Sten MK III Parts Sets
by Frank Iannamico

Although crudely constructed, awkward to fire and (arguably) aesthetically unappealing, the World War II British Sten series of 9mm submachine guns (or machine carbine in British parlance) have established quite a loyal following among US collectors and shooters.

I base these assumptions of popularity on the prevalence of the Sten boards on the web, and sales of The Sten Owners Manual for Shooters and Collectors (available from Moose Lake Publishing). The Sten Manual that was first published in 1997, and now in its second printing, continues to be one of my best selling books. I guess what make the Sten appealing to many is its simplicity as well as its rugged reliability. Although close to the bottom of the food chain when it comes to Title II firearms, from my observations the Sten is very well liked by those who possess them. No doubt the combination of history; select-fire capability, common 9mm cartridge and price combine to make the Sten an attractive choice.

The basic Sten Mark II version is the most popular, and most of them out there are “tube guns” that were assembled with “new-manufacture” receiver tubes by Class 2 manufacturers and individuals back in the pre May 19, 1986 “good old days.” Although there are a number of original Curio and Relic (C&R) Stens in the system they are certainly not as common, or cheap as, the Sten “tube gun.”

The Sten has a enjoyed a renewed interest recently (more accurately the Sten’s RECEIVER tube) spurred largely by SAR writer Bob Bishop’s article on how to convert the “lowly” Sten MK II into a streamlined Sterling Mark 4 aka the L2A3 (See SAR Volume 2 Number 12). I admit after reading Bob’s article I too was inspired to have my Sten MK II under go the transformation into a “Stenling”, and did so shortly after the article appeared. After my Sten was returned from the metamorphosis, I was quite impressed with my new “Sterling” conversion. The Sterling is an awesome, accurate gun. However, I soon began to miss my somewhat inelegant Sten MK II model and soon felt that I just had to acquire another one. After acquiring another Sten MK II, I then of course would need a number a spare parts “just in case”. I had regretfully sold off all of the spare parts that I had accumulated for my other Sten when I had it converted into a Sterling submachine gun.

Most every owner of a Title II firearm likes to have a number of spare parts on hand “just in case.” The Sten is no different, although the price of a complete kit in decent condition for a MK II model will generally exceed $150. Additionally, the availability of Sten MK II kits have dried up a few times over the past few years, but the surplus dealers always seem find new sets somewhere and they reappear, although this will no doubt end someday. A disadvantage of purchasing a complete Sten parts set is that some of the parts will generally not be used as spares. These parts would usually include the trigger housing, the buttstock, barrel retaining nut and the magazine housing.

Recently a large number of Sten MK III (three) kits have appeared on the surplus market. The MK III model of the Sten differs in construction somewhat from the MK II version, and there are very few live, transferable MK III Stens out there in the hands of U.S. collectors. The World War II Sten MK III was originally designed because it could be manufactured even faster and cheaper than the MK II version. However this was not true for Class 2 manufacturers and individuals who were constructing “new” Sten subguns. As a result most individuals and Class 2 manufacturers stuck with making the Sten MK II model. Thus, there is little demand today for a Sten MK III spare parts, so the kits are being offered at under $30 and even cheaper when purchased in quantity. Many of these kits are being purchased by those individuals, who wish to build an inexpensive non-firing MK III Sten for display purposes.


The Sten MK III, officially introduced in the spring of 1943, was the least expensive Sten model to manufacture. While the Sten MK II was manufactured by a number of different prime, and many subcontractors, the Mark III variation was made only by Lines Brothers LTD. Lines Brothers was a metal fabrication firm with no pre-war gun making experience. The company however, was a contractor for the Sten MK II, Sten magazines, and various parts for other weapons during the war. The MK III was originally intended to replace the MK II model. After comparing the advantages and disadvantages of each individual model, the British decided that there were many more attributes to the MK II version that made it decidedly superior to the cheaper MK III. Production of the MK III was terminated only eighteen months after it began. During the weapon’s brief manufacturing run there were over 876,000 examples of the MK III produced.

The MK III receiver extends out almost to the end of the barrel. A spot-welded seam forms a rib that protrudes from the top and runs the length of the receiver. This rib also forms the front sight, which makes it a non-adjustable or replaceable part. Few parts could be replaced in the MK III including the barrel that was not readily removable from the receiver. The magazine housing was permanently fixed to the receiver tube by spot welding and could not be rotated as a dust cover over the ejection port as on the MK II model. Internal parts and operation principles were the same as the MK II models.

Although the Sten MK III model had the advantage of being able to be manufactured rapidly and was inexpensive to produce, it was extremely difficult to repair and maintain in the field. This was attributed to the fact that the weapon could not be easily disassembled for cleaning, or to replace parts. During the war when a Mark III Sten encountered any functioning problems or was damaged, the entire weapon was usually scrapped, excluding the few internal parts that could be salvaged. Few replacement parts were even made available to repair a MK III. One of the Sten MK II’s main attributes was that it could be repaired, disassembled and concealed. These features were what made the Sten MK II ideal for soldiers in the field and resistance groups operating behind enemy lines. With the MK III model this was not possible. The majority of the Mark III Stens were issued to the British Home Guard units.

The good news for registered Sten owners is that many of the parts of the MK III are the same as a MK II. These parts would include the bolt, extractor, extractor pin, selector, trigger assembly, trigger pin, sear, sear pin, trip lever, recoil spring, spring cap and lock, buttstock, magazine catch and spring. Other parts although not readily interchangeable, can be modified to work in the MK II model. The bottom line is that the MK III parts sets can be used as inexpensive parts source for the Sten MK II or MK V owner. Owners of the MK V model should be advised that the bolt, trip lever and lower cover are different, but the parts, with the exception of the cover, can be modified rather easily to work in the MK V gun.

Most of the MK III parts sets are in very good condition with a few observed that appeared to be brand new! Another interesting observation is that many of the Mark III kits that were imported have the heretofore-rare cast aluminum-bronze bolts in them. The bronze bolts had previously been quite scarce in the U.S. in past years. Although they are not suggested for use, as the sear surface will wear rapidly with consistent firing in semiautomatic.

The barrel of the 9 mm MK III Sten was designed differently than the MK II or MK V. The MK III barrel was not originally designed to be readily removed. It is more or less permanently riveted inside the receiver tube. The MK III barrel can however be used in the MK II model by fabricating an adapter. The adapter is a fairly simple affair and could be easily made by a competent machinist or a gunsmith.

Simply support the receiver end containing the barrel in the jaws of a vice. Do not tighten the vice jaws, simply use the jaws to support the end cap. The long end of the barrel should be pointing toward the floor. Take a soft piece of wood and support it with your hand on the muzzle end of the barrel. Using a hammer, tap the wood placed on the barrel’s muzzle downward. This will shear the two 1/8-inch rivets that are securing the barrel to the receiver remnant, and the barrel will drop out of the receiver.

The MK III kits are cheap in abundance today, but as any seasoned machine gun collector/shooter knows with machine guns and parts the situation is often “here today - gone tomorrow”. Classic example: try to locate a complete spare parts set today for a M1928A1 Thompson Submachine Gun. These were once plentiful as well.

This article first appeared in Small Arms Review V7N2 (November 2003)
and was posted online on October 4, 2013


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