Well it's pouring rain here so with nothing much else to do I went to the barn and took a couple pictures of the 28cc cylinder for you. They are completely interchangeable with the 31's as are their crankcases. The one piece crankcase is a much later version and only 31cc engines were made at that time. The earlier style crankcase's are stepped towards the front and take a slightly different and smaller dia. reed housing and back plate. Also the crank itself has a slightly smaller counterweight dia. as the inside of the case is more compact than the next version. Next comes the unstepped crankcase with the later style reed houseing/back plate. It has a little larger inside dia. and it's crank won't go into the stepped case but it's piston and rod will, though the earlier piston cannot be used on this crank as the notch in the skirt isn't quite large enough. The 3rd. version of this engine comes with a one piece crankcase and flywheel housing and it's internals are the same as the second. How's that for a rainy day lesson? Now don't get me started on the Stihl FS 36 and 44's which are also Ryobi's but with some other changes........Images too come as I try and get a bit smarter......Bob