The blonde lady and the 710

Just few days ago i was at the gas station, when a blonde woman drove in and asked for a "710".
All guys present there looked at each other puzzled. Someone couldn't stand the embarrassment and asked : "What's a 710, lady??"
The woman answered "Come on, you know... the one that is located near the middle of the engine. I must have lost the previous one and i need a new one a.s.a.p."
"What is this 710 for? What is it's use?" some other guy asked..
"Well guys... i don't know exactly it's use but i have noticed that it is mostly located in the middle of the engine" the woman answered.
Everybody was so puzzled. Noone could understand what she was talking about. We all felt so awkward since men mostly, are familiar with the inner parts of a car's engine.
The guy at the gas station had no other choice than to give her a pen and a paper and asked her to draw that piece of equipment.
The lady started drawing a circle about 6cm diameter and then she wrote "710" at the center of that circle.
When we looked at the paper it finally made sense :