Sir I have overhauled a diesel engine and face the block it has blowby, after I open the head and check the piston top piston is 1mm. Over the block it has less clearance bitween piston and head that's why engine has blowby I think this is the reason because combrtion area is less, so now I open the piston and machine the piston from top as same level with block thain it will OK. Please advise me.