Front door lock larva: how to buy and replace with your own hands

Recommendation points

Thanks to the compact key and simple installation procedure, cylinder-type locks have become the most common type of secret mechanism. Today he will tell you how these devices work, how the cheap ones differ from the expensive ones, and why an incorrect replacement can cause a hack.

Front door lock cylinder

How does the cylinder mechanism work

Cylinder locks consist of a secret and a locking part, that is, a body. Resistance to opening by intelligent methods does not depend in any way on the properties of the latter, requirements are imposed on these nodes mainly in terms of reliability, wear resistance and durability.

Certain hacking methods bypassing the secret part are rarely practiced by intruders due to the significant noise and duration of the process. However, the owner of a locked home is provided with a relatively simple option of non-criminal burglary of the door if it is blocked..

The device of the cylinder lock mechanismThe device of the cylinder lock mechanism

The secret part of the lock is represented by a cylinder mechanism or a larva. It consists of two parts – a body in the form of a cylindrical sleeve with a rack and a so-called plug – an internal freely rotating core, mechanically connected to the drive cam. The plug and the inner surface of the sleeve have seats in the form of through and blind holes. Inside them there are pairs of code and thrust pins with springs, all this together forms the code mechanism of the lock.

When the key is inserted, the code pins rest against the grooves on its working part, while the stop pins are located with their edges exactly on the border between the plug and the body, without interfering with the rotation of the mechanism. The security class of the lock and the maximum possible number of code combinations depend on the number and configuration of pins, while the strength properties of the case determine the lock’s resistance to burglary by force methods..

The lock

In addition to pin there are also disc and rack and pinion cylinder mechanisms, the name of which comes from the type of code elements. Despite the somewhat different principle of building the code combination, the essence of the cylinder’s operation does not change from this, and no type of locks can boast of any outstanding qualities, it all depends on the quality of the structure and the presence of additional protections.

How locks are broken

There are two types of methods for breaking cylinder locks:

  1. Clean (smart, manipulative) methods have the goal of choosing the correct combination so that the edges of all stop pins are located on the border between the plug and the body. Each lock has its own tolerance for the dimensions of the parts. The larger it is, the easier it is to ensure the stable operation of the mechanism, but at the same time the resistance to intelligent hacking decreases..
  2. Forceful or vandal methods involve the destruction of a certain part of the mechanism in order to disable the secret of the lock or significantly simplify it. The bodies and plugs of locks are usually made of rather soft, but at the same time fragile brass, which allows burglars to literally turn the larva into shavings and directly affect the drive part of the lock.

Opening the front door lock without damaging

Pure autopsy methods include:

  • Busting with picks: the pins are moved one at a time with a thin tool until they all stop interfering with the scrolling of the plug.
  • Vibration or shock bumping: a special shaped key is inserted into the well, with the help of which all the pins are “shaken up” simultaneously. At some point, they inevitably end up in the right position..
  • Making a duplicate key from a mold.

Opening the lock with master keysOpening the lock with master keys

Opening the lock with vibration bumpingOpening the lock with vibration bumping

Among the forceful hacking methods, the most popular are:

  • Drilling out the lock post where the main code elements are located.
  • Spot drilling along the line of joint between the plug and the body, where additional code elements are located.
  • Breaking the lock for the part of the larva protruding from the door. Any cylinder mechanism has the weakest point at the point where the fastening screw is screwed in at the bottom of the rack, breaking along this line completely deprives the lock of the secret part and opens access to the drive cam.
  • Pulling out the larva by the screw screwed into it.
  • Hacking with a convolution: a rigid steel plate is introduced into the well, sharpened along the profile of the key, then the plug is turned by force with the destruction of the code group.

Picking locks by milling and pulling out the larvaBreaking open locks by milling and pulling out the larva by the screw

Each of these methods can be used only if the lock does not provide additional protection and strengthening of problem areas.

Weaknesses and design flaws

The main disadvantage of cheap locks is the material from which they are made. The keys and parts of the code mechanism in the cheapest devices are made of brass, which is not capable of resisting vandal destruction and has an extremely low wear resistance. Because of this, the resource of the lock is very limited and over time it may not be possible to open it even with a native key. In a high-quality lock, the code elements and the key are made of nickel silver.

Opening the lock by drilling the larva

The second problematic part is the loosening in the area of ​​the post where the hole for the mounting screw is located. In high-quality locks, the connecting neck is made of solid, but at the same time, sufficiently ductile metal, for example, stainless steel.

Key profile – its shape in cross-section without taking into account the code grooves. For keys with a common profile, blanks are available for sale, but if the profile is patented, it will be possible to make a duplicate only in the company workshop using an individual cutting code. An unnecessarily short neck can also be attributed to the problem of keys, which does not allow installing a full-fledged armored plate on the door.

Examples of quality cylinder larvae

The understanding of which locks use really effective solutions, and what to consider as useless bells and whistles, appears after getting acquainted with the highest quality samples of cylinder mechanisms. Here are some examples:

ABUS Bravus MX PRO– German locks with enhanced protection against burglary. The hard-alloy shackle at the beginning of the hole prevents drilling and screwing in the self-tapping screw. The body is artificially weakened by transverse cuts: when trying to break off half of the larva, only its edge will break off, while part of the secret mechanism will remain in working order and can be opened with a key. A hardened insert is pressed into the body post, turned under a taper, which makes it impossible to center the drill, there is also a carbide element at the interface between the plug and the body.

Cylinder ABUS Bravus MX

Cylinder ABUS Bravus MX

Kale Kilit OBS Series– one of the best locks in the budget segment from a Turkish manufacturer. It has two rows of locking pins, which significantly complicate intelligent burglary methods, plus all the parts of the lock are made with fairly small tolerances, although the overall seriality of the code elements is low. To protect against drilling, a pair of hardened pins are provided on the edge of the rack, breaking the lock is also not easy: the body material is viscous and at the same time quite strong.


Apecs N6– another representative of inexpensive locks, in which protection against various types of burglary is well implemented. The first few pins are made of hardened steel and are resistant to drilling. The rest have anti-lock design: the shape of a dumbbell blocks the pin during an attempt to move it with a loaded plug. Carbide inserts are pressed into both the body post and the core, making it difficult to drill by all common methods. One of the code elements is located in a specially shaped channel that limits the protrusion of the pin, which makes it impossible to bump the lock. The neck of the lock is reinforced with a steel bracket, and the body is weakened by transverse slots, which eliminates the possibility of breaking along the cam line.

Cylinder mechanisms APECS N6

EVVA (Series 3KS, EPS, MCS)– modular cylindrical larvae with a slider operating principle of the code group. All pins have anti-lock grooves, the lock is not subject to bumping, reinforcing inserts made of tungsten carbide protect from drilling, the post is reinforced with a hardened steel core and is not subject to breaking. Products of this Austrian brand are rightfully considered the best cylinder mechanisms, but the price is appropriate: some models will cost the Russian consumer $ 200-250.

Cylinder mechanisms EVVA 3KS

How to choose the right cylinder mechanism and install it

In addition to protective properties, the size of the cylinder larva is also important, which is determined by:

  1. Cross-sectional dimensions.
  2. Body length.
  3. Offset of drive cam and mounting hole.

In the CIS, DIN standard cylinder mechanisms with a case diameter of 17 mm and a height of 33 mm are widely used. The length of the larva should be such that its protrusion from the door or armored lining is no more than 2 mm, otherwise the lock is potentially vulnerable to breaking or being pulled out. It is better to determine the displacement of the cam using a sample, otherwise it will simply not be possible to correctly install the core. In matters of choosing a standard size, locks with a modular device have a great advantage: with a relatively small warehouse content, the seller will quickly assemble a mechanism with the desired length and offset without restrictions in choosing a secret part. Also, instead of the cam, a drive gear can be installed for compatibility with some models of lock cases.

Replacing the front door lock cylinder

The installation of the larva is extremely simple. It is necessary to unscrew the fixing screw from the end of the door located at the levels of the lower part of the rack. The free exit of the mechanism is impeded by the cam, which is turned by 10-15 ° when the key is removed. The key must be inserted and turned slightly towards the opening. The installation of a new larva is carried out in the reverse order..

Rate the article
( No ratings yet )
Add comments

;-) :| :x :twisted: :smile: :shock: :sad: :roll: :razz: :oops: :o :mrgreen: :lol: :idea: :grin: :evil: :cry: :cool: :arrow: :???: :?: :!: