Should I Polish Combustion Chamber?

The combustion chamber is exposed to high heat (uo to around 1800F). Any casting flash can create glowing hot spots that can pre ignite the air fuel mixture. Polishing can help eliminate hot spots and some say even reflect heat back. Often unshrouding the valves improves flow, but only done correctly.

Is a bigger combustion chamber better?

Smaller combustion chambers allow you to create a higher compression ratio; however, they can also create clearance issues with pistons and valves. Conversely, larger combustion chambers will accommodate larger dome-style pistons.

Does oil get into the combustion chamber?

Oil collecting on the stems of intake valves is sucked into the combustion chamber during normal operation. Hot exhaust gases burn oil on stems of the exhaust valves. If there's too much clearance between the valve stems and guides, the engine will suck more oil down the guides and into the cylinders.