Although this isn’t entirely necessary for moderate (or lower) levels of port work, there can be a few degrees of inaccuracy with any method of finding TDC. One of the most common ways is the “split-the-the difference” method- where you use a hard stop (even a longer spark plug!) above the piston, rotate the crank in both directions until it jams firmly, and center your degree wheel rotation to halfway between the two points that the crankshaft cannot rotate past. In an ideal situation this would be almost entirely accurate- however many 2 stroke engine designs- whether motorized bikes, chainsaws, and even most full size four strokes include a slight cylinder and/or wrist pin offset (ie. not directly aligned vertically) for the sake of reducing piston slap/rock. This will induce a slight error- and although I wouldn’t say out loud that “this is a big deal!”, I know that whenever I usually degree a cylinder, I check and write down the Blowdown degrees in BOTH directions- and often there is a difference of a couple degrees! This is entirely caused by the variable alignments between the crankshaft, cylinder bore, and (most importantly) the piston/wrist pin offset. All mass produced engines have *at least* a wrist pin offset to reduce piston rock/slap noise and wear during warm up!
so at the end of the day- what it boils down to is- yes, there is a slight inaccuracy in any methods we use for porting these things, but it will probably not be a noticeable difference in final result. For those who are OCD like me and want to use multiple methods to verify timing- I’ve made some adjustable (note-light duty) piston stop tools so that true TDC can be found as closely as possible. Selling cheap as this is mostly experimental- but would love to hear feedback!
[As you allow the piston to rise closer and closer to actual TDC, the error in the “split the difference” method will grow smaller and get closer to the actual tdc position. In the end, when the piston is only lightly tapping the stopper, you will be (of course) as close to TDC as reasonably possible. The “split the difference” method, unless you purposely stop the piston very close to the top like here, may be off by a few degrees due to the pin offsets mentioned above]