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SMK QB/XS/TH 78 79 tuning kits and parts

For Professional trigger tuning.

First remove all parts, smooth off all the mating surfaces with the wet and dry supplied in our trigger tuning kit. Reassemble with our spacing shims one either side of the trigger, and fit our allen screws, and put the brass spring guide at the front of the trigger spring. You get a short and long trigger spring adjuster. I usually use the shortest one to give me a lighter trigger. Using the longer one can give a harder trigger pull.

Our spring guide supplied in our kit is made to smoothly ride the different angles of the trigger when it is pulled.
Below and right is a part tuned trigger with the spring guide fitted and the adjusting screws set to a fine adjustment. This may be too light for beginners. The top allen screw from our kit adjusts the depth of the sear, while the second one adjusts the trigger stop. The trigger stop should be set so the trigger stops soon after the release (firing). I would normally cock the empty gun with the second lower screw wound in, then with pressure on the trigger, gradually unwind the second trigger stop screw until the gun releases (fires). When you are happy with the trigger settings just put a drop of Locktite or even nail varnish on the screw and body to prevent them moving

Right: Trigger sear face, Polished the sear face and sides.

Don’t make the trigger pull too light as the gun may go off accidentally. 


Make sure gun is unloaded

Wear protective gloves to prevent you cutting yourself. You can remove any sharp edges with a needle file if you want.

You will get three new reservoir O rings that are a fraction smaller than the standard to make fitting easy, use the Molycote 33 to lubricate the O rings, You will also get the valve block at .600” of an inch (the same as the block supplied in our tuning kit), and an adjustable piercer, You get a new trigger retaining screw for the front of the trigger.

The XS 78s and QB 78s have two different length reservoir tubes. The early ones had a short tube and the later ones have a longer tube. This means that the front cap could be a short one for the early tubes or a longer one for later tubes. You will find pictures of the tubes on our site under the XS category.

Using the XS/QB 78
To put the Co2 bottles in the reservoir, first unscrew the front cap, Place the first cap in nose first and the second cap in bottom first. Tighten cap fully and then unscrew quickly half a turn to pull the piercer out of the bottle to allow easier gas flow.
Never use anything other than your fingers to remove the front cap! If it is hard to unscrew this is usually because there is still too much pressure inside. Keep firing (Safely) until the remaining pressure is very low. When the pressure in the tube is low then the front cap should unscrew easily with your hand.
On a few rare instances we have found that some XS/QBs can lock up when you tighten the top screws that hold the breech down, if yours does this then it is worth checking that your hammer operating pins are not too long. There is a square pin at the back and the round one at the front. If yours is locking up when you tighten the top screws then taking 2 to 3 mm off the bottom of these may free it up.

If on rare occasions you gun goes too high then you can cut half a coil or so off the hammer Spring. But make sure that you are checking with a good Chrono.
Tips for changing t
he standard reservoir to our XS 78 QB 78 reservoir (longer tube).

To find the piercer adjustment, us 4 dead Co2 bottles to get the correct adjustment. Using dead or used Co2 bottles will prevent you accidentally piercing one of the new ones, which could be dangerous.

Adjust so the piercer is at its shortest and then test with 4 used bottles. If the cap is tight and the bottles can still be heard to move up and down inside then it will have to be adjusted out a little at a time until you are satisfied they are held solid with a few threads still to go on the outside of the tube. This is to ensure that when you tighten the front cap all the piercers will pierce the forward three Co2 bottles, the first one you put in is pierced when you fire the gun by the standard black piercer or the stainless steel piercer we supply in our tuning kit. 

The adjustable piercer can be adjusted to allow the front cap to engage in most of the threads as the pointed parts puncture the Co2 bottles. Do not adjust so that there are only a couple of threads engaged, but try to get as many threads engaged as possible, before the cap bottoms out.

The piercer can be adjusted to allow all the Co2 cylinders to be pierced. The cap is tightened just as the original reservoir. Don’t forget to undo quickly half a turn to pull the point out of the Co2 bottle to allow Co2 gas to flow out easier. This also applies to the standard gun.

Use the new O rings supplied for easier fitting.

When assembling, put the piercer and brass body loosely together and push in from the rear, this is important for easier assembly. Do not use the standard O rings. Do not push in from the front.

Never undo the front cap with anything like pliers. If it is hard to undo then it is usually because there is still some pressure remaining in the reservoir (tube), you can just keep firing the gun without a pellet in to deplete the gas.

Venting excess Co2

Our reservoir has a small hole near the back to give access to the hammer; the hammer can be pushed forward to force open the exhaust valve when the gun is not cocked. This is so the exhaust valve can be depressed to let the last amount of Co2 out.  Make sure there is only a small amount of Co2 pressure left. Do not use the venting hole when there is still a lot of pressure in the reservoir tube, as this can freeze the valve.

Once all the pressure has been released the front cap should unscrew easily.

When the reservoir tube has been fitted, you can check the number of shots that you get with 4 X 12 gram Co2 bottles. If you get 80 shots at full power then when you go shooting just put around 70 to 80 pellets in your pellet pouch (pocket) and when the pellets have all gone you know it is time to replace the Co2 bottles.

Message from customer

XS 78 Exhaust Valve is pressed together so you cannot change the seal. Ours is made to give more power and unscrews so you can change the seal. When replacing the seal make sure you put the new seal in with the smooth side facing outwards. The stainless steel shaft can be gripped by placing it in drill chuck, and then unscrew the brass head, if it slips then gripping with a piece of wet and dry would help.

hi terry,
just a note of thanks, i bought a th78d tuning kit from you and with the aid of your excellent video have fitted it and energy has gone up from bearly 7 to 11.5ftlbs.
it is disappointing that these guns are advertised as "full power", "10 " even "11.5ftlbs" when clearly they are not when sold, but at least it is easy to tune with the video.