I am replacing the locks on a property and want to change the deadlock. Firstly there are two depths to the locks: 64 mm (2.5") and 76mm (3") under the same BS3621. After returning the first lock I bought and buying the correct depth (with the same BS3621) the new lock has other subtly different dimensions:

  1. the body of the lock is taller by a few millimeters (89mm vs 76mm) so it doesn't fit in the hole already there.
  2. the position of the key hole is a different distance from the door edge by 4mm and is lower by 10mm.
  3. the deadbolt is taller (36mm vs 30mm) so doesn't fit into the strike plate's smaller recess.

I'm sure there are other things that are different but this is clearly not a standard, or it is a standard that has changed or been narrowed over time.

I tried to look up the old installer / suppler but they've both closed down.

My question is: before I buy the third lock, how do I tell it's going to fit? Clearly BS3621 defines some thing but exact dimensions is not one of them. What can I do? (You can imagine what I think bs stands for right about now)

2 Answers 2


The British Standard is about the quality of the lock ie :

its burst strength,

how difficult it is to pick etc

The standard is not about the size, length depth etc.

As an example cars have to meet British and European standards but come in different sizes...

This means you need to measure any new locks you look at before buying so you know that they will fit, much easier than doing many round trips to the store and getting refunds....

  • Thank you @SolarMike I have been buying them online so it hasn't been possible to measure them. Will give online one last go then will have to revert to the old fashioned approach and use my legs.
    – AJP
    Commented May 31, 2019 at 11:11

The classical way you handle this situation is to pull the old part out and take it to the store, precisely so you can compare dimensions and show it to a clerk to tap his experience.

You prefer mail order. That works much better if sizes are standardized to one size, because then, you would not have to learn things about lock fitment. As a direct result, you have formed the expectation in your mind - because that is what will work the best for you.

It's like "surely the speed limit must be 120kph, because I'm late".

That is why you are concerned with BS 3621 - you desire size standardization to exist, you hope there is a standards document that calls it out, and so you latched onto (heh) the first standards document that seemed to relate. It says nothing about lock dimensions.

I recommend you make yourself a sceptic of thought processes like these so you can stop being snared by them.

Also I suggest you run, do not walk -- I mean walk, do not type to the nearest appropriate hardware store, old lock in hand, and have the clerk help you, then cheerfully pay his retail price for the needed part. While this will be more expensive than 1 mail order lock, it will certainly be cheaper than "education via buying the wrong parts".

P.S. I just spent a half day fiddling around with a fuel pump which arrived with the wrong fittings necessitating a fuel line reroute, and the bar was bent wrong so it wasn't even touching the eccentric (hence not pumping). A quick search of the Web reveals all modern fuel pumps are similar. So I feel your pain. I would have much rather just walked into the Stihl dealer, however this is not a Stihl.

  • Yeah you nailed it @Harper. It's completely my misassumption. I looked at the face plate and found a "standard" stamped on it. I removed the lock body and found there were no other reasonable descriptors. Together thought that would be enough: clearly wrong. > sceptic of thought processes so you can stop being snared Thanks for your message. It's always a balance. Sometimes you're too sceptical. Other times not enough. This ones the latter. > nearest appropriate hardware store Agreed. They've closed down, but I should have searched for another. Good luck with the fuel pump.
    – AJP
    Commented May 31, 2019 at 14:52

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