If a step-grandfather is the step-father of your father, what would the father of your step-father be? What about the step-father of your step-father?
I have never contemplated these issues before but The Relationship Calculator by Stephen Morse seems to handle them without difficulty.
stepfather of your father is your father's mother's spouse = step grandfather
father of your stepfather is your mother's spouse's father = step grandfather
step-father of your stepfather is your mother's spouse's mother's spouse = step grandparent
Stephen seems to share the view of ColeValleyGirl in her answer that there is no need for any further complexity once you reach a non-blood relationship.
If you have an incredibly complex example that is very well-documented, you might be able to invent a name that will be famous enough for everyone to adopt.
Personally - and slightly tongue-in-cheek - I'd disagree. The step-father of your father is called "the step-father of your father".
I would suggest that a lot of these more obscure relationships are determined in each family and each family will come to a different conclusion. I've heard people refer to their father's step-mother as "gran" pure and simple. In that case, their father's step-mother had died long before they came on the scene, so there was no possibility of confusion. (And I called her "aunt", even though she was my great-aunt.)
To be slightly more helpful, I'd suggest that anything more than the straight blood relationships need to be explained in each case and written out in full as "father of your step-father" or whatever, precisely to avoid confusion. Once the relationship has been defined in that particular case, then feel free to refer to them by the family name (e.g. me referring to my Uncle Bob once I've defined that he was my father's uncle.)
I'm tempted to say that anything more than the straight full-blood relationships need to be explained, as I know there are those who don't accept the concept of a half-cousin. Dick Eastman, for instance, writes (see http://goo.gl/jIdfo) in his Online Genealogy Newsletter that "There is No Such Thing as a Half-Cousin!" and in support quotes Black's Law Dictionary. Sorry Dick. Don't agree - that's an American dictionary and it's a Law Dictionary. The fact is that the term is in the Oxford English Dictionary. But - and here's the important thing, while the OED defines half-cousin, it defines it as either what I would call a second cousin or a first-cousin-once-removed. So I can't claim it clearly has its (currently more usual?) meaning of just one grandparent in common. I'd have to define it before I use it.
Now - what we really need a term for is the relationship between two couples whose children are married....
The father of your step-father would also be a step-grandfather: see Wiktionary. The step-father of your step-father would probably also be a step-grandfather.