QB78 Action Rebuilt

The QB78 XS78 and the TH78 are basically the same rifle as far as the action is concerned anyway. The XS78 have a 10mm barrel and the TH78 and QB78 have a 16mm barrel, The other differences are really just cosmetic.

I can’t comment on the XS79 as I think there may be differences to allow more room under the barrel for the 88g tank.

I like these guns, yes they have their weak points, but for the money they are difficult criticise to strongly. The power lets them down as standard the only produce 6 – 8 ft ilbs and some of the engineering is suspect like in the safety catch for example.

I upped my QB78 power with the tuning kit from T R Robb, which brings then up to a much more acceptable level. And I have had my share of problems with the QB78 which I think was a mixer of a couple of factors which hopefully I have now solved.

There are tons of strip down video’s for these guns on Youtube. But I find pictures are more help than videos for me anyway So I decided to take some pictures of my qb78 while I am in the process of putting it back together again.

This is actually an uprated probe from the tuning kit provided by ThRROBB. I decided to nickel plate mine as they do have a tendency to rust pretty quickly if they are not used straight away as mine wasn’t.

There is an o ring on the probe which I always replace whenever I strip down one of these guns, In fact as a matter of course I always replace all the o rings.

The actually bolt handle is a custom item I bought from the web, though I did tru oil the wooden part. There is an indent that the Allen screw  locates with in  the back of the probe. which may need finger tightening and twisting of the bolt till it locates properly. Then it does need a proper tighten up as I have had  these come loose in the past, I have used a stainless steel grub screw here as I had one available, but the Mild steel screw supplied is certainly good enough.

This is the assembly put together. You need to make sure the slots are visible underneath the breach and that the bolt moves freely. I used some ptfe grease on this part as it lubricates and in the case of a steel probe helps to keep the rust at bay. Not the countersunk hole at the end of the probe. A countersunk screw and shaped shake-proof washer are used her to secure the breach to the cylinder in due course, Actually the Chinese screw is not to bad, but I decided to swap it out for an Allen headed countersunk screw myself and it is less likely to slip and mangle the head of the bolt.

This can be put to one side for now as the work needs to move to the cylinder now.

 

I have laid the

I have laid the cylinder out so you can see where the item is secured to the cylinder, The stock screw has only been screwed in to show where it goes and there are 2 threads on the hammer block that it screws into. The stock screw will only fit one of these threads so it has to be aligned so that the largest thread is at the bottom of the cylinder. This is the original Chinese cylinder. The tuned parts do away with the wadding, washer and spacer. The black piercer and the body are replaced by the parts in the kit, as is the needle valve and the hammer block.This is the valve assembly put together, but it is only on a couple of threads as I don’t want to compress the o ring yet. The crew is again for illustration only  as it is screwed into the cylinder with that countersunk screw. Again I replaced that screw for an Allen headed countersunk screw to give it a more positive lock.

This is the Hammer assembly and when it is put together the holes need to line up with the slots in the cylinder.

These parts are now just about ready to be fitted into the Cylinder

This is a new valve body, spring, Piercer, needle and a new valve block which as I mentioned earlier does away with the Wadding filter, Washer and spacer. Note the valve is not tightened up. Also I put it in from the fill end so that the holes and slots do not tear the o ring.

Now it is time to line up the transfer port and tighten the valve block into the cylinder.

You can just see the needle through the hole that mounts the hammer block.

The hammer block shown here is round the wrong waythe threads should be near the valve block or the holes wont line up. Also remember one thread is smaller then the other. The larger thread is at the bottom for the stock stud to screw into.

I have loosely screwed in the stock stud to keel the hammer block in place.

Now using the special tool to tighten up the valve block. It needs to be tight enough to expand the o ring, but doesn’t want to be over compressed.

The spring guide, spring and hammer are next and the holes in the spring guide will need to line up with the slots in the cylinder.

The two pins that cock the rifle are fitted in the two holes the square one ant the back and the round one ant the front.

The transfer port seal is now fitted and now comes the awkward part Lining it up with the breach and loosely assembling the breach to the cylinder not forgetting the shaped spring washer under the countersunk screw.

The pins need to locate with the 2 slots. Patience may be required as it can be fiddly.

Once the rifle is secured  finger tight. The trigger is used to mount the rear end of the breach Cover removed on trigger so a clear view of the parts can be seen

The rear trigger bolt goes through the 2 end caps. The top one of which is threaded. The little securing screw on the other side of the trigger should not be overtightened, as it only holds on a couple of threads which can be easily stripped.

Once this is secured  the front breach screw should also be tightened up properly. The action can then be tested. It should dry fire as normal. If not the pins are probably not lined up properly.

The Barrel should have the o rings changed and it can be twisted and pushed into the breach. This hides the countersunk screw that has just been tightened up and is secured with a grub screw.

The barrel ban just needs fitting now and the action is ready to go back in the stock and have the safety refitted.

 

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