Ok, TOP TIP time.
Working on one cylinder at a time, bring it up to the top of the stroke and seal the gap around the edge with grease. Use WD40 and a kitchen scourer to clean the black gum off the piston top. You can use a feeler guage but be careful not to scratch the piston as sharp edges will cause pinking.
Again using WD40 and a scourer clean the face of the cylinder head. You need to get it checked for flatness. Also clean the inlet and exhgaust tracts but don't go to town. Make sure there are no bits left though, remove any loosish flakes.
I'll assume you've taken the valves out, put them into a drill and rotate them slowly, brush the back faces with a wire brush (in a secong drill if yo have one) to get them lovely and clean. Regrind the valve seats.
If you want to know if the heads have already been skimmed, look for a pointless little upstand on the front face of the RHS head (back of LHS head) that has a hole drilled into it. If the hole is nearer the bottom face than the side, the heads have been skimmed.
