Memorable records and various mechanical arrangements please consider the work performed by Jean-Pierre Pirault and Martin L.S. Flint as it is the most complete reference almanac available from where engine designers and enthusiasts can nourish their knowledge.

This book explores the opposed piston (OP) engine, a model of power and simplicity, and provides the first comprehensive description of most opposed piston (OP) engines from 1887 to 2006. Design and performance details of the major types of OP engines in stationary, ground, marine, and aviation applications are explored and their evolution traced.

The OP engine has set enviable and leading-edge standards for power/weight refinement, fuel tolerance, fuel efficiency, package space, and manufacturing simplicity. For these reasons, the OP concept still remains of interest for outstanding power and package density, simplicity, and reliability; e.g., aviation and certain military transport requirements.

Using material from historic and unpublished internal research reports, the authors present the rationale for OP engines, their diverse architecture, detailed design aspects, performance data, manufacturing details, and leading engineers and applications. Comparisons to four-stroke and competitor engines are made, supporting the case for reconsidering OP engines for certain applications.

http://books.sae.org/r-378/

This website has been prompted by the availability of a considerable amount of researched OPE information for a technical book.
Unfortunately a large amount of the information did not make it into the book. "Opposed Piston Engines" - Evolution, Use and Future Applications"-
There were limitations on the word count and number of images.

http://www.opposedpistonengines.com/