I am going to try my hand at developing plans for an Offenhauser Mighty Midget Racing Engine. There were many variations manufactured through the decades and my version will likely end up being an amalgamation of many of those. I will also be taking liberties for ease of manufacture/assembly and to increase the chance of it being "a good runner"--or at least a runner. Inspiration came from Terry Mayhugh's build of Ron Colona's Offy and I will be taking advantage of his engineering insights shared in his (Terry's) build log. Why don't I just build that engine? Well, it is a very complex model and I don't feel ready to tackle that level of sophisticated craftsmanship. Second, I want to have control of the plans and have the ability to freely distribute them if they ever get to that point. The variant of the 97 Cubic Inch Midget Offy I will build will have two valves per cylinder instead of the large Offy's four valves per cylinder, the crank will be supported in three places instead of the large Offy's five and the 97 cu in Mighty Midget Offy is smaller overall. I will design in 1/4 scale so the model engine will be about 5.375" long, 5.5" tall and 4 inches wide.
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Source: Fred Offenhauser Photos and Premium High Res Pictures - Getty Images
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Source: Offenhauser: The Legendary Racing Engine and the Men Who Built It: White, Gordon Eliot: 9781626540415: Amazon.com: Books
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I will use the line drawings available in the book noted as a source for the attached pictures, then create 3d solid models of the major components, and I literary mean solid as there will be no internals initially. I will layout the moving components, timing gear train, cam shaft, crankshaft etc as simple stick models to define their geometry. Then I will add increasing detail to the engine, realizing there is a high degree of interdependence between all the components; a small tweak in one place will ripple through the whole engine. Once I am happy with the 3D CAD model I will begin developing the plans themselves. Most plans I have seen are manufacturing method agnostic, that is, they can be used for manual machining methods as well as CNC machining. My plans will be developed specifically with some limited CNC machining in mind. Home grown CNC routers are more common now and mine has given me greater flexibility in producing a complex part in a reasonable amount of time. What this really means is that I will not only produce a set of dimensioned prints, but some IGES files designed for specific machining operations. If someone chooses to build to my plans, they will not need to create their own 3D CAD model from a set of dimensioned prints, they can use the supplied IGES files.
I have scoured the internet and downloaded lots of pictures, enough I think to guide me in producing a realistic replica.
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I scale a 2D drawing and import it into my CAD program.
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Then I extrude the major components.
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And create an assembly.
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Then begin dimensioning the major components.
I will start nailing down the models specification's, for example I think it will have a bore of .75"