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I have my heads off for replacement. While they are off and it takes a while to clean the block do I need to oil the cylinder walls?
Also, does the carbon need to come off the tops of the pistons?
I think they both fall under the "can't hurt...." category. As long as the heads are off, yeah, very carefully clean a good amount of the carbon off the piston tops. Also, get at the ridge of carbon around the top of the bore, if there is any. Can't hurt to wipe the bore down with some motor oil, be careful not to scratch anything, try not to let any crud get down in between the piston and the cylinder wall, (compressed air works well for that) cover the lifter valley up really well with clean shop rags, along with the other bores you are not working on at the time, (by clean, I mean never used, not washed; a washed rag can still have metal shavings and who-knows-what in it) that's about all the advice I have for ya.
if the carbon is quite hard ... I'd leave it ... helps typically low compression.
fwiw ... most local CT race rules require flattops & min 70cc chambers (track techs do not cc the pistons during protest) ... most racers don't blow much oil but a little builds atop crowns ... usually, it's intentionally undisturbed during freshening.