alternator and during the work, they found that the carb heat box was bad. The description of the problem I got was this: Air box for carb heat and air intake was broken at carb heat inlet. Also, there was evidence of looseness and wear where box hooks to carb.

We are being told that this will require some sort of rebuild and that only one place in the country does it. Now I've found carb heat boxes on Aircraft Spruce and some other sites for the O-320. What would need to be rebuilt? Is there some sort of mounting system that could be worn or broken?

I haven't seen the problem in person, so I'm only going off of what I've been told to this point. And I realize it's difficult to diagnose a problem from a two sentence description, but any insight would be greatly appreciated.

Thanks.
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Those heat boxes fail exactly as described. And Aircraft Spruce's boxes will either be non-PMA approved, introducing legal issues, or they won't be the right fit, or something. That box from the OEM is really steep (don't ask) but you can buy a new, PMA'd, legal box from Acorn Welding in Edmonton, Alberta.

That old box might be repairable, but sometimes the time spent comes to more than a new box, and you still have a box with oversized rivet holes and other worn-out junk.

Dan
 
The heat box is simple sheet metal, rivets, and metal rods, etc. Perfect for owner 'maintenance'. You learn some new skills - and if you keep track of your actual working time get a new appreciation for your mechanic's bill.
 
ScottK: I HAVE a complete muffler assembly which came off my 1971 C-172L Skyhawk(see avatar) in 2005. It was when I had the Powerflow Tuned Exhaust installed. The muffler and the attached carb heat box HAD PASSED ANNUAL INSPECTION, but carb heat box is not used when the PF-TE is installed(don't ask me mechanical questions as to reason). The above muffler/carb heat box is in my hangar, in the box in which the PowerFlow unit arrived. More questions? Ask.

HR
 
ScottK: FORGET my post. I just checked with Twitchell's Airport. Dale Twitchell told me the following: What I have "is perfectly good, but it's not what he needs. The air box is located beneath the carb heat box and that's not part of what we removed when we installed the PowerFlow." He also told me I should put what I have on Barnstormers because it's perfectly good for someone who needs a replacement. Sorry, Scott.

HR
 
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Dr. O said:
The heat box is simple sheet metal, rivets, and metal rods, etc. Perfect for owner 'maintenance'. You learn some new skills - and if you keep track of your actual working time get a new appreciation for your mechanic's bill.
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Ya know ... with our aging fleet and very expensive OEM parts to keep them running, someone, somewhere is going to make a pile of money by producing fabrication DRAWINGS that the owner can buy, sign their name at the bottom, hand to a sheet metal fabricator, and viola, an owner produced part that is legal to install.

Somebody is going to make a pile of money just drawing this stuff up and creating a library of parts that break on a regular basis, like a carb heat box.

Just planting the seed for thought, mindya ...

Jim

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Dr. O said:
The heat box is simple sheet metal, rivets, and metal rods, etc. Perfect for owner 'maintenance'. You learn some new skills - and if you keep track of your actual working time get a new appreciation for your mechanic's bill.
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The 172's heat box has several die-formed pieces, including the mounting flange, the side plates, and the filter flange. Making them from flat stock isn't easy for anyone, and that mounting flange leaves very little room for working. One would need to use weldable aluminum and weld it up to get anything remotely useable. The shaft has needle bearings in the original; the aftermarket PMA'd bearings are elastomeric units that don't rattle and last longer. The flapper valve must be attached to the shaft with monel rivets. No screws or nuts or blind rivets are used anywhere they might come loose and get sucked into the engine.

In short, it's much more hassle than it appears.


Dan
 
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Dan Thomas said:
The 172's heat box has several die-formed pieces, including the mounting flange, the side plates, and the filter flange. Making them from flat stock isn't easy for anyone, and that mounting flange leaves very little room for working. One would need to use weldable aluminum and weld it up to get anything remotely useable. The shaft has needle bearings in the original; the aftermarket PMA'd bearings are elastomeric units that don't rattle and last longer. The flapper valve must be attached to the shaft with monel rivets. No screws or nuts or blind rivets are used anywhere they might come loose and get sucked into the engine.

In short, it's much more hassle than it appears.


Dan
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Not for a decent machinist. The mounting flange can be made from 2024 and bent quite easily. The rest of it can be 5052H32 and quite weldable. Bearings are bearings. The flapper valve does NOT have to be done with monel; there are a dozen different ways to do it without letting little tiny pieces getting sucked in.

Again I say, somebody can figure out a good way to build this box and make a hell of a lot of money selling the plans.

Jim
 
I stopped by the shop yesterday to see what was involved. The box itself doesn't look that complicated, but the flanges make it a bit more interesting. Apparently, not a lot of people/companies make these. We found a shop in Canada that said they could do it in a week. I'm hoping that's the case. A couple of the other places they called were talking about months.
 
There is a subset of people who like to have/learn skills. Here on the farm we cut, weld, braze, bend, machine, and sometimes swear at, metals and other stuff every day. We make parts from scratch, or rebuild parts, that Mother Deere holds too dear for my pocket book. (not cheap, just thrifffty, laddie) Ans sometimes not available at any price.

And Jim's idea is a good one for a person with energy (not me) and time (not me) and a desire to have a business - the wife says if I start another company it will be arsenic in my tea. I don't think I would like arsenic in my tea.
 
Dr. O said:
There is a subset of people who like to have/learn skills. Here on the farm we cut, weld, braze, bend, machine, and sometimes swear at, metals and other stuff every day. We make parts from scratch, or rebuild parts, that Mother Deere holds too dear for my pocket book. (not cheap, just thrifffty, laddie) Ans sometimes not available at any price.

And Jim's idea is a good one for a person with energy (not me) and time (not me) and a desire to have a business - the wife says if I start another company it will be arsenic in my tea. I don't think I would like arsenic in my tea.
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I to love to build and learn new things. But on an airplane I will not be learning these new skills.
I have seen some mighty strange repairs from farmers. Not sure I want one working on my airplane. Bailing wire and ducted tape are used for all sorts of repairs on the farm in my area. You do have one in a hundred who have a nice lathe and some skills. But most believe a box full of tools makes them a certified mechanic.

Rivets in a heat box. Not a real good idea. My engine just about in jested a couple rivets from a heat box going bad. My machinist was so happy I pulled that unit from service after he laid eye's on it.

Today my carb heat box has not one rivet.

Tony

P.S. I am speaking of an EAB....
 
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1600vw said:
I to love to build and learn new things. But on an airplane I will not be learning these new skills.
I have seen some mighty strange repairs from farmers. Not sure I want one working on my airplane. Bailing wire and ducted tape are used for all sorts of repairs on the farm in my area. You do have one in a hundred who have a nice lathe and some skills. But most believe a box full of tools makes them a certified mechanic.

Rivets in a heat box. Not a real good idea. My engine just about in jested a couple rivets from a heat box going bad. My machinist was so happy I pulled that unit from service after he laid eye's on it.

Today my carb heat box has not one rivet.

Tony

P.S. I am speaking of an EAB....
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many piper carb heat boxes are riveted together
 
JHW said:
many piper carb heat boxes are riveted together
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And most Cessna boxes. But the use of blind rivets (pop or Cherry or whatever) as opposed to bucked rivets is frowned upon because they can rattle loose and fall out, or drop their internal mandrels into the box.

Dan
 
JHW said:
many piper carb heat boxes are riveted together
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If it was just riveted together I could deal with that. It was the flap on the inside that was riveted to the shaft. Those rivets was just about ready to break. They had just a couple more cycles left before breaking. Maybe one or two more times using carb heat and the engine would have ate some rivets.

No more rivets on the inside of this box or anywhere on this box. Its all welded together now.

Tony
 
1600vw said:
If it was just riveted together I could deal with that. It was the flap on the inside that was riveted to the shaft. Those rivets was just about ready to break. They had just a couple more cycles left before breaking. Maybe one or two more times using carb heat and the engine would have ate some rivets.

No more rivets on the inside of this box or anywhere on this box. Its all welded together now.

Tony
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I'd be interested in the basis for FAA approval of that repair, i.e., where do they draw the line? Can loose rivets on the wing be welded too? Welds can crack, split open and be the source of corrosion. If welding can be approved, can epoxy? Why not just glue it together with 'JB Weld', like I used to fill in the rusty windshield channel of my pickup? :) That's great stuff.

dtuuri
 
dtuuri said:
I'd be interested in the basis for FAA approval of that repair, i.e., where do they draw the line? Can loose rivets on the wing be welded too? Welds can crack, split open and be the source of corrosion. If welding can be approved, can epoxy? Why not just glue it together with 'JB Weld', like I used to fill in the rusty windshield channel of my pickup? :) That's great stuff.

dtuuri
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Nice thing about experimental..Don't need no stinking FAA approval. If you seen the quality of this heat box you would not be saying a word. It was built by an x-military aviation A&P who's family has owned this Machine shop since the 1800's.

I have gone into this shop and found large landing gear cylinders that came from Capitol Airport setting on this mans floor.

Tony

P.S. This is an aluminum carb heat box.
 
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