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Beginner project/modifying.

Discussion in 'Anything Airgun Related' started by thomasaurus, Aug 4, 2008.

  1. thomasaurus

    thomasaurus Engaging Member

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    Well, the title says it all really :).
    I have my old smk b2 lying around. No power, but still shoots 1 inch groups at 15 yards (for such a cheapy i can't really moan). Instead of binning the thing I was thinking of making a few modifications to make it shorter, get the power up and re-varnish the stock to make it look a little smarter. Any recomendations on other modifications and how much do you think I could cut the barrel down without sacrificing too much accuracy? I think I could shear about an inch off the stock to get a little extra. Also, if i cut the barrel down a bit would I still be able to get a silencer on it? Only really want it for short range ratting and as a bit of a learning curve without running the change of messing up anything expensive, lol!
    Would love any feedback on the matter as your all a fair bit more knowledgable at all this, cheers!
    Tom
     
  2. Rockabilly

    Rockabilly Big Poster

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    spend your money on a better gun mate, if you cut the barrel down it with need to be recrowned!
     
  3. a1gunner

    a1gunner Pro Poster

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    the power level will also drop;)

    if you want to modd it just do a bit of stock work make the internals work better (just give them some TLC) put washers into the spring to make it more powerful (try not to take it over the limit)

    make she stock heavy if you can.
    springers seem to work better if they are heavy
     
  4. thomasaurus

    thomasaurus Engaging Member

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    hey, thanks for the info. will definately look into that. so no new spring would be needed, just a few washers?
    already got a remington genesis, but just want to learn how to tweak bits without any financial worry, at the end of the day if i break it i wont be fussed in the slightest lol
     
    Last edited: Aug 4, 2008
  5. Darren Petts

    Darren Petts Temporarily Alive

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    You can go as low as around 7 inch of barrel in .22 before you get power loss at sub 12 ft/lb in a springer though with each powerplant type that figure will vary slightly. Only way to be certain is to remove a bit at a time and stop at first sign of power loss.

    As it's a B2 it'll only go over the limit when lobbed off a cliff. They only make around 8 ft/lb in good order.
     
  6. thomasaurus

    thomasaurus Engaging Member

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    ah, good info darren. will have a bash at that i think. recrowning seems easy enough with a decent drill or lathe from what ive seen, got a couple of very good quality countersink bits in the van whihc will work a treat.
    will never make it into a good rifle, but its the start of the learning anyway.
     
  7. Biffo1262

    Biffo1262 Active Member

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    I beg to differ. A club colleague was messing about with one just for the hell of it. When he'd removed all the grease, shimmed up the spring and polished the internals it was almost 20ft/lbs !! :eek: He had two choices (well three actually but the third choice is just too obvious) - deshim it and repack the cylinder with grease or sell to the Chinese Air Force as an anti-aircraft gun. He made enquiries about the latter but they told him they already had far too many of them already !! :D:D So it was down to a tin a of Moly grease and flog it. The grease cost more than the gun!
     
  8. thomasaurus

    thomasaurus Engaging Member

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    haha, not bad for something worth less than a crate of beer! i cant see myself getting such good results but it should be interesting
     
  9. Darren Petts

    Darren Petts Temporarily Alive

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    Sorry Biffo but I can't believe that figure. The B2 simply doesn't shift enough air to achieve that. The HW80 only just makes that sort of figure and it's vastly bigger in swept volume. The B2 shifts a Meteor sized air volume.

    Either:-
    Wrong weight went into the chrono
    It dieseled
     
  10. Darren Petts

    Darren Petts Temporarily Alive

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    Here's a rebuild of a B2 I poached from AGF. The part that concerns the 20ft/lb comment is under Testing



    Trying to improve the Chinese B2 Air Rifle

    In the beginning

    A couple of years ago while attending the Bisley Phoenix meeting with an old friend, he slipped his leash and went and purchased a .177 B2 with 4x20 scope attached, from Modern and Antique Firearms, Bournemouth.

    It, together with a very thin gun bag and a tin of pellets was somewhat oversold to him as a 'combo' and at what I considered to be an inflated price (over £100), but he insisted that he didn't want an 'expensive' air rifle.

    The first thing I did when we got home was to remove the scope (I had suspicions) and my instinct was correct. The rear mount was shimmed with a piece of credit card (?) and the clamp had been screwed down with much vigour, crushing the scope tube. We went our separate ways the next day with my friend returning to Bisley to see MAF. After much argument and unpleasantness, the damaged scope was part-exchanged for another poor quality 4 x 32 unit with my friend being, once more, the loser in my opinion.

    Subsequently, the rifle proved to be very inaccurate and this was found to be due to two identifiable causes (apart from the lousy scope). First, the breech jaws were slack, allowing the barrel to wobble slightly. This was corrected with a bit of tightening of the pivot screw. Second, the output power over a chrono showed wild shot to shot variations together with loads of smoke. Using Hobby pellets, even after settling down, shot-to-shot muzzle energy (ME) varied between 4 fpe and 6.5 fpe.

    The rifle was consigned to the cupboard for the next couple of years and my friend bought a Falcon FN19 and pump to continue his occasional shooting forays.

    On a recent visit with him, full of bonhomie and whisky, I offered to 'take a look' at his B2, to 'see if it could be improved'. Oh, foolish me!

    Anyway, that brings us to the here and now and the fun and adventures with a B2.

    Appearance

    The rifle is a very basic break-barrel unit, fitted with a dark wood 'De Luxe' stock. Open sights are standard but scope grooves are present on the cylinder. A lever projects vertically from the left front of the cylinder, just in front of the jaws. This lever, when pressed, releases the detent catch that holds the barrel in line with the cylinder. This saves having to 'whack' the barrel to slip the detent and is a nice touch. Metalwork is roughly finished, especially where bearing surfaces exist (eg. breech jaws) and the blacking is patchy. The trigger is a truly dreadful single stage unit. Fore-travel, comprising slack in the mechanism is accompanied by the groaning and twanging of the spring that sits under the trigger and once this is taken up, the trigger acts directly on the single sear that retains the piston's bent. There is a considerable overlap between sear and bent and as the trigger is pulled, the angle of engagement alters progressively as the sear is dragged off the bent giving a release that is both heavy and creepy. After release, there's about a half inch of over-travel. Well, what can one expect for a rifle (with basic stock) that I have been told sells into the trade in quantity at around £8 to £10 per unit?

    The De-Luxe stock is of some mongrel timber, stained dark brown and lacquered, but with traces of grain showing through. Chequering is unevenly pressed in and an ill-fitting, thin solid plastic butt pad is screwed on. Surprisingly, the inletting is excellent, showing no loose whiskers of wood and is more cleanly executed than most of what I've seen before. Overall, not a bad little stock, but of a lower quality than that fitted to bottom of the range, £100-ish rifles.

    Disassembly

    The two front stock screws were removed. One was loose and did not have a lock-washer under it. Removal of the front trigger guard screw allows the stock and mechanism to separate. The plastic cylinder end cap then drops off. I've heard these things are lubricated with some sort of brown 'goo' and I was soon covered in it! I think, however, it's an inadvertent mixture of rust (well, blacking is a form of controlled rusting and it's pretty poorly executed on this rifle) and oil/grease that's applied as a lubricant.

    After an initial clean-up, the barrel release catch was pushed forward and the barrel was released from the detent, but not cocked. The screw holding the barrel release was removed, together with the dished Bellville washer under it (note which way it is placed on the screw) and the release catch removed.

    The barrel pivot pin that runs through both jaws at the end of the cylinder and the barrel block should have had a small locking screw next to it, but that was missing because when my friend tightened the jaws to remove slack, the pivot pin's position made it impossible to use the locking pin - so he lost it! The pivot pin unscrews easily when the barrel isn't locked up, but don't try to move it with the barrel locked in position by the detent - it is under stress and could be damaged by excessive force. Once the pivot pin is removed, the barrel and underlever, which is permanently attached to the barrel block by a stamped-in roll pin, can be removed. A wee bit of wiggling may be needed to remove the underlever end from the cylinder's cocking slot, but it does come out easily.

    Next, the 'trigger block' needs to be removed in order to remove the spring and piston. Looking inside the cylinder from the rear, it can be seen that an inner sleeve is used to retain the rear spring guide and this, together with the trigger mechanism, is held in place by a single pin that runs through both sides of the cylinder and sleeve and acts as a fulcrum for the trigger. Under the trigger is a spring that is held in place at one end by a small lug projecting from the sleeve and at the other end it is centred on a small dimple pressed into the trigger face near its base. The spring was hooked out with a probe.

    Holding the cylinder firmly and vertically on a flat surface, it was found that the retaining pin was easily pushed out from one side, when the trigger dropped out. A bit of wiggling removed the pin from the far side of the cylinder, releasing the inner sleeve. There was only about 6 or 7 cm of pre-load on the fairly weak spring, so its tension was released easily and the sleeve, spring guide and spring dropped out onto the bench. The retaining pin was examined and was rather distorted at one end, probably caused when it was stamped into the cylinder, so I'd been fortunate in my choice of direction of removal! The pin showed some signs of wear and had rough spots on it.

    It was also noted that the stock fixing bolt, just in front of the trigger, would pass through the lug, cylinder and inner sleeve if the threads were cleaned up. This might help in reassembly by holding the inner sleeve in place against spring tension while re-fitting the trigger and retaining pin.

    The main spring looked OK but the rear guide was made from a piece of seamed rolled tube with washers welded to the end. The seam running the length of the tube (guide) was open.

    Removal of the piston took rather a lot of pushing with various implements to persuade it to exit at the rear of the cylinder, as the piston washer was extremely tight within the cylinder. This washer was held onto the front of the piston by a long cross-headed screw and was about 1.5cm thick. As far as can be ascertained, it is a deep cup washer made of leather, but the cup has closed completely. Although the outside is oily and has some metal swarf embedded in its sides (where it slides within the cylinder), inside the washer is stiff and dry. The piston was a truly nasty piece of construction, again being made from rolled tube with an open seam and with the cocking slot machined in the side opposite the seam. It is slightly swaged down in its mid-portion to prevent drag against the cylinder's inner wall. The misshapen head of the piston looked like it had been forged with a lump hammer and all metal surfaces were blackened and pitted as though exposed to a coke fire! Only the most cursory smoothing had been done, barely sufficient to get the piston to move within the cylinder.

    A generous quantity of Gun Scrubber was used in the removal of as much muck as possible from all parts before progressing further.

    img-resized.png Reduced: 86% of original size [ 532 x 277 ] - Click to view full image
    b2_pic1.jpg
    1. Compression cylinder.
    2. Part of piston just visible inside spring.
    3. Spring.
    4. Spring guide.
    5. Inner sleeve.
    6. Rear stock retaining bolt.
    7. Retaining pin/trigger pivot pin.
    8. (There isn't one.)
    9. Trigger and trigger sear.
    10. Trigger return spring.

    Because of the swaging mentioned earlier, the piston tapered slightly internally, so some of the springs I could have tried only passed half an inch into the piston before binding. Comparing the spring with those recently removed from an HW77 and a TX200, it was much shorter at 19.5cm, but was acceptably true with regular coil spacing and a total of 27 coils.

    The pressed steel trigger had a sear made from three strips of metal riveted to the trigger, thus creating a sear that is able to exhibit a crude degree of self-alignment on the piston bent (notch). The sear acts directly on the piston bent with no intermediate sears. Once the bent/piston is released, the trigger mechanism can continue on over-travel until it is eventually blocked by some part of it coming to bear on an unmoving part of the rifle. Suffice to say, no means of controlling over-travel was designed into this amazingly basic mechanism the design of which probably dates back to the earliest of crossbows.

    The three metal strips forming the sear showed some signs of wear polishing on their faces, so a needle file was used on a non-contact (safe) portion to establish metal hardness. This metal proved to be very hard, so was case-hardened. The metal comprising the piston's bent was also found to be case-hardened. With proper lubrication the trigger action should last a long time and be reasonably reliable (although heavy, due to its simplicity). I was told that on earlier models the surfaces comprising the trigger had not been hardened, but have no other evidence of this.

    It's worth studying this trigger mechanism to see just how simply a release mechanism can be effected.

    img-resized.png Reduced: 95% of original size [ 482 x 246 ] - Click to view full image
    b2_pic2.jpg
    1. Leather piston washer.
    2. Piston bent (engages with trigger sear).
    3. Trigger reassembled. Trigger goes inside inner sleeve and is retained by single pin. Spring sits on dimple behind trigger and on lug at rear of sleeve.

    The leather barrel-sealing washer lies in a groove in the barrel and was perfectly flush with the barrel block surface, so it would need checking to see if it provided a good seal, but that could be done later.

    The refurbish

    My first problem was to find a suitable replacement spring. As the diameter was smaller than those 'in stock' I decided to pay a visit to ISP Air Guns to see what Shaun had in his workshop.

    At this point, Shaun took over the job. I didn't stand a chance!

    He discovered that a BSA Meteor spring would fit reasonably well as it has a smaller diameter than most. He decided that the full-length spring would be too long for the rifle's short piston travel and would become 'coil bound' (that is, when the spring is fully compressed so that each spring coil is touching its neighbours, the spring would still be too long for the space available). The Meteor spring was shortened to 28.5 coils and 21cm in length. (When tested later, the coils were very nearly touching their neighbours when the rifle was cocked, confirming Shaun's judgement in the matter).

    b2_pic3.jpg
    Original spring (top) and shortened Meteor spring (bottom).

    Next, the piston, piston guide and chamber were polished and burrs removed.

    The piston washer was very dry and a very tight fit in the bore, so the diameter was reduced a little by wrapping some wet and dry paper around the washer and rotating it a couple of times. The washer was then left to soak in some motor oil to soften it up and make it more flexible.

    A little polishing of the trigger sear and piston bent was also undertaken, but there seemed very little that could be done to improve the action of this basic mechanism.

    On looking down the compression chamber, considerable burring was noted around the angled transfer port. With the aid of a couple of long drill bits, the burrs were removed and all loose debris was removed.

    Reassembly

    After removing surplus oil from the piston washer, especially from the face, the greased piston, spring and spring guide were re-inserted into the cylinder. The spring projected from the rear of the rifle somewhat and this is where the fun began.

    Everything is held together by a single retaining pin. That pin passes through the cylinder, the inner sleeve, (which retains the rear spring guide) and the trigger before passing through the other side of the inner sleeve and finally the other side of the cylinder. There's a lot of strain on this single pin and Shaun was of the opinion that if a stronger spring than the Meteor one were to be fitted, it would eventually bend it. Although the rear stock fixing screw could project through the cylinder to act as an additional retainer, with the stock in place it was not quite long enough.

    Due to the spring's compression, it was a difficult task to line up the holes in all the various bits, especially the trigger and segmented sear and push the retaining pin through it all. After cleaning up the rear stock fixing hole thread, the piston, spring, guide and inner sleeve were compressed into the cylinder and the stock fixing screw was inserted to hold it all together while taking a breather. The new spring was a lot stronger than the original!

    Eventually, the inner sleeve was pressed a little further into the cylinder while the trigger was inserted through its slot and the pivot-cum-retaining spring was reinserted. This was done in stages, first getting the pin through one side of the cylinder and sleeve then far enough in to hang the trigger onto it, then through all the trigger segments and to the far side, then eventually through the other side of the inner sleeve and cylinder. This was done without a spring compressor, which would have been most useful on this occasion.

    Had the pin been straight and the machining had been to a higher standard, it would doubtless have been easier, but after a bit of a struggle the pin was tapped home. The trigger spring was then replaced and coated in moly grease to stop its creaking and groaning. After greasing the pivot areas and applying a little oil to the pivot pins, the cocking lever/barrel assembly was re-fitted and tightened up. The lack of the barrel pivot pin locking screw offended Shaun, so a replacement was quickly located from 'the box' and the jaws were tightened up and locked off.

    The barrel lock detent is held in by a small screw beneath the front end of the cylinder. This was removed together with the detent and its spring for cleaning and lubrication. Cover it with a cloth when removing as there's a fair amount of pressure from its spring. When reassembling, ensure the machined flat portion of the detent faces the locking screw and is greased. Adjust the locking screw so that the detent just slides to and fro when pressed.

    Next, after lubrication, the barrel lock release was re-fitted together with the dished belville washer and finally the stock was re-fitted. Everything functioned smoothly.

    img-resized.png Reduced: 65% of original size [ 697 x 133 ] - Click to view full image
    b2_pic4.jpg

    Testing

    The rifle was then cocked - with only a faint whisper of sound from the spring and associated mechanism - and a pellet loaded into the breech.

    Firing was a much smoother action than before, without the previous accompaniment of complaints from the trigger return spring. There was a teeth-rattling recoil, bang and cloud of smoke, which was due to some of the oil on the face of the leather piston washer exploding under compression. Dieseling!

    A peek at the spring through the cocking slot revealed no disasters. The spring looked fine, so several more shots were fired. After half a dozen or so, the dieseling ceased and the muzzle report of the rifle was positively quiet. The cloud of smoke disappeared, leaving only the smell of oil after each shot.

    The cylinder/barrel interface was checked for leaks as the original leather barrel seal was suspect but had been treated to a spot of light gun oil that soaked in fairly quickly. This probably caused the leather to swell slightly as there was no discernable leak, so no replacement was necessary.

    After a break, the dieseling was again present for the rifle's first few shots and this seems to be the expected pattern of things for a while until surplus oil is eliminated from the piston washer. The rifle had, by this time, fired around 50 shots.

    A hundred shots were put over the chrono and, with Hobby pellets, was shown to be running reasonably consistently at just over 7 fpe. The first three or four shots had to be disregarded due to impressive dieseling (one shot ran at 16fpe) but thereafter, the rifle settled to around 680 fps +/- 10 fps, provided it was not left to stand for a while. If left to stand for more than a couple of minutes, dieseling would again occur for the first two or three shots (but only to the tune of 854 fps / 11.18 fpe). With further use, this should settle down but where a leather washer is used, it seems advisable to disregard the first few shots of a session and to be mindful of the length of time between shots. Due to the small swept volume of the compression chamber and the previously described mechanical weaknesses of the design, Shaun reckoned that this was about as good as it could get. Further attempts to increase power would probably result in some serious bone-shaking and any claims that 12fpe is achievable should be treated with caution.

    As expected, the single sear trigger remained heavy, but released consistently and fairly predictably with a progressive increase of trigger finger pressure. The over-travel remained excessive, but could be reduced somewhat by positioning a small block inside the trigger guard. After a hundred or so shots had been fired, everything on the rifle seemed much smoother; barrel release was easy, cocking was light and quiet and even the trigger eased. Firing produced some recoil and noise, but neither was excessive.

    Acknowledgement and conclusion

    Grateful thanks to Shaun of ISP Air Rifles for advice freely given and expertise in matters of spring, piston seal, polishing etc.

    The B2 is a cheapie, and that is reflected in the final finish of some components. Simply re-finishing and re-lubricating all the original parts and reassembling them all back together was rewarded with consistency. Fitting the cut-down Meteor spring to increase the power output may have been unnecessary in view of the marginal power increase. How much it contributed to the smoother cocking and firing cycle is difficult to assess without a further strip and rebuild using the original spring.

    It's not possible to make a silk purse out of a sow's ear, but the work and small cost involved certainly carried a large bonus by turning a rough machine into a good little plinker.

    DaveL

    © July 2002
     
  11. thomasaurus

    thomasaurus Engaging Member

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    excellent info! going to print that out later and use it as a guide to start with refinishing the internal, thanks :)
     

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