There are two common problems that develop with the coil coupler over time; the plunger spring becomes weak and the knuckle won't stay closed, or the knuckle spring breaks or loses its tension and the coupler won't come open without outside intervention.

Here are the components of the Lionel coil coupler.

Here's the method I use to replace those springs.

1. Use a pair of sharp wire cutters to straighten the peened end of the rivet and pull the rivet out from the top. Hold the knuckle in place until you can gently ease it out of the coupler head. The coil plunger, plunger spring, knuckle, and knuckle spring are freed once the rivet is removed.

If this method doesn't work for you, you can file or grind the head off the rivet and then pull it out from the bottom.

You will need a replacement knuckle spring, a new rivet, and a new plunger spring if yours is weak or compressed. My recommendation is to replace both springs as long as you have the coupler apart. The expense is minimal.

2. Follow these steps to reassemble the coupler.

a. Ensure the plunger spring and plunger are in the coupler head (spring goes in first).

b. Insert the new rivet in the coupler top hinge and through the knuckle spring loop. You can insert the rivet most of the way to contain the spring. The long arm of the knuckle spring is against the coupler head side.

c. Slide the spring underneath the top hinge; keeping the rivet inserted just far enough to hold the spring.  Apply a tiny bit of lightweight grease on the rivet to help hold the knuckle spring in place.

d. Smear a small amount of oil on the rounded end of the knuckle where it engages the plunger. This will make closing the coupler easier. Now ease the knuckle between the hinges keeping the spring at the top of the knuckle. Rotate the knuckle into the head and push the rivet through the hole in the knuckle and then through the bottom hinge.

e. Test the operation of the coupler by attaching one lead from a test transformer (dialed to 12 volts) to the coupler frame and touching the slider shoe rivet with the other test lead. The coupler should open. Close it and test again to ensure smooth operation. You can oil the pivot points of the knuckle with a lightweight oil to reduce friction.

f. Turn the coupler over and rest the rivet head on a solid metal surface (like a vise) and peen the bottom of the rivet with a center punch or gently tap the end with the rounded head of a small ball peen hammer. Flatten the rivet end enough to keep it from coming out.

The magnetic couplers are similar in construction to the coil couplers except the knuckle is locked in place with a pin through a hole in the bottom of the coupler head, thus eliminating the coil, the coil plunger, and the plunger spring. The knuckle and knuckle spring are still utilized so the technique of replacing the spring is very similar.