How do you loosen a Honda crank pulley bolt?

Use a 50mm holding tool and a couple of breaker bars. I have a 1991 Honda CRX HF and it does not have an internal 50mm hex for the holding tool. Why is this so?

I'm not sure why it doesn't have the 50mm hex, but what we did to both tighten and loosen the bolt on my 1991 Accord is put the car into gear (5th for 5 speed manuals) and have someone step on the brake. Also put on the parking brake. Then using a big breaker bar and a few extenders put pressure on the bolt. Use a jackstand to support the extenders. It's going to take a decent amount of effort so stay with it. Honda torques those bolts to 300 foot-lbs. PB Lube or WD-40 help a lot. And if you break the bolt it's a 14x1.25mm bolt that's about 2in long. Honda sells them for $20 or so, so don't break it.