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Mathew Stage 04-13-2009 12:44 PM

Steam Piston Rings Replacement
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Well today I took apart one of our McBride steam engines to see about replacing the piston rings, and not knowing exactly how they should look, or what is the best type of rings to replace them with I took some pictures and came on here.

From what I can tell they are a copper rings that sits in a grove about a quarter of an inch, to push the piston back in u squeeze the rings together where the gap is. The rings look fairly decent as far as I know, let me know what you think.


Steven Harrod 04-13-2009 11:19 PM

Evaluation of Ring Condition

My initial response to the photos is that the rings are suspect. The gaps seem very large, but otherwise a photo is not an adequate judge. Rings should have a very close fit in the slot, but move freely. When the ring is compressed in the cylinder bore, the gap should measure 0.002 inch. Essentially, as close as possible without touching. The wall pressure on the compressed ring should be about 5 psi, which is much less that the typical 40 psi for IC engines, but should still require about 20-30 pounds of compression force by you to get the ring into the bore. Sounds like the rings are too light a fit by your description. My dimensions are taken from "So You Want to Build a Live Steam Locomotive"

Note, copper would be a horrible ring material. I suspect they are really bronze? Bronze is also a poor wearing ring material. The standard would be cast iron, because it is a good bearing material in contact with a cast iron cylinder, and its porous nature absorbs oil and reduces friction. Of course, it can rust, so winter layup should include a liberal dose of oil just before shutdown and blowing out with compressed air.

The real test is how much compression you achieve in the cylinder volume. Assuming the piston is detached from the crosshead, push the piston into the bore, and then push on it with a wooden board. Do you feel any resistance? Do you hear any air leaks? Get really fancy and measure the psi with a gauge. How long will it hold before bleeding off?

Mathew Stage 04-15-2009 02:33 PM

Steven, thank you! I'll be heading back home today from NYC/School to finish working on the engine and start the other. Where the piston is in that picture is as far as I could move it. It is disconnected from the piston, however I think I need to loosening up the packing, because it just won't come out anymore. When I get there this time I will bring my digital camera (rather then my phone) and take some better pictures. I was working by myself as normal and it was rather hard to pull it out anymore without someone else helping to spin it while I pulled.

Hopefully I can get the new pictures up here tomorrow!

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