Here are pictures of my Fire Hydrant Pattern, part of my pattern making collection.
When I bought this pattern it is was purported to be a fire hydrant pattern. I have since learned it is more likely to be a single cylinder, 2-cycle marine engine.

I would be greatly interested in any details about this pattern. I do not have the patterns or core boxes for the rest of it. The guy who sold the pattern to me thought it was from Watertown, NY. I had someone from there look for hydrants that might be made from this pattern (I assumed that the town would use the locally made hydrants). None were found. The description from the seller:
Patternmaker's model of a fire hydrant. I believe this one hails from Watertown, NY. Very nicely made example of what it was patternmakers did all day -- and most unusual.

Outside view. The black above the 'W' represents the hollow portion of the hydrant. The W is 3/4" high
Inside view. The black indicates hollow portions, the light brown would be metal. As is, this pattern would product a solid hydrant! I don't have the corresponding core box which can be though of as a pattern for the interior of the hydrant. The pattern maker who made this pattern painted the outline of the core on the inside of the pattern. This is/was standard foundry practice. The moulder often did not know or care what the casting being produced was for (it might not be recognizable as some internal part of a machine might be). The black paint and the light brown core outline would clue the moulder in that this pattern should be cast with one or more cores and should not be cast solid!

The core outline would also give me a clue as to what the interior core should look like- in case I wanted to cast non solid hydrants

Partially Separated View. To make a casting: one half pattern is placed with the flat side down on a table. A box with no top or bottom (called a flask) is placed around it. Moulding sand is rammed all around it. It is flipped over and the two halves of the pattern are placed together (the index holes are visible in the picture above- the mating half pattern has corresponding pegs. The top peg is visible on the right half near the top. It's white). Another flask is placed over the first and more molding sand is rammed around it. The flasks are sparated and the patterns removed. Now the impression of each half pattern is left in the two flasks. If the flasks are closed back up and molten metal is poured in, the resulting casting will be solid. So before the flasks are closed together a core is inserted. The core is made of sand in the shape of where you don't want the metal to go. If you are casting a simple pipe the core would be cylindrical- since that the void you'd want. For an automobile engine or a fire hydrant the core is a little more complex.

Click here to see a mitre gauge (with a core) being cast in a flask.

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