The Craftmans Shop > Model Engineering
Side Valve i.c. engine from Bar stock
Brian Rupnow:
Todays offering to the machining Gods is a piston. Granted, its a rather strange looking piston, and I have never built one like this before, but remember, we are in the wonderful world of prototype here. The top section of the piston extends up beyond the split between the combustion chamber and the cylinder, so we don't want it to contact the wall of the combustion chamber in that area. Additionally, there is more "hogging out" of material in the rod end of the cylinder than is usual. That is to accommodate the 1/4" sealed needle roller bearing, which is the smallest size I was able to buy. I'm not even certain a needle roller bearing is going to give us any great benefit there in terms of reducing friction, because the rod only swings in a small arc, not a full rotation.
I have to contact the manufacturers of the needle bearings on Monday, as I remember reading somewhere that needle bearings are made deliberately oversize, both on the outer diameter and on the inner diameter. They are intended to be press fitted into a housing, with a hard enough press fit to reduce the bore to the correct size.--That could be quite a trick, when they are going into an aluminum rod with only a thin band of material enclosing the bearings.
Brian Rupnow:
I was getting ahead of myself!! Before I make the piston I have to lap the cylinder bore. Before I lap the cylinder bore I have to figure out a gasket to go between the cylinder and the combustion chamber and permanently lock them together. Before I do that I might as well make the valve cages and install them. So---These are the valve cages, which act not only as a seat for the valve to close against, but also as a guide for the valve stem. People have made these valve cages from many different materials, but I find that brass seems to work well, and an added advantage is that the brass cage, being softer than the steel valve, will "conform" to the shape of the valve face after the engine has been run for a short period of time, providing an excellent seal for compression.--And--If you do happen to screw up a valve seat, then you only have to replace this small item, not an entire cylinder head or whatever it is pressed into.
Brian Rupnow:
Hmmmm---Everything I do has consequences--- I had to go out this morning to buy some brass to make valve cages out of. While I was out and about, I got thinking about the 0.197" diameter on the cages. Started thinking, that if I found out after the fact that the .197" diameter was too large to fit into a spring comfortably, there was no way to change it after it was pressed and Loctited into place. So---I stopped at the tool shop and bought a .197" diameter drill. Then I stopped at my fastener supplier and dug around in his spring collection. I ended up buying a pair of springs "INDUS0615125" They are a tight fit onto a .197" drill. Consequently, I will reduce that diameter on the cage drawing to .193" diameter. In the picture that is a .193" diameter drill setting inside the spring, and it fits well with no binding. The spring is wound from .023" diameter wire, has a pitch of .090 between the coils, and each spring is approximately 2" long, free length, but they will be cut down to a shorter length when I install them.
Brian Rupnow:
So now we have a pair of valve cages installed. The first picture shows one of the semi-completed valve cages laying beside the cylinder, with the "pusher tool' that I use in my mechanics vice for pushing the cages into place. The valve seat is not cut yet in that picture, and the hole which lets fuel or exhaust enter/exit from the cages are not yet drilled.
Brian Rupnow:
In these two pictures you can see the underside of the valve cages in one shot, and the top of the cylinder and valve cages in the other. In the shot showing the top of the cylinder, you can see my "George Britnell valve seat cutting tool". The seats are cut in that picture, but are hard to see because they are very small. They are cut by hand---no power tool involved.
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