Probing through spark plug hole will measure piston's up stroke.... eraser side down will not scratch anything and grip piston crown..... when it approaches TDC the piston pushes the pencil out. Naturally you dont want to break the pencil and should readjust it and turn the crank very slowly. Easiest method is pull ALL the spark plugs then work from the cylinders that are closest to TDC. With all the plugs out you can practically turn the crank by hand yanking on pulley and belts........ this comes from one-time experience with 220Sb.

IMHO, valve seals fall into DIY labor of love category - not the easiest job, but not the toughest either...... probably a "6" on degree of difficulty scale with things that can go wrong - not something for the mechanically challenged, but fun way to lose yourself for at least a full day if you enjoy working on your own car. Poking and pulling out old valve seals was toughest for me. Was lucky to have a very good spring compressor on loan.