Marlin Model 1897 Disassembly

by

Gary Thomann Thomann Engineering Singletrack Publishing

1 Marlin Model 1897

Marlin Model 1897 Disassembly

Introduction This is a manual for taking apart a Marlin model 1897 22 caliber rifle. It is primarily intended for someone taking apart the rifle for the first time, when it is probably dirty and most of the screw threads are stuck. Of course it can also be used for disassembling a clean rifle. The instructions also apply to other old rifles because some time will be spent on how to remove stuck screws. There is another manual available on the internet for about eight dollars that is advertised to be for the 1897. However, it is actually for the Marlin 39A and there are several differences between the 1897 and the later version of the rifle. This manual is specifically for the 1897. There are a few recommended tools and supplies needed so let’s list those next

Tools and Supplies Patience. This is not a one night project. There will probably be a lot of stuck screws that take several days to get the loose. Don’t rush! Assortment of hex screwdriver bits or some screwdrivers. The hex bits are best, try to find hollow ground ones. These will be used to make screwdrivers to fit the rifle. If you are like me you probably already have a bunch of these around. Gunsmith screwdrivers. Actually not needed but if some of them have thin tips they could be handy. The Marlin 1897 has very thin slots in the screws. Bench grinder. The grinder will be used to grind the bits to the shape desired. Some other type of grinding device could also be used. Liquid Wrench, acetone/ATF 50/50 mix, wax. Some other fluid for loosening stuck bolts could also be used. My experience is that the 50/50 mix of acetone and automatic transmission fluid is best. Old drill press chuck. I used this to hold the hex bits and make a screwdriver. You could also use any hex holder. I like the chuck because there is no slop when turning. Mineral spirits. Optional, but I did use mineral spirits for cleaning the magazine tube. For other parts I prefer using dish soap and water. Dish soap and water. Check with your husband, he will know where to find the dish soap. Small brass brush, old tooth brush, tooth picks, Q-tips, cotton rags. Jewelers screwdriver. For cleaning out screw heads. Similar small instrument can be used. Heat gun. Like used for shrink tubing.

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Screw Threads As I took apart the example rifle I tried to determine what threads were on each screw in case it had to be replaced. This was not a particularly successful endeavor. At the present time the United States is under the Unified Thread Form/Unified Inch Thread also referred to as ANSI/ASME B1.1-2003 and by the term UN. The thread flank angle is 60. Normally this standard is just referred to as UNC (coarse) and UNF (fine) and apparently an 8-UN that I not familiar with. There are two modified UN threads named UNR (rolled) and UNJ (aerospace). There is an equivalent standard to UN for metric screws. If you are going to replace a screw in the rifle one of the UNF screws would probably be used. First let us note that Marlin was founded in 1863 by John Mahlon Marlin. The company made pistols until 1875 and then began production of Ballard rifles. The first lever action rifle was made in 1881. The Model 1897 was an improvement of the Model 1892 which itself was an improvement on the Model 1891. So, the screws that were used in the Model 1897 must have been developed based on manufacturing from about 1863 to 1897, although there was probably a tendency to use the same screws in the Model 1897 as had been used in the Models 1891 and 1892. Before the 1840’s there was apparently little standardization in machine screw sizes and threads. In 1841 Sir Joseph Whitworth proposed a thread with a 55 thread flank angle and rounded roots and crests. The Whitworth system did not seem to have small sizes, although of course somebody could use his form with small screws. The standard Whitworth small screws are 1/16 – 60, 0.10-48, 1/8 – 40, 3/32 – 48. The terms BSW (coarse threads) and BSF (fine) are used for Whitworth screws. The standard is obsolete but BSW and BSF screws are still used, particularly in Britian. Standardization in the U.S. began back in 1861 with the specification of the Franklin Institute/Sellers/United States thread. This standard was mandatory only for the military and government departments. The Sellers thread had a 60 degree angle and had flat roots and crests. Apparently the use of the Sellers thread was widespread by 1880. In 1907 ASME defined two series that used Sellers thread and numbered the sizes by gage from 1 to 30. Each gage increase was 0.013 inch. Next came the American National Standard Screw Thread by the Bureau of Standards, also referred to as CS24-30. It became effective July 1, 1930. After that came the B1.1 standard adopted in 1949. It has been updated several times. Obviously the standardization discussed in this paragraph was too late for the Marlin Model 1897. So what was Marlin using? I would guess the Sellers thread. In the screws on the Model 1897 it is the pitch (threads per inch) that don’t seem to match the modern screws. The diameter was less of a problem. My rifle had all fine threads, the modern UNF screw threads are shown in Table 1. The screws in the table are what we would like to use if we had to replace a screw in the rifle. As we will see substitution doesn’t always work. The diameter shown in the table is the major diameter, the value at the outside of the threads. The diameters shown in the table are actually maximum values. A more complete thread table is shown in Appendix A.

3 Marlin Model 1897

Table 1. Modern standard UNF screw sizes and major diameter.

Size Dia, in

0 - 80 UNF 0.060

1 - 72 UNF 0.073

2 - 64 UNF 0.068

3 - 56 UNF 0.099

4 - 48 UNF 0.112

5 - 44 UNF 0.125

6 - 40 UNF 0.138

8 - 36 UNF 0.164

10 - 32 UNF 0.190

12 - 28 UNF 0.216

1/4" - 28 UNF 0.250

Getting Stuck Screws Loose When I began to take my 1897 apart I found that about two-thirds of the screws were stuck. I have a set of the eight Grace gunsmith screwdrivers. The next thing I found was that they would not fit the rifle screws, the slots were much too narrow. I took a couple cheap screwdrivers and ground the blade narrower on a bench grinder. Gunsmith screwdrivers are supposedly hollow ground. The Grace ones were actually flat ground. I tried to grind mine in a flat profile also. I then tried some intermediate pressure on the stuck screws, but they did not budge. I did not want to put any hard pressure on the screws; I was afraid the bit would slip. Some of the screw heads were already damaged. At that point I slopped some Liquid Wrench on the screws and retreated to the internet to find the best way to loosen stuck screws. The classic method to loosen a rusted bolt is to heat it red hot with a torch. While this is ok on a car muffler it was certainly not something I was going to do on a fine rifle I paid $600 for. I was willing to apply a bit of heat using a gun I have for shrinking tubing. I looked for reviews on the best solvent to use. Apparently before they had Liquid Wrench and its siblings wax was used. That was interesting. Then I ran across a review that claimed the best solvent was a 50/50 mixture of acetone and automatic transmission fluid. In the meantime I was giving some thought to the screwdrivers. I decided that using the quarter inch hex bits would be the best. I already had a large collection of them in a tupperware container. Some of them were even hollow ground, although the blades were too thick. I did some grinding on the flat side of the wheel and tried the screws again. I needed something solid to hold the bits that had a handle diameter that fit my hand well. I decided on an old drill chuck I had. Figure 1 shows the chuck and some of the bits I eventually made. The photo also shows the two cheap screwdrivers I initially ground down.

4 Marlin Model 1897

The drill chuck fit my hand well. For each screw I was working on I tried to make a bit that was the right thickness for the slot and with a width to go completely across the screw head. Of course I was making bits throughout the disassembly process. By the time I got all this going the screws with Liquid Wrench on them had been sitting there for about a week. With my new bits I tried again to loosen them, again using only intermediate pressure. No luck, they were still stuck. Apparently Liquid Wrench doesn’t work well on early 20th century corrosion. So I went to the auto store and bought a quart of transmission fluid and in a small wash bottle mixed a small amount with some acetone. I put a drop or so on the screws I was working on and let them sit a couple of days. And some of them came unstuck with just medium pressure! On the ones that were still stuck I heated the area with my heat gun until the metal was just about hot enough you couldn’t leave your hand on it, put some more mixture on, and waited another day or two. And these screws came undone. I continued the process as I took the rifle apart, eventually getting all the screws out without additional damage than had been done by the previous owner(s). Why this mixture works I don’t know; the acetone and transmission fluid are not even soluble in each other. After the bottle sits a while the two fluids separate. So that is the method I would recommend. Make a bit that fits the screw nicely. Put on the acetone/transmission fluid mixture and wait. Apply a bit of heat if needed. Have patience. One thing though. Transmission fluid smells bad so don’t spill any on your wood work bench or you will be stuck with the smell. When I used it I place aluminum foil under the rifle to catch any drops. Other times I put the whole assembly (like the right side of the receiver) in a zip lock bag before applying the solvent.

Figure 1. Homemade screwdriver bits and drill chuck handle.

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The Disassembly

Initial Breakdown Start with the main holding bolt on the right side of the receiver, as shown in Figure 2. Unscrew the bolt, it will not come completely out of the receiver. You should be able to loosen the bolt with just your fingers. If you need to use pliers, wrap a rag around the bolt so you don’t mar it. Put the hammer at half cock or full cock; I found that it works best with the hammer at full cock. Hold the stock in your right hand and the rifle forearm in your left hand. Bend the stock to the right and the rifle should come apart. You can also lay the front of the barrel on the table with the right side of the receiver up and push down slightly on the receiver. The rifle will come apart at the receiver with the right side of the receiver connected to the stock and the left side connected to the barrel. The bolt and firing pin will still be in the left side; slide it to the rear and remove to the right. A photo of the bolt just before it is lifted out is shown in Figure 3. When you set the bolt down the firing pin will probably fall out. That is not a problem.

Figure 2. Main assembly bolt on right side of receiver

Figure 3. Bolt pulled back and ready to be lifted out.

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Next remove the rear most screw on the upper tang. This is shown in Figure 4. The On my rifle this bolt was not stuck but the end had mushroomed very slightly, so I had to loosen and tighten it several times to get it out. As you can see the head already had some damage. When you have this bolt out, slide the stock out of the receiver. Two sight screws are also shown in the figure; these will be discussed later. So now lets start discussing screw threads, beginning with this stock screw. The major diameter of the thread is 0.204 in, a little smaller than that of a 12-28 UNF screw. On my thread checker, the screw fits both the 12-28 and 12-32 holes. This is probably because the stock screw is a bit smaller than a #12 and also because the thread checker is only 1/16 inch thick; it does not provide much resolution.. At this time I do not have either a modern 12-28 or 12-32 screw to try in the tang - I am trying to find them. Another interesting feature of the stock screw is a tapered shaft. At the end of the threads the diameter is 0.208 in, just under the head it is 0.229 in. The taper must be to draw the stock in snug. Obviously it complicates things if you need to make a replacement. Next the magazine tube and forearm will be removed. At the front of the forearm remove the two screws in the collar – the right side collar screw is shown in Figure 5. Although the forearm will be loose you cannot yet remove it; the magazine tube is holding it on.

Figure 4. Stock removal screw and screws for sight.

Figure 5. Right side forearm screw.

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The diameter of the forearm screw is 0.137 in, the same as a #6 modern machine screw. It will go about 2 turns into the 6-40 hole on the screw checker before it jams. A modern 6-40 screw does screw into the forearm mount so that size could be used as a replacement. Next the magazine tube needs to be removed. At the front of the barrel push in the magazine tab, shown in Figure 6, in and pull the magazine tube all the way forward. There will be a click when it reaches the limit, there is a crude mechanism for holding it open. At the front of the barrel under the magazine mounting dovetail that is shown in Figure 6, a hole in the outer outer cylinder of the magazine tube will allows access to the mounting screw. Figure 7 shows what you should see. Notice that the screw on my rifle is a bit boogered up. Using a bit of the proper size bit unscrew it bolt from the dovetail mount in the bottom of the barrel. However, I recommend that you do not pull the screw out of the inner tube, tube; just get it out of the dovetail in the barrel. You can now now remove the magazine tube assembly from the rifle and can then remove the forearm. At this point you should have five parts/assemblies. First, the rear stock and its mounting bolt. Second the right side of the receiver. Third, the left side of the receiver and the barrel. Fourth, the magazine tube assembly with the mounting screw still in it. Fifth, the foream, its collar and the two collar screws. You can lay the wood pieces to the side, we are not concerned with them. It is ime for some initial cleaning. Using a toothbrush, maybe a dish washing brush, and perhaps a cheap paint brush, clean the left side of the receiver and barrel with warm water and dish soap. Wipe

Figure 7. The magazine screw seen through hole in outer magazine tube.

Figure 6. Magazine tab and the dovetail the tube screws into to.

8 Marlin Model 1897

dry, blow it off and wipe with the barrel with a rag with a little oil on it. There is more disassembly to do, but it will proceed best if the receiver is a clean as possible. Do the same with the right side receiver mechanism. Remember I suggested that you not take out the magazine screw. The reason is, if you do, the spring will extend and you may have some trouble getting everything back together. The spring does not have a neat ending and it can jump out around the latch the screw goes through. The magazine assembly was the one part I did not clean with soap and water. I used mineral spirits. To get the inside of the tubes I used a small dowel to push a rag soaked in mineral spirits. There is a slot in the outside tube that will assist in sliding a rag through it. I suggest you clean in the same manner, then lightly oil the tubes and set aside. If you want to take the magazine tube completely apart I think you have to punch out the pin through the locking tab. Considering the bad reputation the magazine tube has for causing problems I did not do this. You may also not want to go any further with the magazine tube assembly. Now we are going to do some more work on the left side of the receiver. You can set the right side of the receiver aside. If you have not already done so, you may wish to put some acetone/transmission fluid solution on the screws in the right side of the receiver, place it in a zip lock bag and set aside. On my rifle several screws in the right side of the receiver were jammed.

Cartridge Stop, Ejector and Guide Spring The inside of the receiver left side with the bolt removed is shown in Figure 8. This picture was taken after everything had been cleaned and put back together. Yours probably won’t look quite as clean even though some initial cleaning has already been done. There are two assemblies you can see. The lower, held by one screw is the cartridge stop or cartridge cutoff. The top part mounted to the receiver with two screws is the ejector and its housing.

Figure 8. Receiver left side with cartridge stop and ejector housing visible.

9 Marlin Model 1897

The mounting screws as seen from the other side of the receiver are shown in Figure 9. These screws all have narrow slots. As you can see if you look closely the slots have been messed up slightly by a previous owner. What you could not see in Figure 8 is the cartridge guide spring. It is at the top of the receiver and not visible from the side. Figure 10 shows the receiver laid flat so the guide spring is visible. The guide spring is mounted to the receiver with a screw that comes in from the top. Figure 11 shows the screws at the top of the receiver. The two screws at the left are for mounting a sight or scope. The third screw on the right is the guide screw mount.

Figure 9 Cartridge stop (bottom) and two ejector housing mounting screws.

Figure 10. Guide spring at the top of the receiver.

10 Marlin Model 1897

Remove the screws holding the cartridge stop, ejector housing and guide spring. You can also remove the sight screws at the top of the receiver. On the example rifle most of these screws were stuck and required several days of work to get loose. The guide spring, ejector and cartridge stop assemblies after removal are shown in Figure 12. Notice that the spring under the ejector housing has the sharp end up and that it mounts in the forward hole. The ejector housing also has another screw head which has a notch in it. I aligned the notch so the ejector fits into it. Is this what you are supposed to do? You can remove the ejector by punching out a small pin, but there is no reason to do so just for cleaning. The two ejector housing mounting screws are very close to 3-48, the major diameter is 0.096. A modern UNC 3-48 screw could probably be substituted. At this point I have not found a modern 3-48 to try in the ejector housing.

Figure 11. Sight (left two) and guide spring mount screws on top of receiver.

Figure 12. Guide spring, ejector and cartridge stop assemblies, screw sizes are approximate.

11 Marlin Model 1897

The cartridge stop screw has a major diameter of 0.138 in so it should be a #6. However, the screw binds in my 6-40 UNF gauge. I tried a modern 6-40 screw in the cartridge stop; it was a bit loose but could probably be substituted if not over tightened. The guide spring is crimped at the front; the short crimped over part goes at the top. The mounting screw for the guide spring has a 0.107 in diameter so should be a #4. I tried a modern 4-48 screw in the spring; it was a bit tight but could probably be used. The two additional screws on the top, placeholders for a sight or scope mount, had a major diameter of about 0.1625 in, which should make them a #8, but I could not get them to fit any of the 8 gauge holes. However, a modern 8-32 screw did fit the receiver, so if you need a replacement that is what to use. Since I am sure you want to know, the spacing of the guide mount holes is 0.889 in as closely as I could determine, or just under 57/64 inch. The reason for this particular spacing may be lost to history. Clean the ejector, cartridge stop and guide spring assemblies with soap and water using the brass brush, wipe with a lightly oiled rag, then put them back into the receiver. Also put the sight screws back in. At this point you can put the magazine tube and forearm back on. Remember to put the forearm and its collar in place BEFORE you put the magazine tube in. Don’t ask how I know this.

Bolt & Firing Pin When you pulled the bolt out of the left side of the receiver and set it on the table the firing pin may have fallen out. Figure 14 shows the bolt, looking from the top, with the firing pin still in. The extractor is also still in the bolt. As mentioned, the firing pin will fall right out. The extractor can be pried out with your fingers and a small screwdriver. The firing pin and extractor are shown in Figure 13. Clean them in the usual method, put them back into the bolt, and replace the bolt in the left side of the receiver. The bolt replacement will be the reverse of the removal. Put it in a couple of inches behind the closed position, hold it against the receiver, and slide it forward. You can now set the left side or the receiver/barrel aside.

12 Marlin Model 1897

Figure 13. Extractor and firing pin.

Receiver Right Side Next we move to the right side of the receiver, which has more moving parts. It is shown in Figure 15 and I have tried to identify some of the screws that will be removed. Some of the names I did not know because the parts do not appear in the later model 39 version of this rifle. So I tried to assign a name that made sense, at least to me. There does not seem to be a rigid order in which the parts are removed, I will simply describe my take apart order. You can again see from the photo that the screws have narrow slots and more custom bit grinding will probably be necessary.

Figure 14. Bolt, firing pin & extractor.

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Figure 16. Friction spring & lever.

Friction Spring& Lever There is a piece of metal that rubs against the lever lever to provide some lever friction and I called it the friction spring. There is probably a different correct name. The screw holding it comes in from the bottom of the receiver. Remove this screw (one of the ones stuck on my rifle), and then remove the lever pivot screw (also was stuck). Use brush, soap and water to clean these parts. After a bit of cleaning what you should see is shown in Figure 16. And now the usual puzzle about the bolt threads. The friction spring screw has a diameter of 0.133 in. The screw would go part way into the 6-40 hole on my modern screw checker before binding. A modern 6-40 screw appeared to fit the spring very well, so that size could be used for a replacement if needed. The lever screw had the same diameter and also went part way into the screw checker before binding. The modern 6-40 screw fit into the rifle hole okay, although it was maybe a bit loose. But it appears a 6-40 screw could be used for a replacement lever screw if needed. Maybe just not tighten it too much.

Figure 15. Receiver right side with screws marked.

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Hammer and Hammer Spring Next you can take out the bolt that holds the hammer spring. To keep parts from flying when the screw comes out, clamp the spring to the tang while you remove the bolt. After the bolt is out slowly release the clamp. Use a clamp with plastic jaws or use some cloth to protect the tang surface so it does not get scratched. As an alternative, loosen the hammer spring screw a bit and then move the spring off the hammer bushing. To reduce the spring tension put the hammer in the completely down position. The hammer spring screw is a bit awkward to remove. The bit that removes it has to fit between the upper and lower tang. I used a short hex bit and held the bit with a quarter inch open end wrench. Of course I had to do some grinding on the hex bit before it would fit into the screw slot. This bolt was not stuck on my rifle. The bolt holding the spring had a diameter of 0.185 in, close to a modern #10. It would screw into a 10-32 gauge a small amount before it stuck. I tried to put a modern 10-32 screw into the hole in the tang; it would only go about one turn and then stopped. So the hammer spring screw appears to be a renegade with no easy replacement. The spring and screw is shown in Figure 17.

After removing the hammer spring unscrew the hammer pivot screw and remove the hammer. The hammer pivot screw has a thread major diameter of 0.206 in, somewhat less than a modern #12, which has a diameter of 0.216 in. On the screw checker it would fit into both the 12-28 and 12-32 hole. The 12-32 seemed to be the better fit. At this time I have not found a modern 12-28 or 12-32 screw to see if it fits the receiver hole. The shaft of the trigger pivot screw is 0.234 in which is very close to 15/64 inch. The shaft of a 15/64 inch drill bit would go through the hammer hole while a 1/4 inch drill bit would not. So if a new hammer pivot screw was needed, it would be made from 15/64 stock. The length of the 15/64 inch section of the bolt is 0.436 in. 7/16 inch is 0.4375 in so maybe that is supposed to be the length. The length of the threaded section is about 0.125 in. As I mentioned, I have not yet determined the screw threads. The hammer and pivot screw are shown in the above figure.

Figure 17. Hammer and hammer spring.

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If you look closely at the bottom of the hammer you can see a pin. This pin goes through a roller that sits on the end of the hammer spring. For normal cleaning and maintenance there is no need to remove this pin; it would only be done if the roller was being replaced. There are two notches in the hammer, to hold it at half- and full-cock. On my rifle the half-cock notch had some damage, with a small piece missing. The full-cock notch, which releases the hammer when the trigger is pulled, appeared to be in good shape. At this time I have not tried to dress up either notch.

Carrier Next remove the carrier pivot screw and the carrier. Then remove the small screw in the carrier itself. The carrier completely apart is shown in Figure 18. From top to bottom let’s call the three pieces the rocker, rocker spring and carrier. Notice the small spring has the sharp end up. When you put it back together set the spring in the carrier and then the rocker on top of the spring. The right end of the rocker fits underneath the carrier. The small bolt, with the toothpick under it, appears to have two functions. One, to hold the spring and rocker in the carrier and two, to limit the carrier movement. The thread diameter on the small screw is 0.104 in, not a size that seems to match anything modern. However, it does fit nicely into the 4-48 hole on the modern screw check and somewhat surprisingly a modern 4-48 screw also fits the carrier. If you have to make a replacement for the screw it will be important to get the head diameter right; it is about 0.21 inch. The carrier pivot screw looks complicated and, not a help, the threads don’t seem to match anything modern. The thread diameter is 0.184 in, a bit smaller than a modern #10. However, the screw does not fit a 10-32 hole on the screw nor will a modern 10-32 screw fit into the mounting hole. The shaft diameter below the threads is 0.162 in and the length of the smooth section is about 0.55 in. Obviously it would be best not to lose this screw.

Figure 18. The carrier assembly.

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Receiver Side Spring I call it the receiver side spring because I have no idea what the real name is. This spring does not appear to be there on the later Marlin 39A for which I have a manual. Anyway the spring and the receiver mounting slot it fits is shown in Figure 19. The side spring goes back into the receiver with the hook at the end of the spring down as shown. The screw that holds the spring in has a thread diameter of 0.106 in, just a bit smaller than a #4. It does not fit the 4-48 hole in the modern gauge, although a modern 4-48 screw does seem to fit the receiver mounting hole pretty well.

Trigger Spring There is still a bit more. If you tilt the receiver and look down where the hammer spring was mounted you see a trigger spring. This is shown in Figure 20 where the receiver is leaning against a sandbag. This is one of those aggravating screws to remove because you have to have a short bit which fits between the two tangs. Also it is easy to mess up the screw slot because you cannot get any pressure on the bit. Also shown in this photo are sight mounting screws on the top tang. They are slightly out of focus because I concentrated on the bottom tang. I will discuss the sight mounting screws in a minute.

Figure 19. Receiver side spring and mounting slot.

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After removal the trigger spring and screw are shown in Figure 21. The mounting screw has a thread diameter of 0.138 in, a bit larger than a modern #6 UNF screw. The mounting screw would not go into the 6-40 hole on the gauge. However, a modern 6-40 screw did fit the mounting hole in the tang, albeit a bit loose. It could probably be used as a substitute if you did not tighten it too much.

Figure 20. Trigger spring and its mounting screw, also sight screws on top tang.

Figure 21. Trigger spring, also the trigger mounting pin.

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I think this is a far as the receiver needs to be taken down. You could punch out the trigger pin and remove the trigger. However, for cleaning purposes I don’t think it is necessary. If you were replacing the trigger then of course the pin would be removed. The pin should be remove from left to right. On later Marlin models apparently the big bolt could be removed; I did not see a way to do it on my rifle. There was a shoulder on the bolt that prevented removal.

Tang Sight Mount Screws As mentioned before, and shown in Figure 20, there are two screws used for mounting a sight. These screws have a thread outer diameter of 0.166 in, slightly bigger than a #8. The screws would go part way into either the 8-36 and 8-40 holes in the gauge. A modern 8-36 would go into the tang about 3 turns before jamming. I would guess the screws are like 8-38. On my rifle these screws do not look like placeholders, they have large heads. Maybe at some point this rifle had another sight mounted on it and the original placeholder screws were lost.

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Appendix A Modern UNC and UNF Screw Threads The table below shows some of the specifications for modern UNC and UNF screws. The thread major diameter has both a maximum and minimum value. The Allowance field is an additional amount that can be added to the major diameter if the screws are plated. 3A screws are of a tighter tolerance than class 2A. The minor diameter is at the thread root.

Allowance Major dia, in Minor dia, in

Size Class inch Max Min

0-80 UNF 2A 0.0005 0.0595 0.0563 0.0446

0-80 UNF 3A 0.0000 0.0600 0.0568 0.0451

1-64 UNC 2A 0.0006 0.0724 0.0686 0.0538

1-64 UNC 3A 0.0000 0.0730 0.0692 0.0544

1-72 UNF 2A 0.0006 0.0724 0.0689 0.0559

1-72 UNF 3A 0.0000 0.0730 0.0695 0.0565

2-56 UNC 2A 0.0006 0.0854 0.0813 0.0642

2-56 UNC 3A 0.0000 0.0860 0.0819 0.0648

2-64 UNF 2A 0.0006 0.0854 0.0816 0.0668

2-64 UNF 3A 0.0000 0.0860 0.0822 0.0674

3-48 UNC 2A 0.0007 0.0983 0.0938 0.0734

3-48 UNC 3A 0.0000 0.0990 0.0945 0.0741

3-56 UNF 2A 0.0007 0.0983 0.0942 0.0771

3-56 UNF 3A 0.0000 0.0990 0.0949 0.0778

4-40 UNC 2A 0.0008 0.1112 0.1061 0.0814

4-40 UNC 3A 0.0000 0.1120 0.1069 0.0822

4-48 UNF 2A 0.0007 0.1113 0.1068 0.0864

4-48 UNF 3A 0.0000 0.1120 0.1075 0.0871

5-40 UNC 2A 0.0008 0.1242 0.1191 0.0944

5-40 UNC 3A 0.0000 0.1250 0.1199 0.0952

5-44 UNF 2A 0.0007 0.1243 0.1195 0.0972

5-44 UNF 3A 0.0000 0.1250 0.1202 0.0979

6-32 UNC 2A 0.0008 0.1372 0.1312 0.1000

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6-32 UNC 3A 0.0000 0.1380 0.1320 0.1008

6-40 UNF 2A 0.0008 0.1372 0.1321 0.1074

6-40 UNF 3A 0.0000 0.1380 0.1329 0.1082

8-32 UNC 2A 0.0009 0.1631 0.1571 0.1259

8-32 UNC 3A 0.0000 0.1640 0.1580 0.1268

8-36 UNF 2A 0.0008 0.1632 0.1577 0.1301

8-36 UNF 3A 0.0000 0.1640 0.1585 0.1309

10-24 UNC 2A 0.0010 0.1890 0.1818 0.1394

10-24 UNC 3A 0.0000 0.1900 0.1828 0.1404

10-28 UNS 2A 0.0010 0.1890 0.1825 0.1464

10-32UNF 2A 0.0009 0.1891 0.1831 0.1519

10-32UNF 3A 0.0000 0.1900 0.1840 0.1528

10-36 UNS 2A 0.0009 0.1891 0.1836 0.1560

10-40 UNS 2A 0.0009 0.1891 0.1840 0.1592

10-48 UNS 2A 0.0008 0.1892 0.1847 0.1644

10-56 UNS 2A 0.0007 0.1893 0.1852 0.1681

12-24 UNC 2A 0.0010 0.2150 0.2078 0.1654

12-24 UNC 3A 0.0000 0.2160 0.2088 0.1664

12-28 UNF 2A 0.0010 0.2150 0.2085 0.1724

12-28 UNF 3A 0.0000 0.2160 0.2095 0.1734

12-32 UNEF 2A 0.0009 0.2151 0.2091 0.1779

12-32 UNEF 3A 0.0000 0.2160 0.2100 0.1788

12-36 UNS 2A 0.0009 0.2151 0.2096 0.1821

12-40 UNS 2A 0.0009 0.2151 0.2100 0.1835

12-48 UNS 2A 0.0008 0.2152 0.2107 0.1904

12-56 UNS 2A 0.0007 0.2153 0.2112 0.1941

1/4-20 UNC 1A 0.0011 0.2489 0.2367 0.1894

1/4-20 UNC 2A 0.0011 0.2489 0.2408 0.1894

1/4-20 UNC 3A 0.0000 0.2500 0.2419 0.1905

1/4-24 UNS 2A 0.0011 0.2489 0.2417 0.1993

21 Marlin Model 1897

1/4-27 UNS 2A 0.0010 0.2490 0.2423 0.2049

1/4-28 UNF 1A 0.0010 0.2490 0.2392 0.2064

1/4-28 UNF 2A 0.0010 0.2490 0.2425 0.2064

1/4-28 UNF 3A 0.0000 0.2500 0.2435 0.2074

1/4-32 UNEF 2A 0.0010 0.2490 0.2430 0.2118

1/4-32 UNEF 3A 0.0000 0.2500 0.2440 0.2128

1/4-36 UNS 2A 0.0009 0.2491 0.2436 0.2161

1/4-40 UNS 2A 0.0009 0.2491 0.2440 0.2193

1/4-48 UNS 2A 0.0008 0.2492 0.2447 0.2243

1/4-56 UNS 2A 0.0008 0.2492 0.2451 0.2280