The Do-It-Yourself Sten: Part Two

My friend bplvr (name withheld for privacy reasons) helped write this article and provided much of the technical expertise. He deserves much of the credit for his instrumental help and hard work.


bplvr is a long-time manual machinist, tinkerer and collector that is proficient in engineering solutions for adapting firearms to fit into the stringent California legal landscape. While working he was employed as a Process Engineer. He is the California Gun Show Coordinator for and just rescued his 101st Golden Retriever, affectionately named “Major Dick Winters”.

The last STEN article left off with deburring all the cuts and slots. This article will continue on with setting the front and rear trunnions, installing the barrel and welding all the critical components.

For those that are unfamiliar with the term, the trunnion is a piece of steel that interfaces between the receiver tube and the barrel. Since the trunnions that will come with your STEN kit are going to be made of soft steel and full of rivet holes, they will be more than likely unsalvageable for reuse in your new STEN so either fabrication or purchase of new trunnions is necessary. Fortunately, Indianapolis Ordnance provides a new rear trunnion with their build kit (which is necessary due to the reduced inner diameter of the semi-automatic receiver tube) and new front trunnions are available for purchase. For most instances, purchasing a new trunnion is highly recommended.

Pic 1: Pictured here is a brand new front trunnion.

Pic 7: Pictured here is a brand new rear trunnion

Pic 2: This is a side-by-side comparison of a rear (right) and front trunnion that were pulled from a Sten MK3 during the de-milling process.. Notice the rivet holes. 

The most critical part of the trunnion setting process is measuring and setting the rear trunnion, as this not only sets the headspace for the bolt but it also aligns the magazine well for proper feeding. The critical measurement for the rear trunnion is between .000” and .060”, meaning that from the edge of the ejection port or magazine well to the face of the rear trunnion can be between .000” and .060” inward. The front trunnion, however, simply rests on a shelf cut into the forward end of the barrel thus relies entirely on the spacing of the rear trunnion.


Pic 4: This is the front end of a Sten MK3 that has not been fully demilled yet. The cap that the barrel sticks out of is the front trunnion.

Pic 6: This is the view of the same STEN MK3 piece but from the rear trunnion.

To set the trunnion spacing, center punch all the rivet holes that are marked on the tube and drill them out to the correct size with successive drill bits (start with a .062” bit, then a .093” and then a .125”), then insert the rear trunnion in the correct orientation into the tube, followed by a piece of milk carton, business card or credit card cut to size (any of these items will serve as the <.060” spacer that is needed) and then a flat aluminum or steel bar through the ejection port and magazine well. To ensure the trunnion is seated correctly for the next step, you will either need to use a 1” C-clamp to clamp the flat bar to the tube or you can shove a bar/tube through the receiver tube, forcing up on the flat bar and setting the rear trunnion correctly (this way would be a two-man operation). Once this contraption is set up properly, you will need to use your .125” bit to drill a single rivet hole through the rear trunnion. Insert a rivet and hammer it about ½ way in. Then drill all the other holes in the rear trunnion. put the rivets in and hammer them ½ way in. It is critical to only pound the rivets in ½ way because it is entirely possible that the rivet length won’t allow the barrel to mount on the rear trunnion.

In addition, center punch and drill out the front trunnion holes on the receiver tube (same style as the rear trunnion) to prepare for setting the barrel.

Pic 13: Pictured here is how one can set the rear trunnion using a flat aluminum or steel bar. Note that the build pictured is a STEN MK 2 pistol but the technique pictured is what we are going for. Where the spring and plastic tube is at is where a large bar or tube would go to hold up the flat bar. This method will also allow you to make sure that the forward edge of the magazine well and ejection port are even. If they are not even at this point, assess the discrepancy and file to fit as appropriate.


Pic 15: Pictured here is using the C-clamp method of setting the rear trunnion. While the tube pictured is not a receiver tube, the principle is the same. Use your calipers to ensure that the space between the forward edge of the ejection port or magazine well and the rear trunnion is <.060”.

At this point, one must think ahead and consider how to ensure how the barrel will be prevented from rotating while in use. The more modern and potentially more aesthetically pleasing way is to file a small flat on the barrel and use a 10-32 set screw that is drilled and tapped through the front trunnion on the underside of the receiver tube. To use this method, you must file the flat on the barrel BEFORE setting the front trunnion.

If you do not want to use a set screw to prevent barrel rotation, simply wait until the front and rear trunnions have been set and follow the instructions there.

Next up is the barrel. To set the barrel, slide the barrel into the rear trunnion and then slide on the front trunnion over the barrel and into the front of the receiver tube. Slide a steel pipe over the barrel and press down on the front trunnion to ensure a tight seat on the barrel with the front trunnion. Drill out a 1/8″ rivet hole in the front trunnion and pound in a rivet all the way. Repeat with the opposite position (so if you pound in one rivet at the 12:00 position, then pound in the other rivet at the 6:00 position). After you pound in the second rivet, you can now remove the steel pipe sheath and finish drilling and pounding the rest of the rivets on the front trunnion and then finish up all the rivets on the rear trunnion.

If you didn’t cut a flat in your barrel for the 10-32 set screw to prevent barrel rotation, now is the time to pursue the old school (yet effective) method.You just need to use a big center punch (not the spring-loaded kind, the solid kind) and give the front trunnion a good whack at the 12:00 and 6:00 positions on the face where the barrel and trunnion meet. See the picture below to see what is being described.


Pic 16: This is the nose end of a STEN MK3. Notice the two big divots at 12:00 and 6:00. These were made to prevent barrel rotation.

Pic 17: This is the underside of the front end of a STEN MK3 build where a set screw was used. This is an effective but somewhat more complicated method of fixing the barrel.

Next up is setting up and welding in the magazine well and then the receiver tube itself. For the welder, you are going to want to use a minimum 140 amp MIG welding setup for all the welds. Do not over weld, as warping the tube is a real possibility.

Assuming the magazine well has been properly demilled (see photo below for example), just correctly orient and insert your bolt so it rests against the rear trunnion (the closed position). Insert your charging handle as well since this will locate the bolt to the correct 9:00/3:00 position.To keep the bolt in the furthest position to the left (against the magazine well), insert a piece of milk carton between the bolt and the inside edge of the main tube, then insert your magazine well. The bolt is inserted during this process to ensure sufficient minimum and maximum ejector clearance for the bolt. Once the magazine well has been squared and trued, you can TACK WELD the magazine well to the tube at the corners of the magazine well (.125” weld maximum at each corner) then check to make sure that the magazine well is sitting square, that the ejector is sitting squarely in the middle groove on the bolt, that the top of the ejector isn’t hitting or grinding on the bolt, etc.. If there is a problem with the fit, you will have to cut the magazine well off at the welds and grind or sand everything smooth then try again. Do not weld to welds, this is a recipe for failure. When everything is set and trued, weld the magazine well permanently to the receiver tube.

Pic 18: This is an example of how a STEN MK3 magazine well would properly fit into the magazine well slot. The gun pictured is a modified German MP3008 build.


Pic 19: Pictured here are two MK3 magazine wells. The one on the left has had the tabs removed and the one on the right has not. Both will work, but the one on the left will be much harder to fit. The one on the right will look more original after it has been welded.

After welding up the magazine well comes welding the receiver tube to the trigger housing unit. Simply slide the tube into the properly de-milled trigger unit and weld along or through the “ears” and (if available) the top section of the tube. Keep in mind that proper placement of the tube is critical for sear and selector switch placement and making sure that the buttstock can be inserted into the back of the trigger housing (the receiver tube should be flush with the back of the trigger housing). Do not over weld, as warping the tube is a real possibility. In addition, do not cause a situation where weld material gets put into the cocking slot, as the ear on the right side is oftentimes close to the cocking slot.  Before you weld anything make sure that the trigger, selector lever and the sear are working properly and not getting snagged. Before final welding, use .125” diameter tack welds FIRST!

Pic 21: This is a photo of the trigger housing ear on a STEN MK2.

Now that the welding has been completed, we turn to some of the smaller details like the selector safety lever and trimming the extractor.

In the (very simple) trigger pack, there is a selector lever that rides a slot on the underside of the selector switch that, while no longer useful as a selector switch since the gun is a semi-automatic only firearm, can be converted into a functional safety. To do this, you will need to take a ½” diameter round object (a piece of metal, a cartridge case, etc), mark it with nail polish and insert it through the selector switch hole in the trigger housing unit and mark the location where you must grind the selector lever. After marking, remove the selector lever from the fire control group and use a grinding wheel (the selector lever is hardened steel) to cut a ¼” radius into the area, as depicted below. Make sure to deburr and size to fit then reinstall.


Pic 25: How to correctly mark your selector lever for cutting.

Pic 22: Exact measurements per bplvr’s STEN on the cuts that needed to be made.

Pic 30: Selector lever cutting completed.

Then there is the matter of the extractor. When one does a new semi STEN build, the inner diameter of the receiver tube is reduced from 1.375” down to 1.316” (at least for the Indianapolis Ordnance build) to prevent usage of the original full-auto bolt. In doing so, the extractor will not sit flush with the edges of the bolt and, unless sized to fit, will not allow proper functioning in the new receiver tube. In order to fix this issue, mount the extractor into the new bolt but use a 10# nail instead of a roll pin since you will need to test fit and then permanently install. Once mounted, color the exposed portion of the extractor and use the backside of an x-acto blade to scribe a mark along the edge of the bolt. What is above this mark will be what is needed to be carefully trimmed with a Dremel tool. You will only need to remove .060” from the extractor itself. See the before and after photos below.

Pic 26: Pictured here is an original STEN bolt (left) and the new bolt on the right.

Pic 28: This is what the untrimmed extractor looks like popping out of the new bolt.

Pic 29: This is what the extractor should look like after trimming.

All that is needed at this point is to insert the bolt, trim the springs as necessary (any kit will come with details on this part), test fire, tune and have fun! You can Aluma-hyde or Parkerize your STEN after final assembly if you choose or you can go old school British on it and simply put black house paint on it.

Below are a few photos of bplvr’s STEN builds and a video of Nathan test firing and tuning his STEN MK2/MK3 hybrid

Pic 23: This is a STEN MK2 pistol build. The extra assembly atop the magazine release is a custom fabricated bullet button so that this pistol will comply with California’s “assault weapon” laws.


Pic 31: This is a STEN MK2/MK3 hybrid build. The wood handles were put there to block the hole that is in the buttplate assembly so as not to potentially be afoul of California’s draconian laws regarding thumbhole stocks.

– pullnshoot25 and bplvr


*Do not attempt without suitable knowledge and experience. Human Events accepts no responsibility for resulting injury. Firearm laws are subject to change as well as interpretation on federal, state and local levels. Be aware of federal, state and local laws before attempting, and/or consult a licensed local attorney for applicable laws.  YOU MUST ABIDE BY ALL LAWS: STATE, FEDERAL AND LOCAL.

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