AirGunTech: Re Engineering Spring Powered Air Rifles for smoothness, consistency, and to better suit UK power levels.

Weihrauch HW35 LGV

Back in July2012 I had the idea of building a Weihrauch HW35 fitted with a Walther LGV rotary piston and that’s just how it stayed until Christmas — just an idea.

Then in December I was fed up one Sunday  and decided to de-braze a HW35 cylinder from its breech block in order to show how the cylinder was made and to highlight the grooves round the breech block which give rise to the porous cylinder problem on the 35.


As can be seen the original Weihrauch breech  it pretty long with 20mm of the breech brazed inside the cylinder with a total transfer port length of 26mm. With the cylinder de-brazed and armed with a few LGV piston measurements from Tony I was able to work out if the conversion of the HW35 to rotary LGV piston was possible. Everything looked good to go so I ordered a 16joule Walther piston kit from Germany (via Tony) and a short length of 30mm OD X 25mm ID Hydraulic tube. Prior to machining the breech block plug down I was already aware that I was going to break through into the breech latch slot and the breech stop hole — I was quite happy that these problems could be overcome by welding.

So I machined the breech block down by about 20mm:-


As can be seen I took the precaution of bolting a spacer in between the breech jaws to stop them distorting when machining or welding.

Once the holes in the rear of the block had been welded shut I machined the breech down to leave a 0.5mm ridge on which to locate the 25mm ID tube.


I could then clamp the 25mm tube onto the breech block and weld it on and then machine down to end up with a breech block with a 6mm long transfer port attached to a 25mm ID X 30mm ID cylinder.


Now before welding this back into the original HW35 cylinder I decided to machine the piston to suite its new home.

With the original LGV piston seal the piston was too long and would cause a large gap between the cocking lever end and the rim on the rear of the LGV piston. So I decided the simplest option was to machine the seal retaining hub off the front of the piston and machine up a new bronze O ring piston head.


With the piston sorted I was then able to weld the new breech arrangement into the original HW35 cylinder.

I then machined the weld down and linished the joint.29012006709

So I ended up with a HW35 cylinder with a 25mm bore which opened out to 30mm at the cocking lever slot enabling an altered Walther LGV piston to be used

Machining the front of the breech block not only enabled the Walther piston to be used but it reduced the gun’s transfer port down to the length of a HW77 transfer port at 6mm.



  • How much heat to release the braze?

  • Hi Paul

    I had to heat the breech and cylinder to red heat for the braze to release — I used Oxygen Propane cutting gear to get it hot enough.

    ATB Mick

  • Hello, been having a read of your excellent site. I saw your hw85 threads. Do you think you could do anything for mine, the same one? It has bent breech jaws. No it’s not the barrel which I replaced with a new hw80 one. The actual breech jaws are bent upwards and even worse sideways! I was looking at this hw35 post and saw how you un brazed the jaws to work on the port, maybe you could do something with mine. It’s a lovely handling rifle but not much good past 25 yards as my groups shoot off to the left! Let me know.

    Regards, Richard.

You can follow any responses to this entry through the RSS 2.0 feed.

Assign a menu in the Left Menu options.
Assign a menu in the Right Menu options.