I should have mentioned yesterday -
The ports should be smoothed, but not polished, and finished with very fine (1,000 or 2,000 grit) wet or dry with a bit of light oil on it.
Polishing creates turbulence, whereas a smooth surface encourages laminar flow (= more inlet charge).
The key objective here is to smooth the inlet tract, so any gaskets should be trimmed back, and inlet ducts aligned with the inlet ports. This might mean ADDING material such as Devcon, rather than cutting it away.
The idea here is to assemble the engine exactly as the designer intended (AKA 'Blueprinting'), rather than how the assembly line does it.
If you are really keen, the compression ratios of all six cylinders should be matched (by measuring head capacity at TDC), and skimming a fraction off of the piston crowns as required.
You might also check the 'squish band' around the periphery of each piston, using a piece of solder to measure the gap at TDC. Anything more than 25 thou encourages detonation and pre - ignition, but most assembly lines work to much wider clearances for safety. Don’t forget to check the gap between the valves (when fully open) and the piston crowns at TDC too.
Most petrol engines will safely run with much higher compression ratios (= more power for the same amount of fuel) if the squish band is correct.
Many competition engines are blueprinted, rather than 'tuned', as this provides greater power and economy with better reliability. The difficulty is that production lines simply don't allow for this kind of assembly.
NN
On my Sunbeam I had 60 thou machined off the head - in fact it right up to the edge of the plug thread.
I considered a head skim on the 2.6 but that was too difficult being both V and over head cam
I assume that was a Sunbeam car?
60 thou is quite a lot, but many production engines don't run anywhere near as high compression ratio as the manufacturers claim.
We had the head skimmed by 20 thou on our old 1.6 16v Astra (when the head gasket went), and it made a noticable improvement to both power and economy.
I had about 25 thou skimmed off the heads of my 249 TZ Yamaha, and bronze squish rings fitted. This allowed much higher CR, and prevented seizing caused by overheating - a constant problem in engines running at 12,000 RPM.
I used to run it on a mixture of 50% 115 LL AvGas, regular pump fuel and an Acetone/Toluene mix (to further boost octane and prevent the Castor Oil from dropping out of solution).
It was quick (155 + MPH), and also used to smell gorgeous (so I am told), especialy during early moring practice on closed public roads.
NN