TboltBeardie

I am rebuilding the engine in my AE NTC3. I have aircraft grade sandpaper going up to 12000 grit at my disposal. This stuff is capable of polishing metal to a literal mirror finish. I would like to know whether polishing the piston and/or sleeve prior to break-in would be a good idea, or is the chrome plating on the sleeve too thin to do so? The sandpaper doesn't remove enough material to register a difference when measured with a micrometer or a high-end dial indicator, so I don't see any risk of affecting the contour or dimensions of the bore.

Anthoop

Why would you want to do this...what are you looking to achieve?

Of course you know that the liner/sleeve is tapered and the "seal" for compression/combustion is attained by the fit between the piston and liner......
When the piston/liner are manufactured they are made to certain dimensions....however they are never exactly correct.....so everything is made within manufacturing tolerances.....and then (generally) parts are matched up to provide a piton/liner fit that is appropriate, not too tight/loose.

When you break the engine in then the piston/liner will wear so that they fit nicely together...of course the exhaust side of the engine gets hotter than the inlet side so you have different expansion rates....and the piston/liner are manufactured round (within tolerance).

Not only that, the honing/machining process leaves grooves in the piston/liner....these grooves retain oil...
Also, whilst a really fine paper may not remove much material....you would not get a mirror finish on the piston/liner without removing a lot of material....

So....basically...no...do not touch the side of the piston or the inside of the liner with any flatting paper.

There are ways to speed up the break-in process if that is what you were hoping to achieve but I have never used such methods.