Farm Collector

Fusible Plug Safety Tips

As much as I enjoy auctions and good flea markets for antique steam traction engine parts, I no longer consider them acceptable venues for buying fusible boiler plugs, especially in the absence of sufficient information to assure me that what I am buying is right for the boiler I’m going to use it on. The following are some steam engine safety tips, specifically boiler safety tips, pertaining to fusible plugs.

If you purchase a fusible plug at an auction, a flea market, old-stock in a supply house or from anyone lacking knowledge of these critical devices, I suggest you know the answers to the following questions:

Is it a fireside plug or a waterside plug? Do you know the difference? On some plugs the only way to tell is by noting the location of the small end of the tin. The small end is always on the fireside.

Does the plug have an American Society of Mechanical Engineers (ASME) stamp on it?

Is the ASME stamp on the fusible metal or on the brass? If the stamp is on the brass, try to figure out why it is not on the fusible metal.

Is there anything about the plug that might indicate the melting point may be something other than 450 degrees F? The 1946 and 1992 codes state that plugs may be filled with metals other than tin that melt at temperatures exceeding 450 degrees. However, such plugs shall not be marked as ‘ASME Standard.’

Is the head on the plug square or hexagonal? I have seen very few references to square-headed fusible plugs dated after about 1930. The 1924 ASME codes specified a hexagonal head, but the 1946 code did not mention the shape of the head. The two plugs that could not be melted at 1,000 degrees F (see Figures 2 anf 3 in the Image Gallery) both had square heads. When I see a square-headed fusible plug I assume it is very old. I would not consider using one in one of my boilers.

Is the plug long enough that it will project through the crown sheet at least 3/4-inch? Very early fireside plugs, short pattern fireside plugs and any waterside plugs I have seen, would not meet this requirement.

Is your plug rated for more than 50 psi? Presently, Conbraco Industries offers an ASME fusible plug rated at 250 psi and another plug, without the ASME stamp, rated for only 50 psi. On their Web site (, they use the same illustration for both plugs, available in 1/2-inch and 3/4-inch sizes. I have not seen their 50 psi plug, so I don’t know if it is stamped to indicate the lower pressure rating.

Is there any indication the plug has been previously installed in a boiler? Even if I knew what boiler it had been in and how long it had been in use, I would not want to reinstall it in one of my boilers.

Is there any indication the plug has been repoured? If so, I don’t know of any practical way to find out whether it is lead, babbitt, solder, tin or something else. When I did my research on the melting temperatures of old fusible plugs last year (Iron-Men Album, March/April 2002), it cost me approximately $200 apiece to find out that two plugs in my possession had been repoured with solder. Since at least 1943 ASME code has specified that “Casings which have been used shall not be refilled.”

Is the fireside surface of the fusible metal flush with the surface of the brass? ASME changed from this configuration sometime between 1935 and 1943. If you find a plug like this in a boiler, I suggest you consider it to be fully oxidized and incapable of providing any protection. As soon as you remove it from the boiler, take a torch and melt the tin out of it. If it melts out you can be assured no one will reinstall it. If it doesn’t melt out you will have demonstrated the danger of using an old fusible plug. It is a good idea to melt out any fusible plug that is removed from a boiler to assure it is not reused.

Sources for Fusible Plugs

Presented below is a list of companies we presently know where steam engine owners can purchase fusible plugs. This list is by no means complete, and we encourage readers to forward additional sources to our attention.

– A. Turning Arms 250 Road 12 Sedan, KS 67361 (620) 346-2235

– P.O. Box 1070 Standish, MI 48658 (989) 846-9591

– Conbraco Industries 701 Matthews Minthill Road Matthews, NC 28105 (704) 841-6000

– Kramer Power Equipment Co. 2338 State Route 726 Eaton, OH 45320 (937) 456-2232

– D&H Spring, Machine and Welding 3919 Montana Ave. Billings, MT 59101 (800)382-3917

– E.H. Lynn Industries (800) 633-2948

– Ernst Flow Industries P.O. Box 925 Farmingdale, NJ 07727-0925 (732) 938-5641 (800) 992-2843

– Oliver’s Boiler Repairs R.R. #1, Auburn ONT, Canada N0M 1E0 (519) 526-7640

– Ernst Gage Company 250 S. Livingston Ave. Livingston, NJ 07039-4089 (201) 992-1400

– Stellar Industrial Technologies 1918 Yorktown Court Lancaster, OH 43130-1242 (740) 654-7052

– Globe Technologies Corp. 1109W. Cedar

– Vierk Industrial Products 3521 Coleman Court Lafayette, IN 47902 (800) 428-7548

  • Published on Sep 1, 2003
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