[MagicRat]

The balancer does have an 'interference fit' on the crankshaft so it's like being pressed-on. However, that tight fit is there to prevent harmful vibration and looseness. The tight fit is not intended to hold the balancer on all by itself.

The bolt is necessary. Without it, you could break a crankshaft. IMHO you have been lucky so far.

If you cannot see the threads or see a hole in the front of the crank it's possible the bolt head has snapped off, leaving the shaft plugging the hole.

Get a balancer puller and remove the balancer. If the bolt-gods are smiling, there may be some protruding bolt left. If so, you can get a good grip on it with vice grips or use a stud extractor, or weld a nut to the end of the broken bolt to turn it.

Then get a new bolt and install it. Go to an auto parts place to source the correct bolt for the application. Don't use a cheapie hardware store bolt.

If it has snapped off flush with the crankshaft itself, you could try drilling a hole in the broken bolt piece and using an extractor to get it out. However, if you have never done this before, it may be better to take the whole engine or car to a machine shop and let them extract it for you and fit a new one. Often, efforts to extract bolts by people unfamiliar withthe process only makes matters worse.