What is 710?

A few days ago I was having some work done at my local garage. A blonde came in and asked for a seven-hundred-ten. We all looked at each other and another customer asked, "What is a seven-hundred-ten?"

She replied, "You know, the little piece in the middle of the engine, I have lost it and need a new one.." She replied that she did not know exactly what it was, but this piece had always been there.

The mechanic gave her a piece of paper and a pen and asked her to draw what the piece looked like. She drew a circle and in the middle of it wrote 710. He then took her over to another car which had its hood up and asked "is there a 7 10 on this car?"

She pointed and said, "Of course, its right there."