M1942 and M1950 NRV

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M1942 and M1950 stoves

Frustration . . . that’s the word which describes the M1942 and M1950 NRV (Non-Return-Valve), a.k.a. check valve (The Department of the Army Technical manual, TM10-708 calls it the air check valve).  The problem is that the original rubber gaskets get old, dry out, and then fail to seal, allowing fuel to enter the pump tube.  Some people say that this is one of the two primary causes for stove fires on these two stoves.  The other is the pump-tube-to-tank gasket (TM10-708 calls it the filler cap gasket).

The check valve on these two stoves is different than most gas pressure appliances (GPAs).  Typical stoves and lanterns use a ball-type check valve with a stem.  The stem is a positive stop to any fuel getting into the pump tube.  The check valve on these stoves is a spring and cup which is submersed in fuel installed on a removable pump.

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M1942 NRV (top), M1950 NRV (bottom)

I purchased replacement gaskets, which for some reason are commonly referred to as pips, from several places.  The material used is generally not stated.  What I received were gaskets which were oddly formed when compared to the originals and some turned hard after a short time.  The rubber material needs to be not too hard and not too soft, as you’ll see later.

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Malformed NRV pips, except the two on the right which are original

So, I set out to determine which material works well and which ones have problems. I read some forums where people gave suggestions on materials and how to form them, and experimented using the M1942 style NRV.  This is because I could only find the M1950 style gasket for sale, which is about 4.5mm in diameter and doesn’t fit in the M1942 NRV, because the M1942-style is 4mm.  Most people suggested 1/8” thick material, though for the M1942 NRV, I found that 3/32” material more closely matches the original size and so far, worked better in testing.

For an explanation of Durometer Hardness and other rubber properties and comparisons, check out the write-up at McMasterCarr and click on the About Rubber and Polyurethane section.

What follows is a list of what I bought and tried, with some details of how the material worked.  But first, I noticed that the gaskets I purchased were concave on the sides, whereas the originals were square.  This is because everyone is pretty much using a leather punch which, if not really, really sharp, compresses the material when cutting.  I know this because I did the same thing.  Some people said they sharpened leather punches of the grip-type but I already had a set of the type you use with a hammer, so I inserted the tool into my drill press, sharpened it, and was able to cleanly cut the gaskets with nice square sides.

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The 4mm leather punch sharpened and ready to cut a gasket with the drill press

Viton Foam, from McMasterCarr.  Shore hardness: not specified (see that same McMasterCarr Page, except under the Foam section), but it’s really light and really soft. Thickness: 1/8”.  One user said they liked this, and even though it is really expensive, I bought a 6” x 6” square. After cutting them, and installing them, I pumped the stoves and everything sealed great. Then, I set the stoves aside and came back two days later. I removed the pump and discovered a ¼” to ½ of fuel in the tube. Upon inspection of the gasket I found it to be completely compressed and no longer sealing. This was a disappointment and a failure. Fuel resistance: currently testing by soaking in fuel.

Viton Solid. Shore Hardness: 75A (durometer). Thickness 1/8”. One person cautioned against using it, stating it was too hard to seal properly, but then I saw it advertised by people who make and sell stove gaskets that this is the best, mostly because of its claim to superior oil and fuel resistance.  I cut it and installed it. Immediately, the stove leaked fuel into the pump tube.  What I found, was that if I let it sit for a while, installed the pump, it seemed to leak less fuel as time went on, as if it needed to seat.  Fuel resistance: currently testing by soaking in fuel. I would consider this material marginal and agree with the one forum poster that it is too hard at 75A. I will likely try more testing later.

Buna-N, Medium Strength. Shore Hardness: 60A? Thickness: 1/16”. I bought this to make custom fuel cap gaskets, but included it here only to make the comment that some people say that the fuel resistance isn’t good enough and the gasket material turns hard pretty quickly after long immersions in fuel.

Ultra-Strength Buna-N (nitrile, acrylonitrile butadiene, or NBR). Shore Hardness: 50A (durometer), Thickness: 1/8”.  The claim is that it “offers outstanding resistance to vegetable- and petroleum-based oils, solvents, and lubricants”.  I cut and installed these, though I trimmed it down to the cup size with a razor-blade.  The NRV sealed up immediately, and even under pressure.  After an hour or so I checked the pump tube for fuel and it was dry.  Currently testing for seal over a few days with pressure.  Fuel resistance: currently testing by soaking in fuel.

Ultra-Strength Buna-N (nitrile, acrylonitrile butadiene, or NBR). Shore Hardness: 60A (durometer), Thickness: 1/8”.  The claim is that it “offers outstanding resistance to vegetable- and petroleum-based oils, solvents, and lubricants”.  I cut and installed these, though I trimmed it down to the cup size with a razor-blade.  The NRV sealed up immediately, and even under pressure.  No further testing has been done at this point since 60A is the same shore hardness as ECH (see below). Fuel resistance: currently testing by soaking in fuel

Ultra-Strength Buna-N (nitrile, acrylonitrile butadiene, or NBR).  Shore Hardness: 70A (durometer), Thickness: 1/8”.  The claim is that it “offers outstanding resistance to vegetable- and petroleum-based oils, solvents, and lubricants”.  I cut and installed these, though I trimmed it down to the cup size with a razor-blade.  The NRV sealed up immediately, and even under pressure.  After an hour or so I checked the pump tube for fuel and it was dry.  Currently testing and is at 6 days with 30 pumps of pressure and the pump tube are perfectly dry.  Fuel resistance: currently testing by soaking in fuel.

ECH (epichlorohydrin).  Shore Hardness: 60A (durometer), Thickness: 3/32”.  The claim is that this “rubber provides even better resistance to fuel, oil, and ozone than Buna-N”.  This material looked to be a good compromise between the highly rated, but very hard Viton Solid and the Buna-N materials where fuel resistance is concerned.  If you explore the link mentioned earlier, you’ll see that ECH falls into the same Excellent category for petroleum-based liquid as Viton.  I bought the cheapest size I could which was a 2” x 36” strip.  I cut and installed these, and they fit perfectly in the NRV cup.  The NRV sealed up immediately, and even under pressure.  After an hour or so I checked the pump tube for fuel and it was dry.  I checked it again 24 hours later and the stove had plenty of pressure and there was no fuel in the pump tube.  Currently, testing is at six days with 30 pumps pressure and the pump tube is perfectly dry.  Fuel resistance: currently testing by soaking in fuel.

Thickness: As mentioned earlier, most people use or make the M1942 NRV pip out of 1/8” thick material.  While experimenting, I noticed that it was more difficult to pump the stove, even to the point that I couldn’t create good pressure in the stove when using 1/8” thick material.  The 3/32” thick material worked the first time without fiddling with the pump.  My speculation on this is that the thicker material causes the spring to be more compressed, even when not pumping.  When you try to pump the spring has almost no compression left and very little air enters the tank.

I need a proper sized cutting tool for the larger M1950 NRV pip.  Once I have that I’ll be able to see how the different materials work with that slightly different configuration.

I plan to update this post as I learn more about materials and use them over time.  Since I consider myself a user more than a collector, I really want the NRV to seal correctly first time and really, every time, or at least until the material needs to be replaced.

Thanks for visiting my blog.

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6 comments on “M1942 and M1950 NRV

  1. Thomas carr says:

    Steve – your blog has been quite helpful as i recently picked up a couple of M1942 Aladdin stoves. Ones is basically NIB and the other needs some TLC. I haven’t been able to track down a good source of parts – i’m in need of either the NRV itself or a whole pump assembly. Do you have any suggestions or have parts you would be willing to part with?
    Thanks, Tom

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    • sklcolorado says:

      Tom, do you happen to be on the ColemanCollectorsForum? If so, you can PM me there under coldwaterpaddler. Either way, what I have are some spare M1950 NRV (cup w/spring). One of my M1942 pumps came with one of these and I believe they work fine, since basically the pumps are the same, except for length and the screw-on cap. Are you the Thomas Carr on Google Plus?

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  2. I just got one of these off of craigslist and it had a bad NRV… and I took the NRV off the new one (NOS) that i had purchased to help diagnose the very much used one, and upon reassembly, it quit working as well… are you supposed to replace these and just leave them sealed up ? I only had it apart a few minutes and now the pump backs out on it. on the older used one it does not do anything but I can hear air leaking and their in fuel within the pump but not the outer tube.. otherwise I would suspect the gasket. I’m a bit new to this type of stove so I am not certain what it might be… might have overcompressed the NRV for all i know during disassembly.

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    • sklcolorado says:

      Hi Michael, I was out of town on business and just returned. Whether it’s an M1942 or M1950, if the check valve seal (the small rubber piece, sometimes called a pip) is more than 10 years old, even NOS, it is likely dried out past the point of sealing well. Is this what you used, the spare one inside the pump tube or one with the pump cup you purchased somewhere? If so, I’ve tried the old ones and they failed to seal well for me, too, even though they appeared new. Get a few new ones from Old Coleman Parts and they should seal up properly. It is difficult to over-compress the air-check gasket because of the spring, unless the gasket is too tall or it isn’t seated properly in the cup. If the pump backs out it isn’t sealing, for sure. If you can’t develop pressure, it can be the air-check gasket or it could also be the larger pump tube gasket . . . or, if it’s a M1950 stove it could be the internal gasket. Given the pump backing out it seems like it would be the air-check gasket. If you want, you can send me an email with a photo(s) of what you’re working with and I’ll be glad to help you through it.

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  3. […] for the M1942 type, you can substitute an M1950 spring/cup/gasket successfully. See my blog post on M1942 and M1950 NRV gaskets for more […]

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  4. […] on the stove is the check valve, also called air-check or sometimes NRV (non-return valve). This check valve design, consisting of a brass spring, brass cup and rubber gasket, was used in one of two forms from 1943 […]

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