Don't cut the spring yet, my best guess is that it is the battery. Try a 9.6 or 10.8 forget about fitting the crane stock for now just try to get it cycling. If the bigger battery does it you know the exsisting battery is dead or underpowered. This is a common mistake most people make and don't try new batteries before ripping apart the gear box. It happened to me (years ago) and a new proven 9.6v battery like a sanyo nicad made the mechbox run sweet. A no name cell custom 9.6v NIMH battery could not even cycle the piston and it would jam.


Your pictures are kind of fuzzy, but are the wires stripped or bare close to the motor opening? They look like they possibly could be. If they are it could be shorting out and why it will not cycle

When you put it together with just the gears in, did you screw down the mechbox?

This is a really good point, if you don't and just finger hold it, it is not the same. Unless you put the screws back in you can never be sure the gears are free wheeling because everyones "grip" is different when thet just hold a mechbox together.

Edit:

Here is a good link http://pageproducer.arczip.com/daedalus03/shim.html

It sounds exactly like your battery is too weak.

Throw a 9.6v large on there and it will work again.

If it worked with this same spring before, you have done something to make the gears tighter that makes the motor have to work too hard to move the gears. Improper shimming (what everyone has said about putting screws in first to see how well gears spin freely) probably. Or your current battery might just be on it's last legs and doesn't have enough discharge rate to turn your motor over at the very peak of spring compression where the most torque is needed.