Sunday, 5 October 2014

More Stuff on the SMK B45-3

SMK B45-3 after lots of messing about.
It's not the prettiest air rifle and i'm constantly replacing the air inlet valve seal, but for some reason I really like this cheap Chinese multi pump. It's simple to dismantle, which is just as well, because finding the right size and material for the air inlet valve seal has been an ongoing mission over the last year or so. Too thin and the sealing collar cuts right through it, too hard and it doesn't hold air. But with a lot of trial and error I have found something that holds a lot longer than the simple rubber flap washer I used originally. Now I use a thicker, harder rubbery plastic, found on a cheap head torch, which I cut to shape. It is holding up well so far.

A selection of different materials and sizes for the inlet valve in the centre of the picture.

The original stock and pump arm are butt ugly and so is the action if truth be told, but at least I can do something about the stock. I have made a new stock and pump arm which lends more weight and balance, and a raised comb making it a lot easier to shoot.

The trigger gets heavier the more you pump as it releases air from a dump valve/exhaust valve. It would be more accurate if the trigger was a lot lighter. And actually, the thing about this little rifle is that it IS accurate, that is why I have persevered with tinkering with it and will continue to do so. If I go down to the woods with an air rifle, this one always comes along. It is great fun for a plink on my permission woods.

Recently I had found that it was not as accurate as it used to be, so I had a look for the cause of this. On close inspection I discovered that the barrel was slipping out of the breech and that it was also moving slightly in the sight housing/support. 

The barrel loose in the breech block has shifted 2 mm.

The barrel has a thinner diameter where it fits into the breech and is held in tight with a screw from above, this screws into a hole that runs top to bottom which acts as transfer port, and a screw hole underneath for holding the firing pin housing in the right position. Some screws were missing when I got the rifle, and the screw that holds the barrel tight was one of them. I found a screw to replace this and turned it into a grub screw so it wouldn't clash with the scope mounts.

Brass grub screw that secures the barrel.

Once I had loosened the grub screw out, I found I needed to deepen the slot for the flat head screwdriver blade, then it was matter of tapping the barrel back to it's true position. 

Tapping the barrel into the breech while the front sight housing is attached.

The crown of the barrel is recessed so is hard to damage, so tapping it through the sight housing wasn't a problem. 

The barrel crown is not really the crown as the rifling stops 5mm into the barrel.

With the barrel back in place, I checked it was aligned by poking a small hex key up from underneath the breech with the firing pin removed; it was, so no adjustment was needed. Then the grub screw on top was tightened securely against the barrel, which, judging by how far it screwed in, was not secured before.

This was when I noticed that the barrel was not such a snug fit in the sight housing and would move around if I applied pressure from side to side. It was barely noticeable but it was movement all the same. As there is a barrel band, it is not supposed to be a free floating barrel, and a lot of force is placed around here through the use of the pump arm. 

Pump arm pivot pin and securing screws removed.

I removed the pump arm pivot pin and securing screws so the sight housing could be taken off, and replaced the plastic cone that fits over the barrel - i'd originally assumed this was there for an aesthetic purpose and had left it off. The barrel still had movement in it with the cone replaced, but it meant I could wedge a sliver of rubber between the underside of the cone and pump tube. 

Small piece of rubber stops the barrel moving about when using the pump arm.

This pushed the barrel upwards and stopped all movement.

The small cross-head screw that holds the pump arm pivot pin secure in the pump tube is not original, this replacement has the habit of working loose and letting the pivot pin move back so slightly, adjusting the length of the pump stroke. There was a marked difference in point of impact when pumped seven times with the pivot pin secure and seven times with the pin loose. So I decided to use Loctite thread glue to hold the screw in place, and this appears to have done the job.

This stuff really works.

I am happy to say that these small jobs have not only restored it's original accuracy, but have even improved upon it. If I could somehow make the trigger pull lighter the accuracy would be even better, as I have the tendency to pull some shots to the left with it as stiff as it is. But saying that, it wouldn't be too hard to build a replacement trigger for it, such is the simplicity of this rifle's design.


Best wishes, Wing Commander Sir Nigel Tetlington-Smythe

No comments:

Post a Comment