Engines > Your Own Design

Building a twin cylinder inline i.c. engine

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Brian Rupnow:
I have built single, opposed twin, and v-twin engines. I want to build an inline twin engine, and I have the drawings for Malcolm Stride's Bobcat engine. I build in the imperial inch system, and I will be upgrading the Bobcat engine to a 1" bore and using imperial inch measurements as close to the original drawings as possible, but keeping the same original stroke. This past year has been a very challenging one for me, and I have three engines designed and built that either didn't run well or didn't run at all. This has shaken my faith in myself, so I'm going back to an engine design that has proven itself to work well. I am just in the very beginning stages of this engine, so if you are interested, stick around. ---Brian

Watching along!

Roger B:
I have built a 12 cc twin (approx 3/4 bore 3/4 stroke) that has run with 180 and 360 crankshafts as well as a CD (SU type) carb and fuel injection. The thread is here:


This is some load trials with fuel injection:


I have attached my electronic 'back of an envelope drawings'

Can I suggest you also scale the stroke to the same proportion as you intend to scale the bore, same goes for everything.

1mm on the original drawings scaled to 0.046" on yours would be the ideal way to treat everything just adjusting for stock material and fixing sizes.

Brian Rupnow:
Everything has to start somewhere, and seeing as the cylinder bore has changed a bit, and the piston stroke has changed a bit, then the cylinder will be the first candidate.-Because---the change in cylinder outside diameter will affect the center to center spacing of the two cylinders and this affects the main crankcase and the sump and the crankshaft and the camshaft. The original drawings asked for an inner sleeve of cast iron and an outer finned housing from aluminum. I find it a lot simpler to just make the whole cylinder out of cast iron.


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