LOL...
it will have to come off if you drop the chain ....LOL

Really, you need to get hold of some good books before you do this... there are many warnings and specific instructions in the manual which are necessary for you to have a good experience with this job....
Did you know that you need to keep CONSTANT pressure on the chain during this whole process ?
Do you have a replacement chain in there already or are you going to have to grind off some rivets ?
Have you removed your tensioner ?
Greg

Greg, I always thought this to be the situation, as well. But, several folks simply drop the chain into the abyss and fish it back out. It won't fall off the crankshaft or the IP timer. Surprising.........but true. All the efforts to keep the chain taut are largely superfluos. This is with the 617 engine.