To check if a padlock will fit a security chain, you need to compare the padlock's internal clearance with the internal dimensions of the chain link - not just the shackle thickness.

It's also important to balance out the security offered by the padlock and the chain. There's no point having a Sold Secure Gold rated chain if you use a £10 padlock. The thieves will usually target the weakest security first.

Why padlock compatibility matters

A padlock that doesn't fit correctly can:

• Be impossible to use at all
• Sit loosely, creating leverage points
• Force the chain to rest on the ground, reducing security

This is one of the most common causes of frustration when pairing locks and chains - especially with thicker, high-security chains.

The two measurements that actually matter

When checking compatibility, ignore brand names and marketing terms and focus on internal dimensions.

You need to know:

• Internal width of the chain link. This is the narrowest point inside the chain link where the padlock must pass through.
  
  
• Internal height of the padlock shackle or locking aperture. This is the usable clearance inside the padlock, not the external size.

If the padlock cannot pass cleanly through the chain link without forcing or twisting, it does not fit.

Why shackle thickness alone is misleading

Many padlocks advertise shackle thickness (e.g. 10 mm, 12 mm, 14 mm), but this figure alone tells you very little.

A thick shackle:

• May still have limited internal clearance
• May not fit through tightly spaced chain links
• May only work with specific chain designs

Compatibility is determined by clearance, not strength on paper.

How to check compatibility yourself

If you already own the chain:

• Measure the internal width of a link at its narrowest point using a ruler or calipers
• Compare this with the padlock's internal dimensions

If you don't own the chain yet:

• Check the manufacturer's stated internal link dimensions
• Compare them to the padlock's clearance, not its external size

If dimensions are close, allow a small margin - chains are rigid and don't behave like rope.

Closed-shackle vs open-shackle considerations

Closed-shackle padlocks:

• Offer better resistance to cutting
• Require more internal clearance to fit correctly

Open-shackle padlocks:

• Are more forgiving on fit
• Expose more of the shackle to attack

Always prioritise fit first, then security level.

To sum up

To check if a padlock will fit a security chain:

• Compare internal dimensions, not advertised sizes
• Ensure the padlock passes through the chain link cleanly
• Avoid forcing tight fits as you will need a bit of clearance
• Remember that a well-fitting lock is more secure than a stronger lock that doesn't fit properly

If in doubt, consult compatibility guidance for specific lock and chain combinations.