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SpringDon

Theoben evolution question

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I bought an evolution in 2011, didn’t use much initially. When I tried to zero it, the pellets seemed to keep hitting lower and lower until they wouldn’t actually leave the gun. I assumed the gas ram had lost pressure but when I went to return it, the factory had closed!

It languished in a cupboard until  impact air guns started, they could fix it for £200ish but I didn’t fancy throwing that much at it so more languishing ensued.

Finally we reach the question. Having some unexpected free time recently, I stripped the gun and found the ram seemed to have lots of pressure but the piston was very hard to move. It seemed like swarf had flaked off the front of the piston (it wasn’t well finished) and was jamming the piston. There were some quite deep score marks on the inside of the cylinder.

My question to the panel is, would you expect the piston to fall easily to the end of the cylinder if there was no piston seal on it? I suppose I’m asking whether the cylinder should be reamed until the piston moves freely?

 

Rather amusingly, I paid extra for a 10 year warranty which is valid until end of 2011.

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Posted (edited)

If there is no seal, the piston should rattle freely in the cylinder and in effect be free fall.

With a seal the piston should slowly slide down the cylinder if held vertically as it pushes the air out the transfer port.

John Knibbs (airgunspares.com) still does new seals.

I would certainly look to polish out the cylinder to remove/soften the scores, but avoid any reaming, if the piston is too large i would sand and polish it (except the bit under the seal) until it does not cause any cylinder wall contact issues (effectively to reduce the piston diameter) so the only thing touching the cylinder walls is the seal.

A picture of the piston with seal would be helpful to see the damage.

Edited by Stonepark

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Thanks, I’ve already polished the damage from the piston. I blued the piston and sliding it in the cylinder seem to indicate that the cylinder is slightly ovalised in the middle of the cocking slot.

only the back of the piston touches as it has the greatest diameter there. So you would turn down the piston rather than ream the cylinder? Should the inside of the cylinder be polished or honed ( like an engine cylinder)?
 

I’ll try photos but electricity isn’t my strong point.

 

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Posted (edited)
14 hours ago, SpringDon said:

Thanks, I’ve already polished the damage from the piston. I blued the piston and sliding it in the cylinder seem to indicate that the cylinder is slightly ovalised in the middle of the cocking slot.

only the back of the piston touches as it has the greatest diameter there. So you would turn down the piston rather than ream the cylinder? Should the inside of the cylinder be polished or honed ( like an engine cylinder)?
 

I’ll try photos but electricity isn’t my strong point.

 

The cylinder certainly shouldn't be oval.

 

The cylinder walls should also not be polished to mirror as grease does not adhere to it and fine wet and dry (1000 grit) is the finest I would go.

Delrin buttons are generally the way around the rear contact issue as piston rear us turned down sub diameter and then buttons installed into piston at rear to act as guides and restore the contact between piston and cylinder wall.

 

http://anotherairgunblog.blogspot.com/2014/03/webley-hatsan-patriot-piston-buttoning.html?m=1

Edited by Stonepark

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Not sure I want to do the buttoning yet, just want to get it working.

There may be a picture of the piston near here. It’s dimensions are

30.408 seal platform

30.093 middle

30.598 rear

I set a bore gauge to 30.598 and it moves freely in the cylinder except at the points where the witness marks are. Almost like the cylinder was squeezed and deformed. As the marks are within the cocking lever cutout part of the  tube so there’s no seal there, I’m going to try polishing the marks out so the piston moves freely. Gun is scrap at the moment which is a shame as it has a nice walnut stock.
 

D0EFAAC5-669F-40DD-A326-73481042270D.jpeg

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On 13/05/2020 at 12:55, Stonepark said:

The cylinder certainly shouldn't be oval.

 

The cylinder walls should also not be polished to mirror as grease does not adhere to it and fine wet and dry (1000 grit) is the finest I would go.

Delrin buttons are generally the way around the rear contact issue as piston rear us turned down sub diameter and then buttons installed into piston at rear to act as guides and restore the contact between piston and cylinder wall.

 

http://anotherairgunblog.blogspot.com/2014/03/webley-hatsan-patriot-piston-buttoning.html?m=1

Having another think about buttoning, I looked at the article and one question I have is what happens if the piston spins, could the button catch on the cocking slot? There doesn’t seem to be anything that keys the piston.

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7 hours ago, SpringDon said:

Having another think about buttoning, I looked at the article and one question I have is what happens if the piston spins, could the button catch on the cocking slot? There doesn’t seem to be anything that keys the piston.

It's not one I have come across, as buttons are slightly domed and only a mm or so protruding shouldn't be an issue if cocking slot edge has been deburred internally.

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