Depending on the engine size and year of manufacture, the BB can be several degrees advanced from TDC, and it's not always the same distance. It is an ignition timing mark, not a true TDC indicator. It's "close, but no seegar". There is a straight line TDC mark on some flywheels, but it's not the BB.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!