Sunday, 20 September 2015

.22 Mk I Sharp Innova, A Restoration Project. Adjusting The Pump Rod And Blow Off Valve

The pump head - when the pump rod is at rest on any multi pump - should ideally just touch the face of the outside of the air inlet valve, inside the pump cylinder. Most leave a little space which, when the rifle is fully pumped, is compressed air and will want to expand, pushing the rod back if the gap is too large. However the length of the pump rod can be adjusted on the Sharp Innova very easily indeed, which my friends is very handy for so many reasons.

To remove the pump rod for adjustment for either stroke length or pressure blow off adjustment, first remove the front sight unit. Knock out the front roll pin, then pin with circlip on it and the unit pulls off the front as well as the pump arm dropping free. 

As I had been removing and refitting the pump arm so much I used a clear plastic bag between punch and pin to avoid the possibility of scratching the blueing.

Next, pull the rod back by the pump arm linkage, and tap the pin holding them together through the same hole the pin and circlip go through; this leaves the pump rod free to be removed out the front of the pump cylinder. Tease the link end out with something, then it just pulls out.

Just used an old bent pair off tweezers to push the pump rod out this time.

The brass unit on which the actual pump head sits is the pressure blow off valve, and the 10mm silver nut behind this is the lock nut which holds the unit in place on the rod. Crack the 10 mm nut loose by supporting the end of the blow off unit with a 14 mm spanner.

If the lock nuts are too tight, use a pair of mole grips on the valve body, it's pretty rare to resort to that though.

With a 10 mm spanner, loosen off the lock nut for the blow off valve unit and pump head.

10 mm lock nut loosened off.

By hand I screwed the blow off unit and pump head about 2 mm outwards to bring the pump head nearer the inlet valve, reducing the amount of pumps needed to make full power. Remember, if it is adjusted too far the rod will bend eventually as it will be too long, so do it in small increments, reassembling and testing each time until you are happy.

The thread on the rod and the thread on the blow off valve run in opposite directions, which does make things a lot easier.

Then tighten the 10 mm lock nut to keep the pump head in place on the rod.

Tighten that lock nut back up.

For adjusting the blow off valve, use a 14 mm spanner to loosen the brass lock nut at the base off the blow off valve unit; the thread runs the opposite way to the thread on the rod so you can use the 10 mm lock nut for leverage.

The lock nut for the blow off valve is actually a ring with two flat edges opposite each other.

If you need to up the ft/lb before the blow off valve kicks in, screw the brass unit inwards, likewise adjust it outwards if you need to lower the full power output. There is a very strong, thick, square section spring holding back the air inlet seal in the blow off valve, so you might need some mole grips or something to adjust the unit inwards.

Screw to the right or inwards to up the power and to the left or outwards to drop the power, it was fine as it was in this case.

I didn't need to adjust the blow off valve this time so I left the valve alone and tightened the brass lock nut back up.

And retighten the blow off valve lock nut to keep the power where it is.

With both lock nuts tightened up, I carefully inserted the pump rod back into the pump cylinder, being mindful of the pump head as it passed any open metal work on the cylinder.

A dab of silicone oil around the pump head makes this go a lot more smoothly.

Reattaching the pump arm is simply a reversal of the removal process; the brass bushing on the pump arm had slipped out a little and had to be hammered back into place before I could get it to fit in the cylinder and sight unit.

I will never understand why they go to the trouble of fitting brass bushings then scrimp on the breech by making it from plastic, still they stand the test of time anyway.

Tapping the roll pin and stud back in with a rubber mallet is always better than using a hammer.

Rubber mallets also vastly reduce the chance of scratching the bluing or paint work.

And do be careful fitting the circlip as it's a bitch to find if it pings off somewhere.

Anyway, that 2 mm adjustment reduced the the 12 ft/lb blow off limit from 7 pumps down to 5 pumps, it is possible to reduce it down to 4 pumps but I didn't want to chance over doing it and having the rod eventually bend.

Now all I had to do was remove my Deben Whisper silencer which I had used for testing over the chronograph.

The Deben adaptor always gets stuck in the sight unit, not scratched it yet though.

And it was all ready to be sent back to it's owner. I am well chuffed to hear he was very pleased with the work I had done for him, it almost made me feel like i actually knew what the hell i was doing (joking!).


Best wishes, Wing Commander Sir Nigel Tetlington-Smythe.

1 comment:

  1. Thank you for an informative guide to the Sharp, I have one myself and have not had any problems with the rifle.
    Although the pump handle sometimes drops if the rifle isn't under pressure, and I have noticed that from time to time the rifle will " pop" after the first or second stroke of the pump and discharge ?
    So now I never load the Sharp unless it's fully charged.