Upshur Opposed Twin

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May 13, 2018
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Scotland, Aberdeenshire

I’m about to start my next build, Harold Upshur’s Opposed Twin. Has anyone attempted to build the crankcase from the solid?

If you build the crankcase in a block, how do you fit the crankshaft and camshaft inside?
I suggest you put the bearings on the crankshaft and on the camshaft
I'm starting this engine too :D

I intend boring a hole to accept the longest web of the crankshaft then fabricating a plate to run the bearing.


I’ve scaled the drawings on graph paper... I reckon I have enough space. I intend inserting the crankshaft then allowing the conrods to dangle loosely, that way I will be able to secure the big end shells from the opposite side using the hole which I will have bored for that cylinder. I will turn a flange just above the bottom of each cylinder and use that to secure them to the casing.

Although I can see that it is possible to creat the crankcase using plates, I think it could be problematic in terms of oil leaks and binding.... I could be wrong! Anyway, I like a challenge.

I did my Upshur Twin crankcase as hogouts with an end cap that lets the crank and camshafts come out endwise. Pics later if I remember.
Here are photos of my riff on Hamilton Upshur's Twin. I have six nearly complete.


  • Upshur Twin Crankcase 1.jpg
    Upshur Twin Crankcase 1.jpg
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  • Upshur Twin Crankcase 2.jpg
    Upshur Twin Crankcase 2.jpg
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  • Upshur Twin Crankcase31.jpg
    Upshur Twin Crankcase31.jpg
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These are oil leakers. I recommend you use oil seals.
Is that true even with a proper crankcase breather checkvalve that puts the crankcase at negative pressure?
Don't know anyone that has put in a check valve breather. I know more than a few people who've built them from my drawings and unless they used seals or sealed ball bearings, they leaked. Especially the one fellow who built it upside down and the big gear flogged around in the oil.
Hamilton was an aircraft designer who did control systems for helicopters. He designed everything to be lightweight as possible. An example of that is his Farm Engine that has the head and cylinder held on with just two screws. Many folks added another two to avoid the cylinder moving around under power. He also usually shrouded the spark plug gap so it could not 'see' the combustion chamber. Most folks who have a well running Upshur engine have moved the plug to fix that problem.

Many thanks for your input.

My riffs on the Upshur Twin have crankcase breathers that are based on a scaled-down Howell V-twin breather.