Factory Specifications usually leave the pistons safely below the deck surface as much as .025". This allows the auto manufacturers to machine the blocks with less risk of pistons hitting the heads. Also the deck heights are usually not the same from one side of the block to the other, nor from one end of the block to the other.
If you are building a high performance engine it is important to have the same compression ratio from cylinder to cylinder. A higher compression engine performs differently and tunes differently from a lower compression engine. If you have an engine with varying compression ratios from one cylinder to the next it makes it impossible to tune the engine to peak performance.
High compression engine are effected more by deck height changes than low compression engines.
|IF||100cc = 8.30 C.R.||50cc = 15.86 C.R.|
|THEN||110cc = 7.62 C.R.||60cc = 13.31 C.R.|
This makes it even more important to machine the deck surface on a high compression engine.