What is a 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 said that she did not know exactly what it was, but this piece had alway s 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 710 on this car?"
She pointed and said, "Of course, its right there."