Well, I just looked on the RB Concept site - the manual that comes with the engines said 0.8mm worth of shims - 2x0.3mm and 2x0.1mm or 1x0.2mm shim which totals 0.8mm.
I heard from people running 1/8 at a track I go to sometimes, that either all engines they had, or Nova's or RB's specifically (not that it really matters) ran well with 0.8mm, but felt (and showed) that they prefer to run with 0.9mm head clearance.
Ask a guy named West03 on the forums here, he might be able to confirm that.
Car-less (Velox V10 or 733) // RB T10.5"+" (JB Modified) // Bluebird BMS-990DMG (St) & BMS-631 (T/B) // ATS 16% // Capricorn Wheels // Protoform
"70% of the time, it works ALL the time"