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Recognising Lock Bumping
Lock Bumping

By Chris Belcher

It has often been said that lock bumping is difficult to detect without the use of special equipment.

With the help of this guide, locksmiths who have to deal with these situations should be able to determine with reasonable accuracy, if indeed a cylinder has been damaged due to lock bumping, using just the naked eye, and a few common tools.

There have been over the years, misconceptions of how the mechanics of bumping works. This article shows how just one part of the process causes enough damage to allow accurate identification of bumping.

Keyway Damage

A few forms of lock bumping cause damage to the front face of the cylinder core; chrome plated cylinders are more resistant to this damage than brass, because chrome plating gives a hard protection to the metal underneath.

The other forms of bumping, which incorporate a spring or spacer to dampen the strike or return the bump key to its start position, may leave little or no marking on the front face of the cylinder core. Because of this, in all cases of suspected illegal non- destructive entry, marked or not, an inspection should be carried out. Also, use of the correct key, over a period of time will have similar effects as seen on the lighter marked cylinders in the photos.

Basic Diagram of Pin Cylinder

To fully understand what causes the internal damage to a cylinder when a bump strike is made, you need to understand what causes the damage. To do that you need to know part of the mechanics of bumping.

This diagram shows the top and bottom pins at the shear line during the lock bumping process:



A: Driver or Top Pin Chamber

B: Spring

C: Driver or Top Pin

D: Shear line

E: Plug or Bottom Pin Chamber

F: Plug or Bottom Pin

G: Cylinder Plus

H: Keyway

Lock Bumping Action Mechanics


  • When tension is applied to the cylinder core, it rotates fractionally, trapping the driver / top pin at the shear line.
  • At this point it is in contact with the plug / bottom pin.
  • The top and bottom chambers are misaligned at the shear line.
  • When the bottom pin is struck it moves upwards taking with it the top pin until it reaches the shear line (they do not separate before this).
  • The bottom pin then strikes the rim of the upper chamber at its entrance because of the misalignment, so causing distinctive marks at the entrance to the driver/ top pin chamber.
  • The lower pin quite often stops at this point and sometimes will carry on moving upwards.
  • In some cases the lower pin can completely enter the upper chamber damaging the spring.
  • As the lower pin passes the shear line the damage to the upper pin chamber causes scratching to the body of the lower pin. (See photographs below)


By putting all this information together, it becomes relatively easy to determine if a cylinder has been bumped.

NOTE: These tests were carried out on a variety of cylinders including: rim cylinders, where the plugs are at the bottom and cylinders such as euro-profile cylinders whose plugs are at the top.

Spring damage

Now we come to the damage to the springs.

Below is a photograph of three springs; the left hand spring in the 1st photograph is an undamaged spring from a bumped cylinder.


The centre spring is a damaged spring from the same cylinder. This is due to the length of the bottom and top pins and the strength of the bump strike.

The photograph on the right shows yet another damaged spring from a bumped cylinder.

Therefore it should be part of the inspection to see that all of the springs are still relatively uniform.