Folks, if you re-read the first post, and remember there are several twin cylinder engines around 25cc, you have to realize he's asking about 0.25 cubic inch engines.

And the answer to his question is that there have been several small multi cylinder engines on the market, none have survived.

Biggest problem is cost, a two cylinder engine has to be more expensive to produce than a single. Next is mechanical loss, the more moving parts the more friction there will be, so (up to a limit) a single will be more powerful than a multi of the same size.

K&B/Allyn made an inline twin, it came in 0.09 cid and 0.15 cid sizes. They ran well when you could get one to start, and they looked good in a P-40 of the right size.

Once in a while you will find a small radial on "The auction site," can't remember the brand, but they were geared/coupled single cylinder engines. Again, hard to start, not a lot of power, and the gears wore out quickly.

If you go to the "1/2A and 1/8A forum, there are several threads about people making their own small multis, not only "Boxer" twins but some radials too.

Go look, and enjoy.

Bill.