QB 78 Tuning Tuning the QB78 / XS78 Strip-down and Rebuild with Tuning Kit

 QB 78 Tuning Guide

Comments will be closing on this site in the near future. Many thanks, Matt

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My Tuned QB78

pixel QB 78 Tuning Tuning the QB78 / XS78 Strip down and Rebuild with Tuning Kit

QB 78 Tuning Tuning the QB78 / XS78 Strip-down and Rebuild

I recently obtained a tuning kit from T R Robb in the UK and have put this article together as a photo record of the procedure for QB 78 tuning and an aid for others doing the same thing. The job can be done with a minimum of tools: Screwdrivers, Allen keys, a punch, some drifts, small hammer and a long screwdriver or home made tool for removing the valve body. Any large flat bladed screwdriver roughly 200mm or more long should do. 115cf439c37071d QB 78 Tuning Tuning the QB78 / XS78 Strip down and Rebuild with Tuning Kit

QB 78 Tuning Kit

The QB 78 tuning kit laid out with my stockless QB ready for business!

WARNING: Remove powerlets before commencing work on the gun!

Also before beginning, bear in mind that the legal limit for airguns in the UK is 12 ft. lbs. The QB 78 Tuning kit may well boost you guns performance beyond this. Always test your guns power after doing any tuning work

We start by taking the stock off, before we can do this it is necessary to remove the safety catch. Set the catch to SAFE then use a drift to punch it out from the left hand side of the gun (as viewed from the firing position).

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QB 78 tuning preparation

With this removed, unscrew the stock screw and remove the action from the stock.

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    3e88af25ca95144 QB 78 Tuning Tuning the QB78 / XS78 Strip down and Rebuild with Tuning Kit

Next we remove the barrel. Use a Phillips screwdriver to loosen the ‘A’ clamp at the front and slide it forwards.

7426bdacf5c5671 QB 78 Tuning Tuning the QB78 / XS78 Strip down and Rebuild with Tuning Kit

Next remove the 2.5mm Allen screw from on top of the rear of the barrel. You can now simply twist and pull the barrel out.

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QB 78 Tuning – The barrel removed

Next we remove the trigger unit which simultaneously releases the two plugs from the rear of the action. Remove the two screws which hold the trigger unit and remove the unit. The front screw is somewhat obscured by the trigger housing and so some careful use of the screwdriver held at an angle is required. The breech plug comes out easily and the tube plug may also come straight out though in the case of my QB it was very tight so I left it until after the next stage. If you remove yours at this point the spring guide and spring and the hammer can be removed now. (See further below)

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To remove the bolt housing, pull the bolt back and remove the Phillips screw from inside the housing just inside the loading port. This screw should come out very easily (take care not to lose the washer that goes with it) and the bolt housing can be removed from the gun. As can the two pins that are left sitting in the top of the tube.

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A 2.5mm Allen key is used to remove the screw that holds the bolt to the bolt handle. The key slides in through the rear of the bolt housing and into the screw in the bolt. With this out, take out the bolt handle and the bolt itself will slide out of the housing.

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We now return to the lower section of the gunand with the bolt housing removed, if you haven’t already done so, the tube plug can be removed maybe using a small drift and hammer to encourage it through the slots in the tube. The spring and spring guide came out easily afterwards as did the hammer in turn. The large stud underneath the gun is also removed. Observe how the short thread goes back into the gun on reassembly and the long thread holds the stock.

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The inside plug can also be removed. Mine came out easily whilst some can be tight but can be removed by working it out bit by bit through the slots in the chamber. It may help if the block is particularly tight to tap them out with you punch and small hammer as with the tube plug.

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We now need to take that long screwdriver to remove the valve Piercing body from the muzzle end of the tube. I didn’t have a screw driver this long with a decent sized head on it and used a piece of flat bar approx. 15x3mm section which did the job nicely as the joint wasn’t tight at all. The piercing body and internals should now slide free from the front of the tube.

The assembly can be further dismantled although the QB 78 tuning kit comes with a replacement that makes this unnecessary. I have included a picture for reference anyhow. Your gun may have a gauze filter in between the fibre filter and the washer. Mine was absent.

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Now we can remove the screw on the bottom of the tube which will free the valve body which can be tapped out through the front of the tube with a long drift.

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Ready for the QB 78 Tuning Kit

The entire Valve assembly, tube and screw removed from the tube freeing the exhaust valve body

So that’s the whole thing stripped down and we can start reassembling with the new parts from the QB 78 tuning kit. First check the fit of the new bolt. Sometimes it is good at this point to do a bit of extra tuning on the bolt housing, removing any burrs with some emery cloth and generally cleaning it up ready for the tuned bolt. It’s best to give everything a good clean at least before reassembly. When done we can fit the new ‘o’ ring which slides on quite easily until it slots into the groove on the new stainless steel bolt.

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The old bolt and the new one from the QB 78 tuning kit.

The new stainless bolt has a different profile which is designed to give better gas flow.

The bolt can now be slid back into position and the bolt reattached. Slot the bolt handle in and insert the Allen screw until it just makes contact with the bolt. You can then turn the bolt handle until you feel the screw align with the hole in it. When it does, tighten it up and we’re done with the bolt assembly for now.

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Looking good with new tuned stainless steel bolt and seal

We now return to the internals of the tube and start by reassembling the valve assembley.The QB 78 tuning kit came with a new redesigned piercing body which does away with the filter, gauze, washer and spacer. It has also been designed to increase gas flow. So with the new assembly all we need is the piercing pin and spring placed inside the body for now and the new remodelled valve stem at the ready.

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QB 78 Tuning Kit

Piercing body, spring, piercing pin, valve stem and valve body with new seal.

We can either replace the old seal on the valve body or if the old one is in good condition it can be left in place. I chose to put the new seal on as mine was looking worse for wear. All we need now is the new valve stem assembly from the kit and the whole thing can be screwed back together making sure that the piercing pin is central with the hole in the forward end of the piercing body. Leave the whole assemby loose for now as it will go back into the tube more easily then expand when we tighten it up.

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QB 78 tuning kit fitted

It needs to go back in with the transfer port facing upwards. Gently ease it up the tube and line the port up with the first hole in the tube being careful not to let it go beyond this point or the seal may be damaged by any sharpness of the transfer port hole. Now replace the valve set screw.

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Now for the inside plug with the threads to the front and the large thread pointing downwards. The new plug supplied with the kit is shorter which allows the hammer to open the valve further, increasing the amount of gas flow.

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This can be a bit tight. Line up the screw holes top and bottom and place the stud loosely in the larger hole to hold things in place. Next we replace the hammer with the flat part facing downwards, the spring and the cocking block. I am sticking with the original spring in order to see the improvement the other tuned parts make though the kit comes with a range of springs to test. Stronger springs = more gas, more gas = more power. Some of the springs in the kit may put the gun over the limit for the UK though so take care if using them and always test on a reliable chronograph after reassembly.

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Everything lined up nicely.

The two pins go in now, the forward one goes in with the thin part uppermost and the rear one has the square part uppermost.

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The new transfer port seal goes in next, this may need some fitting by gently working it with emery cloth as it is supplied slightly oversize though mine seemed to fit well without any treatment. Once happy with it all the bolt and bolt housing can be replaced. Lining the two pins up with the slots in the tube and bolt then slotting it all together with the valve seal projecting into the bolt housing is a bit fiddly but no real problem. The countersunk screw and washer can now be put in through the breech.

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The tube plug and breech plugs with dovetails at the top can now be put in place ready to refit the trigger. The trigger block is refitted with the small screw at the front. Do this up lightly until the back screw is in place then nip the front one up lightly, being aware that it engages with only a few threads in the tube. Some more lining up of the tube plug may be necessary in order to get the long rear screw in as was the case with my troublesome item!

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QB 78 Tuning

So now to fit the barrel but before we do we can change the ‘o’ rings. Unless they are looking pristine I would remove the three old ones and replace with the new ones in the QB 78 tuning kit. Be careful easing these on as they can easily tear against any sharp edges of the loading port. Now is also a good time to tighten the piercing body and so expand the seal. This is more easily done before the barrel goes on. Do not over-tighten this, just nip it up and the seal will seat nicely. Now pull back the bolt and ease the barrel in place. Put the grub screw in and slide on the ‘A’ frame. You may need to fit the stock at this point so that you can find the exact place it needs to be in order to line up with the recess in the stock.

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The stock can now be fitted tightening the stock screw firmly. The safety catch goes back in. In order to slide this back in you will need to use a lever to push the safety catch plunger back into the trigger housing and hold it there. When this is out of the way the catch will slip in easily though it must be put in in the ‘safe’ position.

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And there she is, back together with the QB 78 tuning kit fitted!

With QB 78 tuning kit fitted we can gas her up. I was kind of pessimistically half expecting a dreadful hissing of escaping gas when I tightened down my tube cap but was pleased to find everything held well! The gun tested at 481 feet per second before tuning and when retested was up to 539 ft/s which was also a good result. I’m glad I refrained from trying the stronger hammer springs supplied with the kit as this would indeed have probably put it over the UK legal limit.

 

I hope this article has been informative. I will be looking to do a similar article on trigger tuning soon. The QB 78 Tuning kit I used can be bought from T R Robb who supplies it with a video or DVD guide. I don’t get a commission on sales for the kits but if you buy a chrony through the links to Amazon I do and so thanks very much in advance icon smile QB 78 Tuning Tuning the QB78 / XS78 Strip down and Rebuild with Tuning Kit

Matt

Copyright Matt Baker 27/09/10

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25 Responses to QB 78 Tuning Tuning the QB78 / XS78 Strip-down and Rebuild with Tuning Kit

  1. mark walters says:

    Excellent article…..!
    I have stripped my TH78D and am eagerly awaiting the tuning kit from T.R. Robb.
    I will let you know how I get on.
    When I bought the gun, secondhand, I bought it without testing it – foolish I know….!
    When I tested the gun on an indoor 25yd range, the pellet could be seen during flight and it would barely pierce the paper target…..! Also, with a zeroed scope (laser boresighted), the pellet dropped abot 18 inches from the bullseye. I am hoping this kit will make a huge difference.
    Regards,

    Mark…..

  2. admin says:

    Thanks Mark. Be great to hear how you get on with it. Certainly sounds like your TH78D will benefit!

    Matt

  3. pete says:

    Hi
    Just came across your great article.
    And has given me enough confidence to have go at my own QB.
    Thanks Pete

  4. john says:

    Thank you for such a great write up. I purchased a tuned QB78 with the thumbhole stock and it shoots “hot” so I needed to see how to take one apart and get it back together. You have shown me how to do just that! Thank you.
    ~GOD BLESS~
    john

  5. Dave says:

    Brilliant write up Matt. Just what I needed after purchasing my first SMK.
    Thanks for going to the effort of this article.
    All the best.
    Dave.

  6. Shelby says:

    Good technical article. I have two QB’s and have modified both of them to mid 650′s. I still picked up some tips from this article on how to improve performance. I would never have thought to shorten the stop block to lengthen the hammer strike. I can see how it would work though. I prefer my QB to my Airforce Stealth whenever I see another squirrel or two getting ready to attack the house.

  7. Iain Parker says:

    Hi, ive just bought the SMK’s CO2 TH78D 2nd hand! Now the cap that you unscrew, so you can put new CO2 capsules in, is very tight to take off, (I haven’t taken it off before, this will be the first time!) Have you any tips on how to do it properly, will a certain tool be good?

    Yours Sincerely

    Iain…..

  8. admin says:

    Hi Iain

    Caution! Is there still gas in it? The cap becomes tight naturally when there is gas in the capsules and mustn’t be undone before it is empty. Unscrewing the cap before the gas has been fully released may cause injury and damages the seal.

    If the capsules are fully discharged and it is just tight and I must say I’ve never come across this, I’d try wrapping a bit of leather or something similar around it to protect it and carefully use some grips. If you want to be super careful, I guess you could make up a tool like a miniature oil filter remover (strap wrench) easily enough and try it that way. http://sheridanmarine.com/public/assets/images/shop/stock/1275346800/oil-filter-strap-wrench-1275585611-l.jpg

  9. Iain Parker says:

    Hi, thanks for your reply! I got the cap off easily in the end! Now I bought mine, (SMK’s TH78D), 2nd hand & you know the screw on cap, well the screw in the end, wasn’t the original, so I put one on of my own, which fit snug! When I bought the gun, the guy was telling me I had to tighten it with a spanner, which I knew was a mad idea, but I bought it, as it was going cheap! Now I dismantled the cap end, to clean inside. What should it look like inside/all the parts? (I don’t know how to upload pictures for you to see?) Inside mine, theres that round, cast thing with the spike in the middle, (excuse me, I’m new to the terminology!) & around it, is a rubber ring, is that all that should be there? Well after cleaning it, putting it back together, I placed the two CO2 caps in & screwed the cap on as far as it would go, then it made a hissing sound, then started leaking CO2 out of it, luckily before I did this, I filled my bath full of water, so I placed the gun under water, just were the cap is & the CO” seamed to be coming from the bottom of the cap, were the last of the threads are! (I wish I could upload pictures & give you a closer look!)

    Thank You

  10. admin says:

    Sounds like the parts are right:  photo DSCN3813_zps670e4376.jpg

     photo DSCN3814_zps8d96e067.jpg

    The screw should be countersunk to allow the piercer to move slightly inside the cap when fully tightened. It should not be a solid fit. If this is all OK already you probably just need a new o ring.

    All the best,

    Matt

  11. Iain Parker says:

    Oh, thank you & appreciated! Yes all the parts are there for the end cap, its just not the original screw, that’s all! I’ve ordered all new seals & O rings for the gun, to give it a good service myself! Once again, thanks for your prompt replies, they’ve been a great help to me!

  12. Raymond Watson says:

    I have now fitted the longer reservoir (4 capsule) to my new .22 QB78DL and must say that it has improved the shot count, the rifle balance and so the accuracy tremendously. That along with the shortening of the stop block by 2mm has given the chronoed power to just under the legal limit using the Thunderbolt heavy pellets. Obviously vermin are my only targets and I have effected an instant kill each time. Don’t need to see it land…the Whack’ of the hit and the many following somersalts tells all. I can now see why they are so popular and great fun to use…….

  13. Joe clarks says:

    Hi I just fitted this kit from t r robb and I have noticed that now the bolt handle is not sitting right down in its position when it is pushed forward and ready to fire it almost feels like the new custom probe is made to long and is to tight to turn down into the last couple of mil.it’s fine until I place the barrel into the breach then the bolt/probe goes tight.any ideas??

  14. admin says:

    Hi Joe. I’ve not come across that myself. Strange as you say it’s OK with the barrel out. I presume the probe o ring’s the right size and is seated as it should be on the probe. Did the barrel o rings go in ok? They get shredded all too easily. Anyone else come across this? If no joy your best bet is try T R Robb. Will give it some thought over Christmas when I can. Let us know if you get it sorted :)

  15. Joe clarks says:

    Hi admin.thanks for a response and very much appreciate the above article☺.I have checked all seals and placement of parts and rebuilt the rifle 4 times now and it’s still not sitting right, like you said I will contact t r robb and find out what’s going on. For reference it’s a qb78 deluxe .22 . I’ll update your thread when I find out more.

  16. Simon says:

    I just wanted to say thanks – I went for a thumbhole QB78 to get me back into shooting after a break, and decided to take it apart. Your guide has been invaluable in the tricky bit – getting it all back together…

    Lining the breech assembly up with the main body was the hard part. My top tip is to line up the transfer port and the front bolt peg first, seat them as well as you can. The square headed rear peg will be a couple of mm too far back to let the rear of the breech assembly slide down onto the peg due to the spring, but you can still get the screw bolt that holds the breech assembly to the main body partially tightened down. I used a small bladed screwdriver to push said peg the couple of mm forward it needed then tightened the screw bolt all the way down.

    Hope this helps and again – many thanks!

  17. admin says:

    Hi glad the article helped. Thanks for your information too.
    Matt

  18. admin says:

    Hi Cheers. hope all goes / went well :)
    Matt

  19. Maurice Garrett says:

    Hello all I am new to this site and would like some info please.
    I have been given a QB78 which looked old and obviously hasent been looked after, for instance the end of the stock is chipped also the hard plastic at the end is cracked, also it appears to be leaking gas, but does fire ok.
    I would like to fettle it up a bit and will need a tuning kit can you please let me know where I can get one.
    Cheers all mole5

  20. R T. says:

    I have reassembled the breech assembly without the barrel and trying to cock it dont work it the bolt goes forward so far then locks up? there must be something wrong but I have’nt a clue what it could be! any suggestions? as I am getting fed up with taking it apart then reassembling it with exactly the same result, I could do with a picture of where the two short pins go into the bolt housing slot! Thanks.

  21. admin says:

    Hi I’ve added some pictures of this at the end of the article above. Hope they help. Should be straight forward enough where the tops of the pin go. Takes a bit of a giggle to get the two parts to mate with both pins and the breech seal located properly. Good luck.

  22. R T. says:

    Thanks very much for the info Matt, most obliged, now all OK, Cheers.R T.

  23. R T. says:

    The transfer port hole when the valve is in place and secured to the tube by the small screw is slightly offset which is not good for correct alignment of the seal, other than buying another brass part of the valve, I would have to dispense with the valve retaining screw and relaline the port and use the stock bolt to ensure the valve cant move back towards the trigger area.

  24. admin says:

    Sounds like a bad plan to leave the screw out! Might be worth letting your local gunsmith take a look at it.

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