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The Crankshaft

Building a crankshaft for the 4 stroke model engine

About 2 years ago I was flying my R/C plane and the engine (an ASP 75) seized up in flight. I was not surprised, this engine had bit the dust MANY times. After an examination of the engine I discovered that the crankshaft was cracked. It was almost a two piece crank! I looked at the crank and thought "I bet I could make one of these on my lathe". So I started to work. I made a new crank, put it in the engine and it ran as good as before. After that I thought "I wonder if I could make a whole engine". So I made several attempts at making a crankshaft for a small engine, but none were quite acceptable. I had made no plans at all as of yet. I finally decided to draw a simple plan for the crankshaft before I started on an other one. So I drew a plan for a 2 cylinder crank with two throws. It was to be a one piece crank. After a lot of thought about how to make it, I decided that it would be too much work and trouble for my experiance. I then drew another crank with only one throw that both connecting rods would run on. The crank was just like the one for the R/C engine, only bigger. I made it to use the bearings from the R/C engine. I had a bar of stainless steel 1.5 inch dia., and a 1 inch stroke was about all that could be attained from it. So this is what I came up with. This became the final drawing of the crankshaft.

The crankshaft has very little design to it. The maximum throw was limited to what I could get from the bar of stainless steel that I had on hand. As for the length of it, I really had no idea of how long it should be. Now with the engine almost complete it needs to be at least an inch longer. The counter-balance? I just put on a lot of it. I figure it can always be cut down later.

picture of crankshaft