Pendulums hang on thin pieces of suspension steel normally incorporated in the suspension unit, which vary according to the type of clock. A distorted suspension will greatly hinder the going of the clock so always replace a doubtful one.
British Longcase clocks generally have a pendulum that swings once per second because the seconds hand is directly connected to the escape wheel. This make selection of a replacement very easy. It is a thin strip of suspension spring steel with a brass block at the top and another at the bottom.
The thickness & width of the steel are not critical. The length can be adjusted but generally it's simple to raire or lower the bob as necessary.
English Fusee clocks are also very similar to each other. The length is slightly more critical because most pendulum rods have a slot that must line up more or less with the crutch. Snap off the unwanted part. Place it in position and re-use the original pin. Don't attempt to make a hole in the suspension steel. Use the pin as a punch and tap it with a clockmaker's hammer.
French clocks are quite the opposite. There was no standard size and the chances of finding a suspension 'off the shelf' that will fit are very slim. This is especially true if the clock has a time adjuster near the numeral 12 on the dial that changes the effective length of the steel. Worse still are the early Brocot adjusters that enclosed the suspension in a groove inside brass blocks.
There is little alternative but to file off any surplus brass from the nearest suspension you could find but take great care when doing this. If you don't clamp the two pinned parts together there is a high risk of them coming apart.
400 Day suspensions can only be identified by knowing the make & model. Measuring is pointless because you would need to know the characteristics of the steel as well as its physical size. www.anniversaryclocks.org will help you to identify the model.
American Clocks were manufactured with little thought of tolerances and generally 'One size fits all' where suspensions are concerned. Form your own hook and cut off the surplus. Don't cut it until you are sure that timekeeping is correct. The bob isn't always where you might expect, even when there is an aperture to see it through.
The T-bar type is an example of an exception to this rule.
Early ones were made by flattening the wire into a 'feather'. Later ones were made by riveting spring steel to the wire rod and these are better. Both types are still made mostly for the sake of authenticity.
Repairing Your Own Clocks by Mervyn Passmore