What are the benefits to doing this to your head (milling) and block (decking)???

 

it yields slightly higher comp when u do it

people also do it to make sure the surface is straight

 

How do you know if the head needs milling or if the block needs decking??

 

milling and decking are basically the same operation, i think I can clear it up as follows, one you do to "service" the head that may be bowed or crooked to ensure a tight seal, the other is done in order to increase CR. there is a little nobby cut out that shows the service limit for decking, milling etc to ensure that psitons wont hit valves with stock cam shaft.

 

do u do this when sleeving the block?

 

Generally, when a block is serviced (such as sleeving etc..), is it decked as a precaution to ensure it is perfectly flat.

 

Keep in mind that milling is irreversable and may change the angle at which the valve faces the relief on the piston. (Generally not a good way to increase CR if it can be avoided)