How far does the crank gear need to go on? I cant find this in my manual?

Should go on until it bottoms against the shoulder on the crank.

Are you talking an aftermarket gear? Some of them are too tight - I had my machinist hone mine out so that it would go on all the way.

This is a rollmaster set...

Even on the Rollmaster sets, I've never had a problem getting them to seat all the way on the crank snout after 5 minutes heating up the gear with a mapp gas torch (now taking the Rollmaster gear back off leaves a lot to be desired).

FYI, if you are using a large socket or piece of pipe and using a hammer to seat the crank gear, your #3 bearing will need to be replaced before doing any more assembly.

-D.Idiot

Why just number 3? And what does it do?

That's the thrust bearing for the engine...It takes enough abuse doing it's job, hammering on an installed crank is a sure way to put the crank end play out of spec and trash this bearing.