Why is my LEGO® Technic model stuck or hard to turn?

A Technic mechanism should turn smoothly by hand, without a motor. If it is stiff or blocks, a part is rarely broken. Almost always a part is a little out of place.

The margins are small. If a gear sits a millimetre too far to the left on its axle, it no longer meshes with the next gear. A millimetre too far to the right and it jams against it. From the outside the model still looks perfectly right.

First find where it jams

Turn the mechanism slowly by hand and feel what happens:

It is stiff from the start.
It blocks at one fixed point.
It gets stiffer the further you turn.
It only jams in one direction.

Turn through a full cycle, not just a few degrees. If it always blocks at exactly the same point, something is touching there that should not.

Check where the gears sit on the axle

Do not only check that the right gear was used, but also where on the axle it sits. An axle runs through beams, spacers and bushes. If one is missing or in a different place, everything after it shifts.

Are all bushes and spacers where the instructions put them?
Does each gear sit in the right position, not one place along?
Is the axle fully through the part, and not sticking out too far?
Are the beams the right distance apart?

Two gears too far apart do not mesh properly. Too close together and it runs stiff. A motor notices that sooner than your hand does.

Are parts touching where they should not?

While turning, watch for a gear or axle rubbing against a beam, a panel or the inside of the model. A bush rubbing past another part feels the same as a gear that jams.

A mistake from step 37 shows up at step 80

A beam one hole out of place moves an axle. That moves a gear, and that changes the distance between two gears. So search back to the moment the mechanism still turned freely. Checking only the last step is often too late.

Do not let the motor solve it

A motor will push through a misaligned mechanism. That puts extra load on gears, axles and connectors. Fix the mechanical cause first.

How to go about it

Turn by hand and force nothing.
Find the exact point where it blocks.
Check the distance and position of the gears on the axle.
Check bushes and spacers.
Check that the axles are the right length and fully seated.
Check beams and frames for one hole of difference.
Build back to the step where it still turned freely.

Test while you build

Turn a gearbox or a differential as soon as it is finished, not when the whole model is done. If it turns freely then, the fault is in a later step. On a vehicle, let both wheel axles spin freely before you connect the drivetrain.

With Technic, precision counts for more than force.