Thursday, 24 October 2013

Adjusting the Pump Stroke on the SMK B 45-3

In dire need of a decent stock, and a decent trigger wouldn't go amiss either.

Ideally on a multi-pump air rifle you want the pump head seal to be touching the valve at the end of the stroke, so you know all the air you are pumping is going where you want it. This means lass pumps to get the desired air pressure in the valve, usually there is adjustment on the pump rod to achieve this.

With the SMK B45-3 there is a pin that stops the pump head from rotating and therefore unable to adjust, but on a previous strip down i removed the pin and adjusted it up then locked it with the locking nut.

Hole where retaining pin went.

 But i didn't adjust it enough as there is still compressed air between the pump head seal and the valve, and this is evident when the air forces the arm back when you go to take the forth or fifth pump. So time to take the pump assembly out and tweak it till the valve gets the full stroke of the pump.

First step was to remove the screw that holds the pin the pump arm pivots on,

Securing screw for pump arm pin.

the with the pump arm loosened off the pin came out of it's holes and housing releasing the pump arm.

Pump arm pin.

The two small flat head screws at the front release the housing that supports the arm, and also supports the end of the barrel.

One of two tiny screws.

This now slips out.

Housing for pump arm and barrel.

With the housing removed the pump head assembly can now slide out, but being careful not to mark the pump head seal.

Out slides the pump assembly.

Pump head assembly with locking nut.

Now i've already remove the retaining pin previously it's just a matter of loosening the locking nut, now you can adjust the pump head assembl and tighten the locking nut again. However don't adjust it to much as the pump arm won't close fully.

Ooops, to much adjustment.

So then it's just a matter of taking it out again and adjusting it till it just touches the valve face, which after a couple of attempts i got it spot on.

parachute seal

Short of the air that gets trapped in the parachute seal there was no compressed air to push the pump arm back once it was back together. Hopefully i will only need five or six pumps to get the same muzzle velocity i was previously getting with eight pumps, oooh, the suspence is killing me. I can't wait.


Wing Commander Sir Nigel Tetlington-Smythe

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