I originally intended machining a set of R100 Nikasil barrels to suit the R90 case but darned if I could find a pair at a reasonable price so I opted for the Siebenrock 1000cc power kit knowing that I probably wouldn’t use the pistons.
The following pic shows the Siebenrock piston at TDC in the 7rock barrel (cylinder base shim deleted). You can see the step in the piston that accommodates the smaller diameter horizontal R90 squish band to make them Plug ‘n’ Play. The step is a 1.4mm lower than deck height. To improve squish, I machined the R90 heads with a sloping squish band to R100 dimensions.
To machine a matching sloping squish into the pistons and delete the step, I guesstimate the outer periphery of the pistons’ squish band would be around 2.5 – 3mm below deck height. This means that I would have to remove around 4mm of metal in order to increase the compression ratio to about 9.5 – 10:1. (though maybe not that much since the Siebenrock piston crown seems a bit higher that the stock piston)
This would mean taking material off both the head mating surface and the cylinder base. I prefer to leave deck height stock so my thinking at the moment is to use stock R100 pistons and maybe machine their squish periphery to allow a higher crown to sneak further into the chamber. Have to take a few more measurements…