US Army M2A Burner Unit

Discussion in 'Military' started by Ben Hall, Oct 22, 2021.

  1. Ben Hall

    Ben Hall United States Subscriber

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    Good afternoon all,

    I got out one of my US Army National Stock Number (NSN) 7310-01-113-1285 "Burner Unit, Gasoline, Model M2A" this morning for a photo-session for my October SoTM entry. This particular unit is an M2A made by Armstrong Products Corporation in 1977, making it just slightly newer than I am. ;) It is equipped with the safety device, so technically, it's an NSN 7310-01-113-9172 "Burner Unit, Model M2A, With Safety Device".

    IMG_5440.JPG

    IMG_5433.JPG

    It is a gasoline (petrol) fueled pressure stove rated somewhere around 55,000 BTU/hr, or just over 16,000 Watts. On top is a 2 pint Valor 55 stove so you can get an idea of the size of this thing!

    It is sitting on a wooden stand I built to get it off the ground so I don't have to bend over so far! ;)

    Some may remember my recently post on my US Army Modern Burner Unit (MBU) Version 3 (V3) stove here: US Army Modern Burner Unit Version 3 (MBU V3) If you've read that post, this is the burner that was replaced by the MBU, and it's designed to be used in the M59 field kitchen range.

    In doing some research on the unit, some folks in the Army called them "M2 Bomb Units" as apparently, they had a history of explosions and other accidents. I never did find out what caused the accidents, but I can make several guesses based on info I've found in the Army's Preventative Maintenance (PM) Magazine and other publications. Of course, what I'm going to share are just conjecture on my part.

    As can be seen by the name itself, the M2A is a modification of the M2 stove...and the M2A with "safety device" is a modification of the M2A stove itself. That safety device consists of a fuel tank over-pressure relief valve, set to about 60 psi or so, that would vent excess pressure from the fuel tank into the base of the burner. The M2A does have a pressure gauge that has green, yellow, and red areas, and the manual talks about the importance of never running the stove in the red. I suspect that the first cause of accidents was over-pressurization of the stove, leading to a fuel tank rupture and fire / explosion.

    Another likely cause of accidents is the design of the M59 field range. From what I understand, it can be configured in different ways for different cooking tasks, and the M2A (and MBU) can be installed in different "shelves" at different vertical heights inside the range. There are warning stickers on the range saying that only one M2A / MBU should be inserted at once, so I suspect another cause of accidents were folks installing a lit, running M2 stove underneath a non-running stove when not paying attention...with the lit stove then overheating the unlit stove until an explosion occurred.

    I suspect many of the accidents were due to misuse than faults with the stove. (although, as we'll see, the design is not faultless.)

    These are very interesting stoves. The long tubular unit across the top is the generator, and it is massive compared to anything else I've ever seen. Even the Coleman 457G Handy Gas Plant / Heatmaster rated at a similar 35,000 BTU has a tiny generator tube in comparison, maybe 10mm in diameter. The one on the M2A has to be somewhere around 50mm in diameter.

    The design is both excellent and terrible. As the other gallery post notes, they have a pre-heater burner for heating up that massive generator that itself needs pre-heating. However, the pre-heating burner is mounted at a 45 degree angle, making the spirit cup pretty much useless. Thankfully, burning gasoline, they don't need a lot of pre-heating, so a few seconds with a propane torch and they light right up.

    There is no fuel shut-off between the tank and the inlet side of the generator. So if a fault occurs, the only way to stop fuel flow from the tank is to depressurize the tank itself.

    The vapor metering orifice and fuel valve is built onto the generator assembly. This is both good and bad. It's good that when you install a new generator, you have a fresh valve and orifice. It's bad in that as the generator heats up, it thermally expands, getting longer, and if it gets hot enough, it could pull the orifice out of the manifold, shooting vapor places it was not supposed to go, likely causing a fire and perhaps an explosion. The manual doesn't explain this, but when I replaced the generator in this unit after disassembly for cleaning, I ensured that everything was adjusted so that the orifice was pushed as far as possible into the manifold.

    The design of the tank over-pressurization valve is interesting. I'd expect that in an over-pressure situation, venting fuel vapor from the tank into a running stove manifold would be very exciting / terrifying...likely causing a huge flare-up? I'm not sure, it almost might put the stove out as the raw fuel vapor would cause a *very* rich fuel/air mixture in the burner? I really don't want to find out. ;)

    Of course, on the good side, like many of the Classic Camp Stoves that we all love, it can be fettled with only the most basic of tools - screwdrivers, and adjustable wrench/spanner, etc... There are no electronics unlike the MBU, so I expect it will be operable long after the last MBU has died from bad capacitors or other problems. I also like how the stove has provisions to mount two spare generators...so if one does go bad, you've got two more.

    This stove required a LOT of fettling to get it working. I bought it CHEAP on the e-place, paying about twice in shipping over what I paid for the stove. It came with a spare fuel tank, for reasons that will soon be obvious. It needed a new burner tray pan (the stainless steel part surrounding the burner) which was also found cheaply on the e-place.

    The original fuel tank was a DISASTER. I think fuel had been left in the tank since it was built in 1977, so rather than try and clean out the tons of garbage inside, I just replaced it with the brand new spare tank. :)

    The original over-pressure relief valve had a copper tube going from the valve to the burner manifold which prevented fore/aft adjustment of the burner, and then proper mounting of the generator - there was a sizeable gap between the generator valve and orifice and the manifold. The tubing was cut so short that there was no way to get a good fit. I ended up replacing the copper line with a piece of very high-temperature rubber hose (Viton, maybe?) so I could now adjust the fit of the generator properly.

    The pre-heater was also a disaster, as someone had tried to start the pre-heater with that icky gasoline in the tank. It was removed, cleaned, and tested fine.

    The original feed pipe from the tank to the generator was also a mess since it had been dipped in the original icky gasoline for a very long time. Thankfully, while it had some corrosion, it cleaned up and was reusable.

    Starting one of these units takes a long time and some finesse. Of course, you've got to fuel it and give it some pressure with a tire pump. 6 to 8 psi per the instructions, and "use a hand pump only." ;)
    IMG_5441.JPG
    You can see I've got a propane torch, it makes lighting the pre-heating burner very easy.

    IMG_5442.JPG
    That should do. As you can see, being that there are no finer graduations on the gauge other than 10 psi, it is somewhat of a guess. ;)

    Those with good eyes may be able to tell that the gauge says "MADE IN SPAIN." Interesting. :)
    IMG_5431.JPG
    Up next, spin the lever on the pre-heating burner, pre-heat the pre-heating burner, and light it.

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    It was sunny this morning, so even in the shade, it's very hard to see the pre-heater burner flame, but maybe you can see a little orange at the top of the photo and some blue inside the burner bell?

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    The stove has a cover that gets installed over the pre-heating burner and the front half or so of the generator. This is a nice touch, without the cover it takes much longer to pre-heat. And even with the cover, pre-heating takes a LONG time given the size of that generator! The manual says to pre-heat until the far end of the generator is hot. Took me about 20 minutes this morning to get it hot.

    IMG_5461.JPG
    To light it, I remove the preheating cover, leave the pre-heater burner running, place a flame by the burner, and open the fuel valve. It usually takes a few seconds for vapor to get out of the massive burner. The light-up can be quite exciting. In the above photo, it's lit, but burning fairly poorly almost as if I didn't pre-heat it enough!

    Per the manual, it should ignite off the flame in the pre-heater, but that doesn't seem to work all that great for me. Of course, once the main burner has lit, turn off the pre-heater.

    Here is another bad feature of the stove. If you haven't heated it enough, you'll get liquid gasoline out of the orifice. It drools out the front of the stove, and can catch fire. Ask me how I know. ;)

    Once the generator is fully heated by the flame, you can adjust the fuel/air mixture with the rotating damper on the stove.

    IMG_5465.JPG IMG_5466.JPG
    Fuel/air mixture damper fully closed to fully open.

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    Once fully up to temperature and with the mixture set right, it burns with a very nice blue flame. It's hard to tell in this photo, but the damper is fully open and the mixture is full lean. I am running regular unleaded ethanol-free gasoline. It almost seems like it could use a little more air. I suspect that the gasoline of today is quite different than gasoline in 1977, so perhaps that's why?

    Some additional photos:
    IMG_5423.JPG IMG_5424.JPG IMG_5425.JPG IMG_5426.JPG IMG_5427.JPG IMG_5428.JPG IMG_5430.JPG

    In conclusion, it's a neat stove, but one that I don't use very much due to it's complex and lengthy starting procedure, plus it can be somewhat temperamental. While I can see how it gained the nickname "M2 Bomb Unit," I'm not afraid to use it, as I am careful and understand its problem areas. For sure, once it is running and up to temperature, with the valve full open it puts out A LOT of heat and has very close to no odor, which is always remarkable to me for stoves that are running regular unleaded gasoline.

    Thanks much,
    ben
     
  2. presscall

    presscall United Kingdom SotM Winner SotY Winner Subscriber

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    Thanks Ben for what must surely be the definitive write-up on the M2A. It’s certainly the case that a stove’s ‘quirks’ can prove endearing to a collector but what you describe takes such to excess perhaps, particularly:
    If I remember correctly, that’s a ‘feature’ that contributed to Coleman taking their No.501 stove off the market and recalling those sold, guaranteeing its place in the hearts of collectors, much like the M2A no doubt.

    John
     
  3. Ben Hall

    Ben Hall United States Subscriber

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    Hi John,

    I completely agree - quirks are endearing, but the M2A just has so many quirks. So many that some may find it utterly unappealing. For sure, it's not a favorite of mine, but I don't hate it either. :)

    Thanks much for the link to the 501! I knew there was a Coleman stove that had the same lack of a fuel tank shut-off leading to a recall and it going off the market...but I couldn't remember the number. The lack of a fuel-tank shut off is definitely something I keep in mind when I'm running the M2A.

    I did leave the M2A fueled up. I need to bottle a batch of beer tomorrow, and to do that I need a good supply of hot water for cleaning and sanitizing the bottles. Usually I just get it from the tap, but I figured I'd leave the M2A fueled up and would use it tomorrow morning for heating up the water I need. :)

    Thanks much,
    ben
     
  4. Majicwrench

    Majicwrench Subscriber

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    I bought one locally a couple years ago, mine fired right up, after the long pre-heat, and mine pops off with no yellow/orange flames as your picture shows @Ben Hall Wonder if your generator is full of crud, seeing that the tank was.
    They put out enough heat that they are tough to stand near to.
    BUilt a grate out of two 413 coleman grates so I could cook on it, but have yet to take it backpacking, my wife is a lazy one and refused to carry it.
     
  5. David Shouksmith

    David Shouksmith Subscriber

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    Hmmm, mine must be an M2 because it's got a small Coleman stove as a preheater.

    I (we, really - there was a team of us!) lit it once at Newark. What a palaver! It took at least 30 minutes but we got it going eventually and it burned for hours with a lovely flame pattern. I think filling the fuel tank doubled the value of the stove - a bit like an old Skoda before VW took over.

    The previous owner had welded two grills together which sits nicely on the top and supports the pans...
     
  6. Ben Hall

    Ben Hall United States Subscriber

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    Greetings @Majicwrench - If I remember correctly, when I replaced the tank I thoroughly flushed out the generator...and it wasn't that bad. The pickup tube leading from the tank to the inlet side of the generator was plugged solid, so I think that actually helped save the generator. :) I'm about 99.9% sure that I didn't preheat it long enough when I did that start above.

    About a year ago, I did find a source of new old stock generators on the e-place. In the photos above, you can probably tell that one of the new generators is installed on the stove in one of the spare generator brackets. The other generator installed in the spare generator brackets on the other side

    I used the stove a *lot* today (probably about three hours of run-time in total) for heating up water for cleaning bottles and other beer making equipment. The first start this morning was as you described - a very long pre-heat and it lit immediately. :) I used an IR temperature gun and for that first start, the generator end furthest from the pre-heating burner measured 150 deg F / 66 deg C before I started the main burner. Same for the second start - slightly more than 150 deg F / 66 deg C and it lit right up.
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    My setup. That's a 10 gallon / 40 liter or so pot on my homemade stands. The rusty black thing in the background is my trailer for my lawn tractor turned up on it's end, and propped up to make an ad-hoc windscreen.

    I snagged some photos after it was running nicely today:
    IMG_5469.JPG IMG_5470.JPG IMG_5471.JPG
    Some comments on the photos:

    * As the stove got fully up to temperature, the two "arms" of the burner on the right side were both even - the front "arm" looked like the back "arm". :)
    * When I got the stove, the cast iron burner was quite rusty on the left side. I did quite a bit of cleaning to remove the rust, but appears that additional cleaner is going to be required as some of the burner slits are partially blocked. If I had a tool kit for the stove, there is a "cleaning tool" for doing this. I'll have to make one.
    * The manual says to "Adjust air shutter until flame is green in color." I was able to do that today - with the air shutter about half-way open, I got a nice green color flame. :)

    After using the stove for about an hour, I took a picture showing how the thermal expansion in length of the generator pulls the valve/orifice out of the air shutter assembly:
    IMG_5473.JPG
    When the stove is cold, that hex nut is fully seated against the air shutter plate. It's probably pulled out 3mm or so? Thankfully, there is another 6 or 7mm inside the air shutter assembly, so it's in no danger of coming out, but you can see how if one was not careful when replacing the generator, and didn't snug up that hex nut against the air shutter plate, it could come free once the generator got very hot. That hex nut is part of the orifice, and is NOT adjustable. On the previous M2A I fettled, one of the generators had a weld that was porous, so I condemned it and cut it apart to see what was inside and how it was made. I tried removing that hex nut, but even with a 1/2" air impact gun, it stayed put.

    I should describe the innards of one of these generators, as they are quite interesting...very unlike a standard Coleman generator with just a wire spring. Probably not many people have had one that they wanted to cut apart. The generator is a horizontal tube, with coarse screens about 30mm from each end. When I say coarse, the openings are probably about 4mm in diameter, and there's probably 20 of these holes comprising each screen. Inside the length of the generator tube, between the screens at both ends, is a black-in-color substance. I'm not sure what it is, but it is porous, somewhat compressible with gloved fingers, obviously can take high heat, and fills the entire diameter of the tube. It's not metallic, and it's not stringy like asbestos or rock wool. I should note that I treated it as if it were something nasty like asbestos, and disposed of whatever it was safely. The coarse screens keep the black material centered in the tube.

    I should have taken some photos, but not knowing what it was and thinking it possibly could be something nasty like asbestos...I didn't pause until I had it safely removed and disposed of!

    Once it is fully cooled off I plan to de-fuel it and put it back into storage.

    Thanks much,
    ben
     
  7. Majicwrench

    Majicwrench Subscriber

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    If I recall (big if) correctly, the manual has you use the edge of the preheater cover to clean the slots in the burners.
    Thanks for the fun post. I have a really big, square griddle, I plan on doing LOTS of pancakes at one time on mine :)
     
  8. Ben Hall

    Ben Hall United States Subscriber

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    Hi @Majicwrench - interesting you mention a large griddle! I was just talking with my wife several weeks ago about getting a large cast iron griddle to go on the MBU, which would also fit the M2A. I'll have to look in the manual, but for sure, the preheater cover edge would be very close if not the exact correct size for cleaning the burner slots!

    Thanks much,
    ben
     
  9. Lance

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    Ben, try using the saw blades of an Xacto knife set to clean the slots of your stove. It is what I use to clean the slots of the burner of my Handy Gas Plant.

    My brother who was a cook in the MC said this was Uncle Sam's answer to bad cooks, as you only sent out to light the stove those cooks who had the intelligence of a pizzle ant and even then you might be insulting pizzle ants. I often wondered how he knew how to light it.

    Lance
     
    Last edited: Oct 24, 2021
  10. Ben Hall

    Ben Hall United States Subscriber

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    Good afternoon all,

    Spent some time in the garage this morning looking at / thinking about burner cleaning among other tasks. Turns out that the preheating cover steel is too thick to be used as a cleaning tool...and the manual mentions a dedicated slot cleaning tool. The Xacto suggestion from @Lance was a good one - I don't have any Xacto saws, but I do have some thin Zona saws that turned out to be too thin to be good cleaning tools. I happened to look down, saw a piece of brass sheet scrap...and it turned out to be the perfect width - just thin enough to get in the burner slots - but thick enough to give them a good cleaning...plus I know what sort of saw blade thickness to look for. :) So off to do some shopping. :)

    I also was working on wiring for a SouthWind 978M surplus military heater because Winter is coming...and managed to slip with a pair of small scissors cutting back cable insulation giving myself a rather nasty/deep puncture wound to my left index finger. I'm on anti-coagulants from three unexplained blood clots several years ago...and for about five minutes...was thinking I was going to be going to the local hospital emergency department for stitches. Thankfully, it clotted fine and actually is not giving me any problems with just a normal adhesive bandage. :) No loss of feeling, so don't believe there is any nerve damage. The hematologist is still trying to figure out my blood clots - I have no risk factors other than a previously-unknown single copy of the Factor 5 Leiden clotting mutation. My tentanus shot is up-to-date, so no worries there. :)

    After that little incident, I decided to take it easy for the rest of the day so no more playing with stoves / heaters / etc... :(

    Thanks much,
    ben
     
  11. Lance

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    Ben I really like the fact your friend shown in your picture is sitting on your shoulder. How do you get her/him to stay there long enough for a photo? My cat would be off my shoulder and headed for the nearest food dish as soon as I set her up on my shoulder.
     
  12. Ben Hall

    Ben Hall United States Subscriber

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    Greetings @Lance - his name is Major, short for Major Monogram, a cartoon character from the Phineas and Ferb cartoon show, named by our son when both Major the cat and Robby were younger. ;) It's totally voluntary - he'll come back to my office/ham shack/computer room/stove workshop, put his paws on my leg, and meow until I pick him up. He *loves* to sit on my shoulder like that, and will sit there purring until something like food hitting the bowl or someone turning on the water grabs his attention. ;) Sometimes, he prefers my lap and shoulder to even food and the water. I've never had a cat before him do anything like this. :) Usually, like you say, you put a cat somewhere and they leave immediately. :)

    Thanks much,
    ben
     
  13. Scrambler

    Scrambler Australia Subscriber

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  14. ArchMc

    ArchMc SotM Winner Subscriber

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    I’ve got a cat who likes to sit on my shoulder, too. Lately, he’s taken to sitting on the back of the recliner when I sit in it, often with a paw on my shoulder or neck. Matter of fact, that’s what he’s doing as I write this.

    ….Arch
     
  15. Scrambler

    Scrambler Australia Subscriber

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    My cat pats me more than I pat him, he has 2 methods - a) stand on the back of the lounge chair and lick my head and b) stand on me and kneed whatever he's standing on like it's dough. I'm not allowed to pat him in these situations - results in a bad tempered growl. Then the tables turn and he's sitting on my lap for the payback.

    My daughter's cat almost tolerates people. Almost.

    I've seen photos of cats sitting on people while they do rock-climbing etc. My cat isn't that tolerant - but it sounds like some of yours might be.

    Wow - this thread has gone further off track then most. Hope that's OK!