4-4-0 american worm gear

[harry40]

Hello, I have a 4-4-0 American almost new. The drive shaft broke. I purchased a new one but cannot get the engine
to run. the motor turns the drive shaft but the worm gear backs out and just spins and does not engage the wheels.
If you put the worm gear back in and turn the drive shaft by hand the wheels engage and turn ok. I put in a new
worm gear and it does the same way. It seems like the drive shaft is too short. I purchased both shaft and worm
from Bachmann. I would sure appreciate any help. Thanks. Harry