Unless HPI changed the design there is a head shim. There is one on my .15FE. It is very thin. Odds are it is stuck to the bottom of the head. IIRC the color of the shim on my .15FE is a copper color.
It is ideal to replace the gasket on the backplate when you tear it down. I didn't do it the first time and I ended up tearing it apart again and installing one.