Ohhh Mr Phantom, you have some more gains ahead yet then
Both shims installed is for 30% nitro, I am running 15% nitro so I have the aluminium shim installed (0.3mm) I did try going to 0.2mm but blew 2 plugs and caused sleeve damage (Lucky engine was kinda oldish). You may get away with 0.3mm and 20% nitro but to be safe id get the shim kit (comes with a 0.1mm shim) and run 0.4mm
. Did you get a manual? for 20% it states 0.4mm +/- .1mm (manual at http://www.stsrc.com.tw/products.php
Stock: Cooper shim is 0.2mm the aluminium shim is 0.3mm
The STS shim kit comes with the 0.2 and 0.3mm shims and a 0.1mm one.