It is a good idea to disconnect, and remove if necessary, the displacer. Again the flywheel should spin freely. Final Tests Reconnect the displacer and adjust the height so that it does not touch either the top or bottom plate. The engine should still turn quite freely except that the displacer does have some air fiction. Replace the acrylic cylinder and test it on ice cubes.
Ice cubes seem to give the best delta-T without risking damage to the engine.
If it is not running at this point all that is left is air leaks and timing. I found that balance is only critical when you are trying to get the engine to be able to run very slowly. This is published in Great Britain and contains information gathered from all over the world. It contains a very interesting chapter on the history of the Stirling engine, and its developments and improvements that were made over the years. There are plans for about a dozen engines, as well as several chapters on building and testing practices.