What is true Top Dead Center? (GY6 150 motor)

[CrankingWithCG]

Currently in the process of rebuilding my 150. Last time i had pulled the top end apart, i had rotated the motor to "top dead center" before disassembly. Both valve's open. I dont remember exactly what i did, but upon pulling it apart this time, i think i remember. It as if the piston had rotated a bit past top dead center, so i turned it back a tad once the head was off to ensure the piston was highest in the bore.

However, i regarded the little timing mark by the flywheel and the t mark. My motor won't sit on that t mark, it always rotates a bit further to make the piston go up all the way, even when it's just the bare piston and rod. With the pistom in true top position, the t mark is a little past the actual marking ridge engraved in the case.

So, which is true top dead center? Im pretty sure ive been running it for almost a year with how i put it back together last time, the piston at its highest point on the correct stroke, and i aligned the camshaft according to that. Cylinder, and piston looks good. It doesnt seem to have running issues when my carb tuning is right. Starts decently, max rpm is about 6400, pulls hard as hell, and does 61-62. Have I accidentally advanced my timing by not putting it lined up with the t? Retarded it? Is the piston being highest true top dead center and not the mark on the flywheel?

Im throwing it back together with a bunch of goodies, including a big ol pe28 carb, maybe a cam, intake, cvt work, and possibly a ported head. Just want to make sure my timing is right and wont kill me 😂

[FrankenMech]

True TDC is when the piston is at the top of it's stroke, usually measured from both forward and back rotation to take up slack tolerances. I usually use a fixed piston stop and find a point right between the two stop marks. The marks on other components can vary due to manufacturing tolerances. Once you find TDC mark it. That will at least last until the flywheel is replaced or removed. It is also a good thing to check the flywheel key and keyways.

[90GTVert]

[ThaiGyro]

Big beast...what franken and Brent are showing you is that, it is seemingly easy to find TDC and many MFG's have OEM marks...

HOWEVER, you might have and engine with prior mods or simple rebuild. I can tell you...the torqued thickness of a cylinder base gasket can mess you up. If you understand the principles for knowing what true TDC is, then you can base line an engine case and mark it.

For street and daily driving? Hmmm, not so needed, but helpful. If you want to get the best o-the-beast? New bearings, true crank, piston, pin and base gasket istalled and torqued, that you can measure cold. Some peeps use different methods, but mine is to assemble cold...and mark. Then run it to normal temps and mark. For me all things equal, that is your range. It is simply knowing your zero points... (WE also check the ignition this way, just to understand potential)

Now, maybe I should look at Brents videos...