Piston Surface Finish

[gmac]

Hello everyone;
I'm new to this hobby but have an engineering background - so pardon my stupidity!! I'm in the process (slow process) of building an Upshur horizontal twin using aluminum piston, Viton o-ring and steel cylinder sleeve. I've read a fair amount of info on how people finish the piston to bore surface; but there seems to be wide variation on how fine a surface is created ON THE PISTON. Some simply finish off with fine sandpaper to the final diameter, others go to the trouble of honing. I'm assuming that for a cast iron ring you'd want a fine, honed surface mainly to maintain compression whereas with a Viton o-ring a "less smooth finish" to collect some oil for lubrication is better. At any rate "how smooth" a finish do people work to in practice. The question is not just specific to the Upshur, as I plan to build other cast iron piston ringed engines later as well. Any and all guidance would be appreciated.
Thanks and best regards
Garry

 

[compound driver 2]

On cast iron pistons from about 1.5 to 3 inches I use an as machined finish with a lite smooth with scotch brite paying more attention to keeping the piston round and the sides parralell. On smaller pistons of say 1/2 inch a lite touch with 400 grit wet and dry held on a steel rule of piece of gauge plate.

Id think aluminum would be about the same. The piston to cylinder fit is more critical and that applies to all matterials and all sizes. too fine a finish and my feeling is oil is not retained on the surface as the piston beds in in the bore.