"Oil Cap" joke

A blonde goes to a auto parts store and asks for a Seven Ten cap.
Everybody look at each other and ask, "What's a seven ten cap?"
She says..., "You know, it's right on the engine. Mine got lost somehow and I need a new one."
"What kind of a car is it?" they ask.
She says that it's a Buick.
"Okay, Lady, how big is it?"
She makes a circle with her hands about 3 1/2 inches in diameter.
"What does it do?"
She says, "I don't know, but it's always been there."
One of the guy gives her a note pad and asks her if she can draw a picture of it. So she makes a circle about 3 1/2 inches in diameter and in the center she writes 710.
The guys on the opposite side of the counter are looking at as she writes it...and they just fall down behind the counter laughing so hard.
One guy says, "I think you want an OIL cap."