The mechanical clock pendulum length
A pendulum clock without a pendulum is a shame for sure. Hence, getting the mechanical clock pendulum length correct does take some diving into the clock world.
If there is no idea what pendulum it would take, this is a basic guide. Of course, this is a basic guide to narrow it down to the best pendulum for the clock. This will cover most situations, without special equipment to figure out the length.
The movement CM or PL stamp
On the back plate of the clock movement usually, there are some numbers and or letters for identification. Likewise, there may be an indication of how long the pendulum should be in the mix of these numbers.
It may say CM or PL and this stands for centimeter or pendulum length. Subsequently, this would be the length where it should keep approximate time. Generally, there are a few different ways to measure this depending on the country of origin.
Mechanical Clock Pendulum Length for German made movements
Most mechanical German made clock movements are easy to figure out the pendulum length. In essence, the numbers will let us know or the stamp will clearly state the CM or PL number. If it is German, keep in mind the CM length is not the actual pendulum length.
Altogether, German units measure this length from the top of the clock movement, and this includes the three components of the pendulum all in one length, in centimeters.
This length will include the pendulum itself, the leader that it hooks to, and the suspension spring on the top that the leader hooks on to, all in one CM measurement. To clarify, CM stands for centimeter and PL stands for pendulum length.
If no stamp try to convert
When there is no stamp with the CM on the movement, there are some additional steps that need to happen. You will need to cross reference The numbers on the back plate to the pendulum length that you need. The first step is to identify the manufacturer of the clock movement by using the movement numbers.
The movement numbers indicate who the actually manufacturer is of the movement. Various clock retailers may have a stamp of their name on it, but the numbers are always true to the manufacturer.
Converting numbers to CM length
Moreover, some movements do not have an indication of the pendulum length on the back plate. The unit will have a number that would need to be found on this website first to find out what the correct CM length is for it.
Conversion number example
Generally speaking, let's say that the movement has no CM stamp on the movement and only shows the Urgos number UW32319.
Go to the identification page to look at the movement number examples. In this example, we see that it is a Urgos. Now it is possible to go to the Urgos page and find the number to see what the pendulum length is.
In this example, the pendulum length would be 80cm, representing the full length with the size including the size of the bob.
German grandfather movements
Almost all grandfather clock movements are going to be German if made after 1950. The first thing we need to do is get the numbers off of the back plate of the movement. This is the only way, and we can't cheat by looking at the paperwork or clock case.
It has to come right off the back of the clockworks. The manual and the sticker on the case is of no use to get the pendulum.
American clocks
The American mechanical clock movement manufacturers refer to the pendulum length as a "drop". The drop is the length of the pendulum from the hand shaft all the way down to the bottom of the pendulum rating nut threads.
It is a different way of measuring the pendulum length then the German made way. The basis for both ways are on the smallest bob diameter. If the bob is larger or heavier, the length would be longer than what the stamp indicates.
No luck
If everything fails for one reason or another, the best chance for a pendulum is the wood stick style. This is the only style of pendulum that can be easily modified because it can be cut down.
These types of clocks are usually antiques, or of Asian origin, and information such as pendulum length is simply not available. Sometimes it takes a good guess on where the manufacturer meant to have the bob sit. To do it this way, you only need a pendulum with a stick that is way too long to begin with.
Chop it, try it, chop it, try it. Each time slowly cut an inch and half off of the sticks length and hang it back on. It will only take a couple of times, and if starting way too long it can always go shorter.
Clock makers method for Mechanical Clock Pendulum Lengths
There is another way, which is known as setting the beat rate. This is a more complex way and is usually just done by clock makers. It involves a beat detecting device that counts out the beats per hour, or the beats per minute. To do this, first figure out what the beat rate is suppose to be for that particular movement.
Then keep adjusting the pendulum length until it keeps time. There are cell phone apps out there these days that will tell the beat rate and the cell phone will listen and say if the clock will keep time with that pendulum. Go longer or shorter until the phone says the beat rate is set.
Contacting us for help with Mechanical Clock Pendulum Lengths
Please email the movement numbers from the back plate, and explain what part you need. The email address to send this information to is [emailprotected]. We always welcome pictures but they are not a necessity. If emailing pictures please include the back side of the clock movement where the markings are.
The content of this website is copyright by Clockworks and written by James Stoudenmire in year 2022
