← Glock 21 reference page · This work order as JSON ·
Work order — Glock 21
Taking the firearm apart for a job such as refinishing, and putting it back together to specification. Generated from the Pew-Takedown entry last updated 2026-10-29. Confidence: high.
| Work order # | |
|---|---|
| Customer | |
| Model on the bench | |
| Serial number | |
| Finish / colour | |
| Received | |
| Technician |
Before you startEducational guide only. Some information comes from non-authoritative sources. Always consult your manufacturer's manual and a licensed gunsmith. Verify your firearm is unloaded before every procedure.
- Glock Ges.m.b.H. limits owner disassembly: Field strip only: slide, recoil spring assembly, barrel and frame.
Shop difficulty: Tier 1 of 3
About 37 line items in GLOCK's list including the magazine; pins and a few small springs, punch-only work. The traps are minor and well documented: the slide cover plate needs the spacer sleeve and bearing held down, and the slide stop lever is held while the trigger pin goes in, after the locking block pin. Same as the Glock 19 (the reference Tier 1 pistol). 37 parts (Stand-in: GLOCK's G17 Gen4 list (GLOCK publishes no list for this model); complete pistol incl. magazine). Pew-Takedown's rating, not the manufacturer's.
Bench time: strip ~15 min, rebuild ~20 min (estimate). Actual: strip ______ rebuild ______
Special tools:
- 3/32 in. (2.5 mm) pin punch / GLOCK disassembly tool — Trigger, housing and locking block pins, slide cover plate
- GLOCK front sight tool (3/16 in. hex nut driver) — Front sight screw
- Rear sight pusher (GLOCK rear sight mounting tool) — Dovetail rear sight
Watch for:
- The slide cover plate only goes back on if the firing pin spacer sleeve and the spring-loaded bearing are both held depressed while the plate is pushed up; on removal the internal parts come out under spring tension. (both; GLOCK Ges.m.b.H. (scan hosted on archive.org))
- The spring-loaded slide stop lever has to be held forward in its recess while the trigger pin is pushed through it, the one place a Glock asks for a part held under tension while a pin goes in. (reassembly; GLOCK Ges.m.b.H. (scan hosted on archive.org))
- The pins go in a fixed order: the locking block pin first because it bears on the slide stop lever spring; out of order the lever has no tension and the slide can lock back early. (reassembly; GLOCK Ges.m.b.H. (scan hosted on archive.org))
- The trigger pin is held by the slide stop lever sitting in its groove and will not push out until the lever is wiggled off it; new guns need more force. (disassembly; GLOCK Ges.m.b.H. (scan hosted on archive.org))
- The slide lock (takedown lever) is orientation-sensitive and can be reinstalled backwards. (reassembly; Apex Gunsmithing)
1. Clear and make safe
- Point the muzzle in a safe direction and keep your finger off the trigger.
- Press the magazine release, remove the magazine, and set it away from the work area.
- Retract the slide fully to eject any chambered round and lock it open on the slide stop lever.
- Visually inspect the chamber and magazine well, then physically feel the chamber to confirm it is empty.
Warning: Both checks, every time. The next procedure requires pressing the trigger. - Remove all live ammunition from the work surface.
2. Disassemble
Field Strip
- Complete the Clear and Make Safe procedure, and put on safety glasses.
Warning: The manual requires safety glasses when disassembling — components are under spring pressure. - Release the slide to the forward position, either by pressing the slide stop lever down or by pulling the slide fully back and releasing it.
- With the muzzle pointed in a safe direction, pull the trigger. The trigger must be in its rearmost position for the slide to come off.
Warning: Verify the chamber is empty before this step. If the trigger is in the forward position, the pistol may be loaded. - Hold the pistol with your firing hand, fingers across the top of the rear of the slide in front of the rear sight and thumb on the rear of the frame.
- Using your fingers, retract the slide about 1/8 in (3 mm) and hold it there.
Tip: Pull it too far and the trigger resets to the forward position. If that happens, pull the trigger again with the muzzle in a safe direction and start over. - Using the thumb and index finger of your non-firing hand, pull the slide lock down evenly on both sides.
- While holding the slide lock down, push the slide forward and off the frame.
- Press the recoil spring assembly toward the front (muzzle end) of the slide while pulling it up to remove it.
Warning: It is under spring tension. Keep control of it. - Holding the barrel by the barrel locking cams, lift it and push it toward the front of the slide, then pull it up and toward the rear to remove it.
Tip: Forward first, then up and rearward. It does not simply lift out. - Stop here for routine cleaning. The frame and slide internals do not need to come apart for normal maintenance.
3. Refinish
The components the procedures above separate. Coating, masking and cure are outside this database; follow your coating system's instructions. A part the manufacturer marks "factory fit" should not be swapped or reworked.
| Component separated | Out | Coated | Back in |
|---|---|---|---|
| Slide assembly | |||
| Recoil spring assembly | |||
| Barrel |
| Component | Material | Factory finish | Source |
|---|---|---|---|
| Frame G21 Gen4 | polymer | — | [1] |
| Slide G21 Gen4 | — | matte black | [1] |
| Sights standard front sights (steel sights optional) | polymer | — | [1] |
4. Reassemble
Reassembly
- Hold the slide upside down, insert the barrel, and seat it fully by its locking cams.
- Insert one end of the recoil spring assembly into the slide and press the other end toward the front of the slide to compress the spring, pushing down until the large end rests in the second, semi-circular notch on the barrel locking cams.
Tip: The second semi-circular notch on the locking cams is the seat. If the assembly keeps popping out, it is not in that notch. - Holding the frame in your firing hand with your finger off the trigger and outside the trigger guard, place the slide on top of the frame in front of the slide rails.
- Push the slide toward the back of the frame until the slide lock snaps up and locks the slide into place.
Warning: Do not hold the slide lock down during reassembly. It snaps up on its own. - Release the slide and let it run forward into battery, then perform the function check.
5. Torque
| Fastener | Specification | Sequence | Source | Applied | Init. |
|---|---|---|---|---|---|
| MOS adapter plate screws (2 × M3x6 Torx, T10) GLOCK pistol models in MOS configuration with GLOCK MOS Adapter Plates 01–08 (this model: G21 Gen5 MOS) |
1.5 N·m 13 in-lb · 1.1 ft-lb |
Field strip; place slide flat. Remove the GLOCK cover plate (the removed screws should not be used for fixing the adapter plates). Select the plate, markings facing up. Tighten the 2 screws (the set's M3x6 Torx screws, which come with a drop of glue applied). 'Make sure to use only these screws for fixing the adapter plates.' Verify tightness of the screws depending on ammunition and frequency of use; if you often screw on and unscrew, apply another drop of threadlocking adhesive to the screw threads. Threadlocker: Screws come with drop of glue applied; re-apply a drop of threadlocking type adhesive if screws are often removed |
[3] PDF p. 1 (Cleaning and Service > SERVICE (PDF p. 1) |
Reported by other sources — not the manufacturer
Not a manufacturer specificationConfirm with the manufacturer or a licensed gunsmith before use. Where these differ from a manufacturer value above, the manufacturer wins.
| Fastener | Reported value | Sequence | Source | Applied | Init. |
|---|---|---|---|---|---|
| Front sight screw GLOCK pistols; Night Fision front sight with the Night Fision screw Reported by Night Fision (sight maker) support knowledge base (secondary) — not the manufacturer |
5–7 in-lb 0.56–0.79 N·m · 0.42–0.58 ft-lb |
Threadlocker: "de-grease and apply high strength thread locker to the front sight screw when installing" (Night Fision screws come with thread locker pre-applied per its install guide) |
[4] | ||
| Front sight screw GLOCK pistols; Night Fision front sight with the Night Fision screw Reported by Night Fision (sight maker) support knowledge base (secondary) — not the manufacturer |
max. 10 in-lb 1.1 N·m · 0.83 ft-lb |
[4] | |||
| Front sight screw GLOCK pistols; AmeriGlo front sight with AmeriGlo screw Reported by AmeriGlo (sight maker) (secondary) — not the manufacturer |
10 in-lb 1.1 N·m · 0.83 ft-lb |
Hold the front blade square and firmly during final tightening; the screw must stay seated squarely in the head (sight most likely to rotate off-center then). Threadlocker: "a very small drop of Loctite on the end threads of the screw" |
[5] | ||
| Optic plate to slide screws (M3 x 0.5 x 6 mm) Glock 21 MOS (Gen 4/5) with Forward Controls Design OPF-G, RMR plate (Trijicon RMR/SRO/RMR HD/RCR) Reported by Forward Controls Design (secondary) — not the manufacturer |
13 in-lb 1.5 N·m · 1.1 ft-lb |
Threadlocker: VC-3 (Vibra-Tite) packet supplied with the plate |
[6] | ||
| Optic (sight) to plate screws Glock 21 MOS (Gen 4/5) with Forward Controls Design OPF-G, RMR plate (Trijicon RMR/SRO/RMR HD/RCR) Reported by Forward Controls Design (secondary) — not the manufacturer |
13 in-lb 1.5 N·m · 1.1 ft-lb |
[6] | |||
| Optic plate to slide screws Glock 21 MOS (Gen 4/5) with Forward Controls Design OPF-G, ACRO plate Reported by Forward Controls Design (secondary) — not the manufacturer |
13 in-lb 1.5 N·m · 1.1 ft-lb |
[7] | |||
| Aimpoint ACRO P1/P2 mounting screw Glock 21 MOS (Gen 4/5) with Forward Controls Design OPF-G, ACRO plate, Aimpoint ACRO P1/P2 Reported by Forward Controls Design (secondary) — not the manufacturer |
26 in-lb 2.9 N·m · 2.2 ft-lb |
[7] | |||
| Steiner MPS mounting screw Glock 21 MOS (Gen 4/5) with Forward Controls Design OPF-G, ACRO plate, Steiner MPS Reported by Forward Controls Design (secondary) — not the manufacturer |
18 in-lb 2 N·m · 1.5 ft-lb |
[7] | |||
| Optic plate to slide screws (M3 x 0.5 x 6 mm) Glock 21 MOS with Forward Controls Design OPF-G, EFLX plate (EOTech EFLX) Reported by Forward Controls Design (secondary) — not the manufacturer |
13 in-lb 1.5 N·m · 1.1 ft-lb |
[8] | |||
| Optic (sight) to plate screws Glock 21 MOS with Forward Controls Design OPF-G, EFLX plate (EOTech EFLX) Reported by Forward Controls Design (secondary) — not the manufacturer |
20–25 in-lb 2.3–2.8 N·m · 1.7–2.1 ft-lb |
[8] | |||
| Optic plate mounting screws (M3 x 6 mm FH) Glock 21 MOS with C&H Precision V4 MIL/LEO adapter plate (RMR footprint) Reported by C&H Precision Weapons (secondary) — not the manufacturer |
12–15 in-lb 1.4–1.7 N·m · 1–1.2 ft-lb |
[9] | |||
| Optic to plate screws, Trijicon RMR (4-40 x .320 FHB) Glock 21 MOS with C&H Precision V4 MIL/LEO adapter plate (RMR footprint), Trijicon RMR Reported by C&H Precision Weapons (secondary) — not the manufacturer |
12–15 in-lb 1.4–1.7 N·m · 1–1.2 ft-lb |
[9] | |||
| Optic to plate screws, Holosun 507C/508T (4-40 x .280 FHB) Glock 21 MOS with C&H Precision V4 MIL/LEO adapter plate (RMR footprint), Holosun 507C/508T Reported by C&H Precision Weapons (secondary) — not the manufacturer |
12–15 in-lb 1.4–1.7 N·m · 1–1.2 ft-lb |
[9] | |||
| Optic adapter plate mounting screws (Holosun K/EPS-footprint plate for Glock MOS) Glock 21 MOS with HOLOSUN adapter plate for 407K/507K/EPS Carry/EPS Reported by HOLOSUN Europe (holosun.eu, operated by Picotronic GmbH, HOLOSUN European distributor) (secondary) — not the manufacturer |
1.4 N·m 12 in-lb · 1 ft-lb |
[10] | |||
| Optic adapter plate mounting screws (Holosun K/EPS-footprint plate for Glock MOS) Glock 21 MOS with HOLOSUN adapter plate for 407K/507K/EPS Carry/EPS Reported by HOLOSUN Europe (holosun.eu, operated by Picotronic GmbH, HOLOSUN European distributor) (secondary) — not the manufacturer |
max. 1.7 N·m 15 in-lb · 1.3 ft-lb |
[10] | |||
| Red-dot optic mounting screws (6-32 flat head, supplied) into EGW rear-dovetail sight mount Glock 21 (non-MOS slide only; EGW: 'Does not fit ANY/ALL Glock MOS models') with the EGW 'Trijicon RMR / SRO, Holosun 407c / 507c Sight Mount for Glock' (replaces the rear sight; for Trijicon RMR/SRO, Reported by Evolution Gun Works (EGW) (secondary) — not the manufacturer |
15 in-lb 1.7 N·m · 1.2 ft-lb |
Remove the rear sight, install the EGW mount in the dovetail, tighten the set screw if applicable, place the optic on the mount, then apply blue Loctite and torque the optic screws. Threadlocker: Blue Loctite on the provided mounting screws (EGW instructions) |
[11] | ||
| Red-dot optic mounting screws (supplied) into EGW rear-dovetail sight mount (Docter/Venom/Viper/FastFire footprint) Glock 21 (non-MOS slide only; EGW: 'Does not fit ANY/ALL Glock MOS models') with the EGW 'Glock Red Dot Mount | Fits Vortex Venom, Viper & Burris FastFire | EGW' (replaces the rear sight) Reported by Evolution Gun Works (EGW) (secondary) — not the manufacturer |
15 in-lb 1.7 N·m · 1.2 ft-lb |
Remove the rear sight, install the EGW mount in the dovetail, tighten the set screw if applicable, place the optic on the mount, then apply blue Loctite and torque the optic screws. Threadlocker: Blue Loctite on the provided mounting screws (EGW instructions) |
[12] | ||
| DeltaPoint Pro M4x.7x15.8mm Torx screws through Leupold mount base into dovetail Glock 21 (rear-sight dovetail slide) with Leupold DeltaPoint Pro on the Leupold Glock base/dovetail (SKU 170906) Reported by Leupold & Stevens, Inc. (secondary) — not the manufacturer |
25 in-lb 2.8 N·m · 2.1 ft-lb |
Remove the rear sight, fit the Leupold dovetail (part number up) in its place, set the mount base over it, seat the DeltaPoint Pro on the base posts, then torque the two Torx screws (press-fit). | [13] PDF p. 1 (Installation section; mount li (PDF p. 1) |
Fit or check on rebuild
- Slide stop lever spring tension — If the lever went in without tension (pin order wrong) the slide can lock back early
- Front sight screw tightened snug, not over-tightened — The small screw breaks if over-tightened
- Sight position / re-zero — Drift or pressed sights moved for coating shift point of impact unless witness-marked and returned to the same spot
- Full function check (trigger reset, safeties, slide lock-back, drop of striker/hammer only on trigger) — Standard after any fire-control strip; confirms springs and sears went back correctly
6. Function check
- Confirm the pistol is unloaded with no magazine inserted before each test.
- TRIGGER SAFETY TEST: with the trigger safety not depressed, the safety should prevent rearward movement of the trigger and the pistol should not dry fire.
- TRIGGER RESET TEST: with the muzzle in a safe direction, pull the trigger — you should hear and feel the firing pin release. Hold the trigger back, rack the slide, then release the trigger; it should reset to its forward position. Repeat this several times.
- SLIDE LOCK-OPEN TEST: insert an empty magazine, pull the slide fully to the rear, and confirm the slide locks open. Repeat with each of your magazines.
- With the slide in battery, push forward firmly on the muzzle. The pistol must not fire out of battery.
- Confirm the magazine drops free when the release is pressed.
If any check fails, the firearm is not safe to fire. Stop and see a licensed gunsmith.
Sources
- GLOCK Owner's Manual — Models G17 to G48 (Instructions for Use) — GLOCK Ges.m.b.H. primary
https://eu-assets.contentstack.com/v3/assets/bltf7171cc1cfc1a31f/bltebc27f0f8099f9f4/6a3bd649918bcf1447c2f9f6/54774_Instructions-for-use_Relaunch_Standard_g17-48-BT01_02a_062026-GLOCK-Inc.pdf - GLOCK Owner's Resources — Downloadable Materials — GLOCK Ges.m.b.H. primary
https://us.glock.com/en/owners-resources/downloadable-materials - GLOCK MOS Adapter-Set (Modular Optic System) instruction sheet (© GLOCK 2020, 31798), linked from us.glock.com Owners Resources > Downloadable Materials as 'MOS Adapter Plate Instructions' — GLOCK primary
https://eu-assets.contentstack.com/v3/assets/bltf7171cc1cfc1a31f/blt06337d8ec5ed9ac8/68f7dfbab497b1ddaede6b66/MOS_Instructions_21422.pdf - What is the Glock Front Sight Screw Torque Spec? — Night Fision (sight maker) support knowledge base secondary
https://nightfisionorg.freshdesk.com/support/solutions/articles/154000142823-what-is-the-glock-front-sight-screw-torque-spec- - Sight Installation Instructions — AmeriGlo (sight maker) secondary
https://www.ameriglo.com/support/sight-installation-instructions - OPF-G, RMR (product page) — Forward Controls Design secondary
https://www.forwardcontrolsdesign.com/OPF-G-RMR_p_224.html - OPF-G, ACRO / MPS (product page) — Forward Controls Design secondary
https://www.forwardcontrolsdesign.com/OPF-G-ACRO-MPS_p_265.html - OPF-G, EFLX (product page) — Forward Controls Design secondary
https://www.forwardcontrolsdesign.com/OPF-G-EFLX_p_459.html - V4 MIL/LEO Adapter Trijicon RMR / RMR HD / SRO / Holosun 407C / 507C / 507Comp / 508T ... Fits GLOCK MOS (product page) — C&H Precision Weapons secondary
https://chpws.com/product/glock-mos-v4-mil-leo-to-holosun-407c-507c-508c-508t/ - Adapter plate for Glock MOS (except 43X/48 MOS), compatible with Holosun 407K/507K/EPS Carry/EPS (product page) — HOLOSUN Europe (holosun.eu, operated by Picotronic GmbH, HOLOSUN European distributor) secondary
https://www.holosun.eu/en/p/70189513 - Trijicon RMR / SRO, Holosun 407c / 507c Sight Mount for Glock (EGW product page, installation instructions section) — Evolution Gun Works (EGW) secondary
https://egwguns.com/trijicon-rmr-sro-holosun-407c-507c-mount-for-glock - Glock Red Dot Mount | Fits Vortex Venom, Viper & Burris FastFire | EGW (EGW product page, installation instructions section) — Evolution Gun Works (EGW) secondary
https://egwguns.com/vortex-viper-venom-glock-sight-mount-fits-burris-fastfire-and-docter - Leupold DeltaPoint Pro Reflex Sight — Operating Instructions (Part #120278, artwork #120279P; English text with French translation) — Leupold & Stevens, Inc. secondary
https://www.leupold.com/media/manuals/DeltaPoint_Pro_Manual_French_120279P.pdf - Virtual Patent Marking of GLOCK® Products (Version 1, Released: August, 2026) — GLOCK (linked as 'Patents' from the us.glock.com site footer) primary
https://eu-assets.contentstack.com/v3/assets/bltf7171cc1cfc1a31f/blt1ccd0bce687f1446/6a732cbdd920822d940ca846/Virtual-Patent-Marking_20260805.pdf - GLOCK Armorer's Manual (2013), archive.org scan — GLOCK Ges.m.b.H. (scan hosted on archive.org) primary
https://archive.org/details/glock-armorer-v-2013_202406 - Disassembly Guide: Glocks — Apex Gunsmithing secondary
https://apexgunsmithing.com/disassembly-guide-glocks/
| Reassembled and function-checked by | Date |
|---|
Educational guide only. Some information comes from non-authoritative sources. Always consult your manufacturer's manual and a licensed gunsmith. Verify your firearm is unloaded before every procedure. Full disclaimer: https://github.com/Fundamental-build-LLC/Pew-Takedown
Legal notice: this material is third-party information compiled from sources of varying reliability and is provided for general informational and educational purposes only. It is not gunsmithing, technical, safety or legal advice, it is not verified or endorsed by any manufacturer, and it must not be taken as fact. Any work on a firearm beyond the routine field stripping described in its own owner's manual should be performed only by a licensed and insured gunsmith. You use this material entirely at your own risk. Fundamental Build LLC and the project's contributors make no warranty of any kind and accept no liability for death, injury, damage or loss arising from its use. Full terms: LEGAL.md.