Replacing a suspension spring on a Seth Thomas #2 Regulator

It should have been an easy fix but often when working with clock movements unexpected issues occasionally crop up.

The clock was purchased in the fall of 2018. This Seth Thomas #2 Regulator has had little done to it except oiling the movement and cleaning up the case. It is one of the most accurate mechanical clocks in my collection which is no surprise since these clocks were originally designed for offices and railways.

According to an online database, the lower section of the case was redesigned in 1922. This allows me to date the clock somewhere between 1922 and 1929. 77 weight-driven movements are very common. They were made sometime after 1915 and well into the 1940s. Perhaps the letter “K” under the 77A stamp on the movement is a clue as to exactly when it was manufactured.

It has a very attractive mahogany finish and it is a real conversation piece. There are probably more oak regulators than any other type of wood, so, mahogany, although not rare, is uncommon.

Seth Thomas Regulator #2
Seth Thomas Regulator #2

The suspension spring

A suspension spring is a thin band of steel called a “spring” by horologists by which the pendulum of a clock is suspended. It separates the pendulum rod and bob from the mounting post. Its purpose is to assist in controlling the rate of the pendulum swing.

The suspension spring looked tired when I first inspected the clock in 2018 and it was time for a replacement, sourced from my go-to Canadian supplier, Perrins.

Seth Thomas Regulator #2
Seth Thomas Regulator #2 base design
What should have taken minutes stretched to an hour or more
Seth Thomas Regulator #2
Seth Thomas Regulator #2 (77A (K)), the iron bracket is just behind the movement

Removing the movement prior to replacing the spring

Removing the movement consists of first removing the hands. A screw secures the minute hand while the hour hand is a friction fit and pulls straight out. The second hand also pulls off. Eight screws hold the face in place; 6 outside the chapter ring and two on either side of the second hand. Once the face is removed there is a wood crossbar, held by two larger screws in front of and either side of the movement, that must be removed.

Then the weight, which is hooked onto the pulley, is removed. There are 4 posts on the front of the movement, inboard on the bottom and above the plate screws on the top. They do not come out entirely but once unscrewed, the movement is released from an iron mounting bracket.

Next, the pendulum/rod which is hooked onto the suspension spring on the bracket is lifted out and put aside.

bracket for Seth Thomas #2
The movement is mounted on a heavy cast iron bracket

Replacing the suspension spring

Once the bracket was exposed I thought it would be a simple matter of swapping out the old suspension spring for the new one. If it was not original, it is, nevertheless, quite old and likely weak with age. It probably can be done in place but it is much easier simply removing the cast iron mounting bracket which is held in place by 4 wood screws.

Seth Thomas #2 suspension spring
Seth Thomas #2 suspension spring, old on left, new on right

Laying the bracket out flat simplifies pushing out the pin holding the suspension spring.

bracket for Seth Thomas #2
Movement bracket for Seth Thomas #2

I have come to learn that replacement parts from a supplier often must be made to fit and the suspension spring I bought is no exception. Using a pair of pliers, the pin was pushed out of the block. Once out I reamed the hole so that I did not have to struggle to install it. Except that the new pin did not fit the smaller hole in the post. So, rather than use the old pin a tapered brass pin replaced it.

Re-installation and testing

The bracket is then screwed back into the case with the suspension spring in place. The pendulum attaches to the end of the horizontal pins of the suspension spring but it is important to ensure that the end of the crutch wire, which has a 90-degree bend and hangs down from the movement goes through the opening in the pendulum.

The movement, face, and hands go back onto the clock. Push the hour hand in far enough, otherwise, it will rub against the minute hand and stop the clock.

There is very little space between the hour hand and the second’s hand. This is by design, and if you do not push the second’s hand in far enough you will know soon enough when interference stops the clock.

Level the clock case on the wall and observe the action of the pendulum.

What should have taken minutes stretched to over an hour. Now that the new suspension spring is in place, I have had a chance to look over the movement and it will be scheduled for a full servicing in the next month or so.

In the meantime, the clock is running well; there is slightly more amplitude in the pendulum swing than previously and it is keeping excellent time.


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