Hey guys i've got just about everything on order for my engine rebuild in my car but i was on a website ordering the last parts this morning and hit a lil speedbump. What is the difference between head studs and main studs? i thought they were 2 seperate kinds of bolts but when i clicked on one to add to my cart it would replace the other... now either their website is just messed up or it's the same thing with a different name. a little educational help please

 

they are two seprarate parts