Using a spacer plate on top of the cylinder is a disaster waiting to happen. I use thicker base gaskets under the cylinder to maintain the use of off the shelf domes. Not only does this keep the piston top within the safety of the liners and maintain forced convection cooling charecteristics of a stock engine, it also enhances the port timing for a more performance oriented engine. If real performance is your game though, additional port configuring is needed to tap in to the full potential of a +4 engine...Jim
Jim is shooting you right. Going with the spacer on top of the cylinders will make it more difficult to get the proper quinch area above the pistons. This creates potential (read garaunteed) detonation. Spacing the cylinders from the bottom is the best option, running short or long rod. The port timing does change with the +4 crank, depends on what you want to do with your bike as to what port configuration you need. With the long rod you are using a diff. pin height in the piston, port timing is the same, but your motor is a little slower revver. The trade off is better breathing in the top rs. Hope that helps.