A few days ago a query arose re the durability of the displacer rod bushes.  The engine was designed for speed rather than longevity and, to keep oil drag to the minimum, I had specified them very short - and kept my fingers crossed.  So news of suspected rapid wear from a builder gave me a most uncomfortable feeling and I decided on giving the engine a 'life test' to see what happened. (and I just love an excuse to run an engine anyway!)

  Up to this point the engine had run for approx 30hrs but only with the very light aluminium displacer and total play at the inner bush was 0.0015".   The heavy stainless displacer was now fitted,  A heater, 10 turns of nichrome wire (ex 1kw electric fire) was wound on a strip of mica (ex toaster element) around the hot cap.  Wrapped with a couple of layers of asbestos string and connected to a 20volt transformer this drew 55watts to drive the engine at approx 1500rpm, off load.

  Running was almost continuous, in fact the last continuous run was over 50hrs by which time the family were fed up with the rattling in the conservatory and, after 120hrs, although still flailing round - it was nearly worn out.  Play at the inner bush was now 0.0045" and the displacer very close to scraping the hot cap.  However the wear on the piston had also reached the limit, making it difficult to re-start the engine without help from a blow torch, and the big-ends were in a shocking state.

  So with the whole engine wearing out together I seem to have achieved my aim, and a total of 150hrs or several hundred 20min runs,  seems good enough for a 'toy'  -  Some lawnmower engine builders get away with less!