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Hoping this is the best area to ask this, mostly in the DIY and hobby world I've seen 2 engines coupled together to make a twin engine some so this for the zen of doing, but others do it to align the pistons I've noticed particularly in RC aircraft rather than buy a twin engine where the pistons operate at exactly the same time and I supposed matched this Introduces a reciprocating vibration that causes issues.

I've been hearing if you take 2 two stroke engines and couple them together with 1 at TDC and the other at BDC they cancel each other out and a guy on YouTube showed a video of one with a glass of water next to it barely moving.

What is the name for such an arrangement? Is there any disadvantage of doing this? Links to more information? I shall search for YouTube video as described above.

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Well, the arrangement used for Volkswagen (air-cooled) engines was known as horizontally-opposed when I worked on them. The cylinders were slightly offset from each other and each cylinder had its own crankshaft journal, so not exactly opposed, but perhaps along the lines of what you were looking for.

I've also heard them called pancake engines, which might turn up some other search results.

On blacksmith37's excellent comment, here's what the BMW motorcycle version looks like inside (these days at least).

enter image description here

For some reason I never discovered, these were sometimes called "boxer" engines, which might provide another fruitful search term.

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  • $\begingroup$ BMW motorcycles have used horizontal twins forever , although 4 cycle. And they were copied by the Russian Ural. $\endgroup$
    – blacksmith37
    Jul 19, 2018 at 0:00
  • $\begingroup$ Ah, yes, of course! How could I forget! Many a happy mile putted around by one of those! :) $\endgroup$
    – scanny
    Jul 19, 2018 at 0:05

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