hi adsl,
more head shims mean less compression, less head shims mean more compression. when the motor is brand new, it can be difficult to turn over:
1. because of extremely tight fit with piston and sleeve.
2. heeps of compression.
by adding head shims you can get rid of some of this compression
tempererally.
also when running a higher percentage of nitro some people run more head shims.
i hope this helps.