as DT said : no ...
a pokemon that has the abilety sheer force will get a power boost on a move that causes a secondery efect so for example :
lets say my DW feraligatr (sheer force abilety ) used waterfall instead of makeing the foe flinch it would just get a power boost
to explain better sheer force only boosts moves that causes secondary effects to the foes pokemon and not including recoil moves too
another great feature about sheer force is if the move is boosted by sheer force and the pokemon has the item life orb it would get the orbs bounus power but without the Hp loss :D
so to end this one no head smash isent getting sheer force boost : moves that cause- recoil or stat changes will not get sheer force unless the secondary ffect is to the foe
heres a link just in case ;) http://pokemondb.net/ability/sheer-force