Hello everyone.
My mechanic is not trustworthy so I want to be ahead of his repair cost game.

Question #1 He told me that my engine is leaking oil from the side, where you have the head gasket, and that he needs to take the engine head to a machine shop. That it is gonna cost about $300. Also, that he has to change the head gasket afterwards.

I don't understand why a machine shop is involved here and if what he is proposing is legitimate, what will be the estimated or actual cost?

Question #2 Also, what is the cost to replace the Belt tensioner?

Thanks.
tgif.