QB 78 Tuning Guide
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.
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).
QB 78 tuning preparation
With this removed, unscrew the stock screw and remove the action from the stock.
Next we remove the barrel. Use a Phillips screwdriver to loosen the ‘A’ clamp at the front and slide it forwards.
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.
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)
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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!
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.
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.
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 🙂
Copyright Matt Baker 27/09/10