Guys while cleaning bolts getting ready to put my engine back in I noticed that the exhaust manifold (stock) bolts are different thread types and one a little bit longer than the other. It appears the pitch is the same but the depth of the thread is shallower on the shorter one. The is an even number of bolts 8/8 ov each. I did not notice the difference when removing. Is this normal? Also is there something I can do to the cast iron manifold to clean them up and keep them looking better?