This is a pack of four replacement REDS Engines 0.1mm Head Shims. These are compatible with the REDS R5, R5C, R5T and R5TE engines. The head shim is the thin circular piece that is installed between the head button and the top of the cylinder sleeve and can be used to adjust and fine tune the compression (deck height) of the engine. In general the higher the nitro percentage of the fuel you run, the more head shims you should put in your engine. Please consult the instructions that came with your engine for details about how much your engine should be shimmed!