46 thou Stainless Steel Compression Plate. This really useful item will save you an awful lot of time with a fretsaw and file.
It is not uncommon to have to make these up when fitting new pistons and trying to get the engine to a particular compression ratio. These plates are laser cut from stainless steel (they do not deform or damage as easily as aluminium) and are 1.2mm/0.046 thou), which for a STD bore 79mm barrel equates to a reduction in compression ratio of 1.2:1 per compression ring fitted. Note: Do not forget that when fitting or removing compression plates, it is necessary to check the clearances on the vertical shaft assembly, and if necessary fit a different width top Oldham Coupling. I have a batch of these couplings in the pipeline in various thicknesses. However, the same theory can also make these compression rings extremely useful - if you are building a new engine for your latest restoration and only have one pair of Oldham couplings, then adding these plates may help get the length correct - which is more important than compression ratio!. Final point - always check the oil holes on your barrel match up with holes on the compression plate - there were some variations. If so - just open holes up to match