As young as you are, you really need to be careful. When I was 18, I was working putting in a modifed Torqueflite in a Dodge Coronet when the damn thing slip out of the trans jack. I thought at the time the trans was more important than my back and I pulled some muscles. Took me a week to get over it and to this day (I'm 58) I continue to have problems from time to time. During my pharmacy career, I have seen alot of guys with bad backs.