Slavia 631 tune-up

Here's my new CZ Slavia 631 air rifle. I got a pair of them recently, for the wife and I to do some bullseye target shooting out back. I had read on the Canadian Airgun Forums that these guns typically come with no lubrication for the spring/piston, and figured I might as well get an upgraded spring while I'm at it. Airbuddy3, a forum member, was happy to oblige. Even had some lube to do the job. Great! The polishing, deburring, and lubrication done during this job will greatly improve the 631 even with the stock spring, but I'll be installing a custom one. Also, you don't have to perform all these steps. It isn't necessary to disassemble the barrel hinge, barrel latch, or trigger safety, but I figured I might as well do it all while I'm at it.

These pictures don't necessarily show the state things were in at the time of the descriptions. Some things later on in the process might have pictures that were taken earlier, during disassembly, and I just recycled them to show where to lube, etc. Hopefully I haven't left anything out.

What will you need?

  • flathead screwdriver
  • sandpaper - 220-grit (I also used 400 and 600 to further smooth things, but this isn't strictly necessary)
  • a fair sized piece of glass, or mirror, is helpful as a sanding/polishing work surface, but not necessary
  • moly lube (I used Dow-Corning 70% moly lube which I got from Airbuddy3, apparently SuperLube from NAPA will do the trick, too, but you run the risk of 'dieseling' and potentially damaging the gun, so if you can get the moly lube it is a much safer way to go)
  • gun cleaner/degreaser (I got an aerosol can from crappy tire, Gunslick Brand metal degreaser and cleaner, same aisle as the airguns)
  • gun oil wouldn't hurt either
  • big kitchen knife
  • scissors
  • cotton swabs are helpful
  • latex/vinyl gloves
  • old toothbrush
  • 'orange' hand cleaner
  • paper towels
  • hand towel or rag

As you can see I've set aside some space to do the work. A coffee table with raised edges in my case, so parts won't roll off. Get yourself a tray or bowl or something to hold small parts while you're doing this job, if you don't have a similar space to do the work. There aren't many parts, but you don't want to lose any. Taking the 631 apart is pretty simple. There are three screws on the bottom of the stock that are holding it to the rest of the gun. The front two, and the one at the back of the trigger guard.

Once you loosen those screws you can lift the barrel assembly off the stock. It just lifts straight up off of it, quite easily. Just be careful with the trigger, it has to come past the trigger weight spring. There is a guide resting in the stock that may come loose, so be careful not to lose it.


That metal guide in the stock can be a pain in the ass. I just did my other 631 and had a problem immediately after reassembly. The thing would dry fire on its own before cocking was complete. It appears they dug out the mount point for this guide a little more on my second gun than they did on the first one, and consequently it was not holding the cocking arm firmly enough against the body tube. So, it was lifting out of the slot in the piston and letting the piston fly forward. I took a piece of thick paper just longer than the guide, and wide enough to fold over a couple of times and stuck it under the guide to raise it up a bit. This seems to have fixed it. Surprising that the small amount of material taken off the piston ends caused a problem with this, I guess combined with the slightly deeper positioning hole for the guide anyway. So if yours is dry firing while you're trying to cock it, try raising that guide in the stock up a bit and that should do the trick.

With that apart, the next thing we need to do is remove the trigger. First the drift pin holding it in has to be removed. You can see an existing dimple in mine. I'm guessing this was done to indicate which end it should be driven from when attempting to remove it. Either that, or mine just had a dimple, heh. I think the pin is ever so slightly tapered, too, so make note of which direction you took it out so you can put it back in the same way it was.

Anyway, I took a small punch and was going to get a hammer to drive it out, but mine just came out from pushing the punch by hand. If yours is a tighter fit, a dead blow hammer might be in order, or maybe a screwdriver handle will be sufficient. I don't imagine they're ever in there really, really tight. I put the end of the gun up on the table edge, just to give the pin room to come out the bottom.

With the pin removed, you can now take the trigger out. The trigger more or less just slides out the bottom, but there is a little trick to it. While you're trying to slide it out you have to move it forward enough for it to clear the end of the slot that it's resting in.

Once you've done that, it'll slide right out. There is a little spring at the front of the trigger that could fly out, so take care not to lose it when removing the trigger the rest of the way.

And here is the trigger, and that little spring. Set them aside, and move on to removing the end plug from the body tube.

This part of the trigger/release mechanism, called the sear, can't be sticking out, or you won't be able to remove the end plug. This is because it is actually installed in that end plug, so it won't be able to turn if it is sticking out. At first I wondered why the plug was on there so tight, I couldn't budge it at all. I just happened to turn the slot face-up while still trying to get the plug to move, and since the sear had fallen back into the slot the plug turned quite easily, haha. And you could then see that part turning inside the tube along with the end cap. D'oh!

The stock spring doesn't have much tension on it, just keep a firm hand on the end plug as you get closer to having it unscrewed completely. It took eight or nine turns for mine to come free, and here you can see how far out it came to rest once free.

With the end plug removed the spring guide and the spring should slide right out if you tip the barrel up.

The plastic spacer should also slide right out, though I've heard some people have had trouble getting it out of the end of the piston because something was keeping it in the end. Lube/dirt/something. Mine slid right out, anyway.

Here you can see the stock 8" spring beside the 9.5" custom spring I got from Airbuddy3, with the washers/spacers over on the left. Next to come out is the piston, but this most likely won't just slide out on its own. It'll likely take a small amount of force to loosen it because of the piston seal at the end of it. Once you move it a small distance it will likely start moving pretty easily, so don't over do it.

Take a flathead screwdriver and insert it here, and gently try sliding the piston towards the back of the tube, and once it starts moving take care in removing it. You want to be as gentle as possible with the piston seal at the top of the piston, taking care not to damage it on the threads at the end of the tube. It is small enough to pass the threads without touching them, so just be careful with it and you'll be fine.

And here's the piston and the piston seal. As I mentioned, you don't want that seal to get damaged, or you'll have to get a replacement. It just pops off the end rather easily, although the manual says to pry it off with a screwdriver. Mine just pulled off by hand. If for some reason your seal came off in the tube the manual also states that it has an M5 thread in it, so you can use a rod with an M5 thread to reach into the tube, screw into it, and remove it.

Seal just pops off. You can see how rough the piston's contact surfaces are. The two ends of the piston are slightly bigger in diameter than the middle portion, and the ends are where it contacts the inside of the body tube. The middle portion should be small enough compared to the ends that it doesn't contact the tube, but mine has some wear (on the opposite side) which would indicate it is sliding against something in there. Perhaps there are some blemishes in the tube's machine work. Didn't check. I am guessing that this contact might only occur during cocking. You can see the marks I'm talking about later, in the picture after I've polished the piston ends.

The barrel hinge has two screws in it. The small one is just a type of set screw, to hold the hinge screw in place so it won't become loose. You might want to make a little drawing that shows the direction that the screws are facing, to make adjustment quicker when putting it back together.

The larger hinge screw comes right out after you've removed the smaller set screw, and then you can remove the barrel from the body tube.

To take the hinge latch pin out you simply turn it 180 degrees, and it'll slide right out. The spring will also slide out, but to get the spring guide out you might have to do a little shaking. It might get slightly hung up on the ridge at the end of the pull bar. Don't worry, it goes back in a lot easier than it comes out. The head of it is angled so it slides past the pull bar's ridge just fine. And here are the spring and spring guide.

I forgot to take pictures of the safety removal, but it's just the one piece, held in by a pin and a spring. You can see the silver spring and pin right here. If you turn the plug around you can see a flat portion where you can slip a flathead screwdriver beneath the spring, allowing you to lift it enough to allow the safety to slip past the retaining pin. Reinstallation just involved pushing the safety back in, simple.

Phew, all apart! 

OK, now for the polishing. I borrowed a trick I learned from CPU heatsink lapping. I grabbed a piece of glass, typically a very flat surface which is ideal for lapping/polishing the surface of a heatsink so it will mate better with the top of your CPU. And I laid my sandpaper down on the glass, and polished the end of the main spring, using circular and figure-eight motions across the paper. The goal here is to end up with a nice, flat, shiny surface on both ends of the spring. Something that will, combined with the lube, provide a smoother, more slippery surface when it moves/twists as you cock it and shoot.

Here's mine after going over it with the 220-grit sandpaper. After this I also did it with the 400-grit and then 600-grit to make it even more smooth. Less friction, less unwanted gun movement.

Now onto the piston. As I mentioned earlier, the ends of the piston have a slightly larger diameter than the majority of its body. Those are the areas you'll want to polish up, the machining tends to be rather rough. I did this by first cutting a sheet of sandpaper into eight pieces. I took one of those pieces and wrapped it around the end of the piston, and then wrapped my thumb and forefinger around it. I then squeezed the paper around the end, and with my other hand, spun/twisted the piston back and forth while maintaining a firm grip on the paper.

After doing the same for the other end, you should have a piston that's almost ready to go. Remember, you don't want to remove a lot of material from the piston's sliding surfaces. You only want to get rid of the rough machining job, so it will have a pretty smooth surface to slide on. My piston had some rather deep nicks and cuts here and there, and you can see some of those in the polished ends here. They're just too deep to worry about fixing, as it would definitely require removing more material than you'd want to in order to completely smooth the entire surface. Just make sure the majority of the surface is nice and shiny, and it should suffice. By my thumb you can see the marks I was talking about earlier, that I think are probably occurring during cocking.

Some of these guns have worse spring twang than others. In fact, of the pair I bought, one was pretty quiet, and the other had a pretty loud twang. Fixing this is pretty simple. The spring guide being too loose is the major culprit when it comes to twang, and sizing it to fit the spring properly will take care of the unwanted noise and movement. The spring guide is a small piece of sheet metal formed into a tube, with a seam that you will spread open to better fit the inside of your spring. Grab the kitchen knife and slide the spring guide onto the dull edge. It may help to use the screwdriver to slide the spring guide up the knife towards the handle. I braced the screwdriver between my leg and the carpet so I could push the guide against the screwdriver shaft to slide it up the knife. BE VERY CAREFUL, IT'S A FREAKIN' KNIFE!!!

Once the guide is all the way up the knife, insert the screwdriver tip into the end and pry it up and off the knife. The knife's blade gets thicker as it goes towards the dull edge, so it works rather nicely for spreading the spring guide evenly apart.

You may have to do this several times until you get the guide spread far enough apart to fit nicely inside your spring. It probably shouldn't be so tight that the spring guide won't fall out, but it should be tight enough so that it barely wiggles at all inside the spring. Mine was pretty loose to begin with, and after spreading it out until it barely wiggled in the spring I get no twang at all.

The slot in my body tube for the cocking lever had a rather nasty bump in the middle of it, so I figured I'd better get rid of it. Check the cocking slots on your body tube and on the piston to see if yours could use a deburring, too. If so, the screwdriver will come in handy here. This will only affect how smoothly your gun cocks, not its shooting behaviour. So you can skip this without worry, but it is nice to deburr the slots and smooth out the cocking action if you don't mind spending the extra time. You'll want to deburr the entire length of the slot sides, not just the spot I highlighted. That's just the bump in my slot that I mentioned I wanted to get rid of. Won't hurt to deburr the trigger slot in the body tube either, since you're there already.

I took one of those 1/8-sheet pieces of sandpaper and folded it over twice, and placed the screwdriver into it in order to sand the slot surfaces. This gave me a small and hard 'sander' to insert into the slots to get that job done. Worked decently, though I kept poking through the paper no matter how firmly I held it to the screwdriver blade, heh.

Once all those slot edges are taken care of, well, all the hard work's done. Just messy work left now, hehe. Anything that's been sanded needs to be cleaned. Probably best done outside. Grab the cleaner/degreaser, some paper towels or newspaper (or both) and spray everything down. I soaked down the spring and spring guide with it a couple of times and then gave a quick wipe with paper towel just to get any remaining gunk off that didn't simply rinse away. The cleaner dries very quickly, so you don't have to dry the stuff off. You just want to get the rest of the gunk off. Then I did the same with the piston and body tube.

The body tube had a crapload of CZ's 'preservative' down in the air chamber, so I twisted up a length of paper towel and shoved it right down there, further twisting as I was pushing it down. The cleaner already got most of that crap out of there anyway, but this got out the remains. Since I had everything else apart I also cleaned up the hinge sliding surfaces, hinge screw, hinge latch, and the safety. The cleaner's label said something about it harming certain types of plastic, so I didn't risk using it on the piston seal. Instead I took an old toothbrush and 'orange' hand cleaner and cleaned/degreased the piston steal with that. Be very light when you do this, as the orange cleaner likely has abrasives in it in addition to the cleaning agents, and you don't want to gouge the piston seal. You just want to clean it up. If you're more patient than I am, then I guess you could just use hand soap.

Lubing everything up is pretty messy, so I didn't get any pictures of this portion of the job. I'll make sure to have the wife on-hand to take pictures of this portion of the process when I do her gun, so I can put some up after that's done. Because it's potentially very messy, you'll want to pay close attention to the order in which you lube things. That moly lube easily gets everywhere if you're not careful. Basically, you will be putting the gun back together at the same time you're lubing it, so that lubed parts aren't going to be getting lube all over unwanted places. Now would be a good time to rip off a bunch of sheets of paper towel so you can just pick one up to use when you need to. And throw on one of the latex/vinyl gloves, as you'll be using your finger to apply and spread the lube. Alternatively, you can take a cotton swab, snip off the cotton end so you just get a small stick, and use the end of that. I did both, depending on the size of the part I was lubing.

I put the piston seal back on the piston, and put a very light coat of lube on the sides of it where it will seal against the sides of the body tube. Don't put any on the top face of the seal. Then I put some on that end of the piston, the part that was polished earlier. Then carefully slip the first half of the piston into the body tube, being very careful as you pass the piston seal past the body tube's threads. No seally, no shooty. So insert it carefully. Wouldn't hurt to smear a little bit of lube on the middle part of the piston where I saw the cocking wear marks, if you show the same marks on yours. Leave the bottom polished end sticking out, without lubing it, for the time being. We'll be moving to the main spring and spacer(s) now.

I put a small smear of lube on each side of the spacers as I slid them on the shaft inside the piston. Just enough to see the lube colour on them. Then I took a small amount of lube, about the size of a grain of rice, and smeared it onto the polished end of the spring. With the swab stick I smeared some onto the interior surface of the spring as far as the little stick would allow, in case it ever contacts the piston's interior shaft. Then smear a small amount of lube onto the exterior of the spring, as this will likely contact the interior surface of the piston as it moves. Don't do the whole exterior of the spring yet, you still need to handle it to lube the other end. Just lube as much as will currently slide into the piston.

We left the bottom polished end of the piston sticking out of the body tube all this time, now lube that surface up. Then you can use the spring to push it into the body tube. Push it in just enough to clear the end of the tube, so the lube doesn't get out of hand. If you removed the safety as I did, put a bit of lube on those sliding surfaces and put the safety back in the end plug. Now you can lube the rest of the main spring's exterior, and the polished end. You don't need to worry about the interior of the back end of the spring as you did the front. It will be lubed because you'll lube the exterior of the spring guide. So do that now, lube up the exterior of the spring guide and insert it into the spring. Once you've done that, put a little bit on the end of the spring guide, too. Then we'll be putting the end plug back on.

The end plug will go on pretty easily if you're keeping the stock spring. If you're using a custom spring similar to the one I used then you'll need to use a bit of muscle to get it back on. I had to use a hand towel folded over a couple times to be able to apply enough pressure to push it far enough to get the threads to catch, because the safety was digging into my hand. I guess I could have just put the safety back in after the plug was back on, but I wasn't sure if the pin and spring that holds it in would be able to move enough once it was back in the body tube. So I had the towel folded over a couple times, and the other end of the tube in the carpet, and pushed down pretty hard to get the threads to catch.

Tighten up the end plug until the portion of the slot between the plug and the body tube is just showing, as you see on the left. If it looks like the picture on the right, then you've gone too far and need to back it off so you can see the little bit of that slot. Look in the trigger slot to make sure you've got it lined up just right, and the sear should fall in and out of the slot easily.

If you took apart the barrel hinge, latch, etc, as I did then lube up those friction areas as you're putting them back together, too. Smear some lube onto the sliding surfaces in the hinge area, and onto the shaft of the hinge screw before you put it back in. Before you put the latch pin back in you can pull the pull bar out and put a little lube on it, and then push it back in. Lube up the shaft of the hinge latch pin, and put it, and its spring and spring guide back in. Turn it back 180 degrees to lock it in place. Put a small dab on the sliding latch pin surface, where it rubs the other fixed pin in the body tube.

You can make the trigger a *little* smoother with very careful lubrication, too. VERY careful lubrication. I believe you should not get any lube anywhere near the areas where the sear and the trigger touch each other, or else there won't be enough friction to properly hold the spring back until you are deliberately pulling the trigger. It could make it randomly release, which obviously is very dangerous. You can put extremely small amounts of lube on the side portions of the sear, very near to the pin on which it is hanging from. But keep the lube away from the angled 'bent finger' portion that is supposed to hold the tip of the trigger. Can't stress this enough, don't get any lube in that general area! As for the trigger, it's even trickier because of its design. If you can carefully get some lube on the side portions near the pin that will rub, ok, but stay way from the area that meets with the sear!! And be very sparing with any lube in this area! You don't want any to be able to work its way to the sear/trigger area.

Putting the trigger back in is the exact reverse of the removal procedure. Put the spring in the sear, and into the trigger, and slide the trigger up into the slot. And once you pass that 'landing' portion of the trigger through the slot, push it backward and in. You should be ready to put the body tube back into the stock, but there's a bit more to lube up.

Hold the body tube upside down, so the cocking lever is resting on it. You should smear a line of lube under the whole thing. I guess it was probably oiled from the factory in that area, but I overlooked that, and obviously the cleaning/degreasing step would have gotten rid of all that oil. I put my gun back together without putting any lube there, and it seemed fine until I got maybe 20 shots through it. The cocking action got incredibly hard, very quickly. I took the stock back off and saw a lot of wear under that cocking lever. So don't forget! Hehe. Smear some lube along the edges of the slots you deburred earlier, too. Obviously helps the cocking lever slide smoother through there. You'll want to lube the outer edge of the cocking lever, too, since it will be sliding on that metal guide in the stock.

It might be tricky to get the body tube back into the stock, depending on how loose that metal guide is in the stock. Hopefully it's tight enough that you can lower the stock onto the tube, rather than having to lower the tube onto the stock. Nicer if you can lower the stock onto the tube, because that cocking lever flopping around can get messy, what with the lube on two of its edges. If you look through the portion of the stock where the trigger is going you will see it can go through in two spots. It should go through the one towards the front of the gun. What you see in there is the adjustable spring that gives the trigger its weight.

Snug up the stock screws and you should be good to go! You should now have a gun that operates much, much smoother. And as a result, it will be producing much less unwanted movement. Which, of course, means it will be more accurate than before, too! Woohaa! :)

Thanks again to Airbuddy3, your instructions were enough for me to go on. And the tune-up kit was beyond satisfactory! As I write this I've only been shooting for just over two weeks, and I'm sure the much more accurate gun that I have post-tune is going to make it much easier to figure out errors in my technique. I immediately saw an improvement in my grouping. The targets on this sheet are 3" in diameter, and before the tune, I was usually keeping a group of 10 shots inside that 3" circle, shooting standing off-hand from 10m. But all over the place within that 3 inches. Now, as you can see, just from the tuneup and the spring change I was able to keep most of my shots in the smaller 1-5/8" black area of the target. I have been using these targets ever since I started shooting a couple weeks ago, and have been firing 10 shots into each one. I've been slowly progressing, but post-tune it would seem I'm able to do better than I thought. :) I started in the top center target here, and went clockwise. Except for that first shot on the square's line I didn't do too bad, I think.