MAKING CAST IRON PISTON RINGS

By Colin MacEke

This Baldwin I’m making is my seventh engine and I have always made my own piston rings. Buying rings goes against the grain, particularly when you choose a ring size closest to your design and then boring your cylinder to that size. It’s the wrong way round.

It began because I couldn’t find a suitable one for my first engine, a Crampton 5”, as they were very small and narrow. Volume one of Engineering in Miniature has an article on how to make cast iron rings and I’ve always made them this way.

The method is to calculate the size by taking the diameter of the cylinder and divide by 25. This gives the width and the thickness. These are then cut to split them and in my case I used a Jeweller’s saw with a blade thinner than 0.2mm which gives minimum loss.

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Apparently, the secret is in the heat treatment which is also the problem as the theoretical ideal temperature is 800º C and must be held at that temperature for 15 minutes.

As can be seen in the photos, the rings are clamped together between two thick steel plates and the rings are held apart by a wedge which is four times the thickness of the rings. Iron wire is wrapped around to cover the rings to stop scale forming on them.

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The author used some form of gas torch to try to bring them up to heat and having got it at its hottest, held it there for the 15 minutes. I think it’s doubtful he would have got the 800 but that doesn’t seem to be that important. The end result is an excellent ring and I’ve never had a failure or broken one trying to get it on a piston. In fact, pulling one apart to breaking point is almost impossible.

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In my case, I’m fortunate in that I have a small kiln which I made back in the 1960s when I was making silver jewellery to earn an extra bob or two and most of the work included lost wax casting.  This let me control the heat and I think, more importantly, the cooling down by leaving it in the kiln overnight. My laser thermometer didn’t seem to be accurate above 500º as I knew when I’d reached 660º by leaving a small piece of aluminium in the kiln to melt. The colour chart gives orange-red as the colour for about 800º-850º. Mine actually went to nearly white so my thermometer is definitely not accurate as it went to white which is near 1000º C! Still, it didn’t seem to cause any problems as when I tested one I found I couldn’t break it.

Building the Baldwin engine is the subject of a series of YouTube videos. A search on ‘Colin MacEke Baldwin’ in You Tube should find them and other videos of interest to the model engineer.

 
https://www.sarikhobbies.com/model-engineer-builder/

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