Q:

a single word for someone who does a routine but important job

someone who performs a job on daily basis, but his job is important and vital for normal procession. like the one who checks the cards at a casino before it is played, one who test drives the bike that is gonna be used in MotoGP, the one responsible to bring the tyres in time for Ferrari so that the assembly line is not slowed down and similar types. 
an assistant doesn't fit the job and a worker is too common. I can recall it being another word, that is less than 6 letters. I can't find it on internet.

A:

You can refer to such indispensable workers as cogs in a machine, bureaucracy, or company. The role of a cog may be minor or even looked down upon but nevertheless "important and vital for normal" operations.

