Thursday, February 21, 2008

RWS 850 AirMagnum PCP Conversion - Part 3

By A.R. Tinkerer

Part 1
Part 2

I can't believe it has been a month since my last post! Sorry about that, I hadn't planned on taking so long!

When I see a DIY project, I often wish that I could find out more about what went on in the design and build process. What options were rejected? Why? Why was a certain material used? Where are the parts available? How were the problems solved, etc.? So today, I will look at the design criteria I originally used for the RWS 850 AirMagnum conversion and explain the reasons for each. After that I will discuss some modifications made to the valve and the results. If you have any questions or suggestions, please let me know.

I started out with five design criteria. Each one is listed below with the reasons for it.

1 - Use off-the-shelf parts
The main reason to use off-the-shelf parts is to keep things simple. The only equipment I had for working with metal were a drill press, Dremel tool, hand tools and welding equipment. That limited what I could do.

Another reason is safety. If you make your own parts, you have to figure out what materials to use, how to calculate part strength, how working the material will change its properties, etc. This was more involved than I wanted to get since the pressures involved are high.

I used paintball parts for everything in the air path. Paintball parts are made for the high pressures. I did modify some of the off-the-shelf parts in the setup shown last time.

2, 3 - Air supply on/off; Removable air tank
These two items are related. I wanted to remove the air tank and the air must be turned off to remove the tank. There are two reasons for removing the tank. The first reason was to make it safer around kids.

The second reason is related to travel. I went to a friend's house for the weekend and drove to work on Monday. After arriving at work, I realized there was an almost full CO2 tank connected to the RWS 850 in the trunk! I had three options. Leave it in the trunk, get permission to bring it inside or empty the tank. Leaving it in the trunk was not an option since it was a warm day. It was in a locked gun case, but getting permission to bring it inside could be a problem. Fortunately, permission was granted. With a removable air tank, I could have brought the tank inside and left the airgun in the trunk - no problems.

4 - Speed of at least 650 fps
The reason for this is explained in Part 1. Basically I needed more power for my specific use.

5 - Price under $150
This one is pretty obvious. The original cost of the RWS 850 was $240 and adding $150 for a conversion would put the total cost at $390. The cheapest multi-shot, PCP rifle at Pyramyd Air was around $390 at that time. I didn't want to pay that much when I could have bought a PCP in the first place!

Valve Modifications
After the disappointing speed of 619 fps @ 1050 PSI last time, I figured it was time to open up the valve. I found an adjustable face spanner wrench (or pin spanner wrench) at TrueValue hardware, ground the pin diameter down so they fit and opened the valve. I used a pin punch to tap out the puncture pin and a filter screen fell out with it. After the puncture pin was out, it was easy to see that it was pressure fit into the valve. Originally I was only going to remove the puncture pin. However, I decided to do a little more. I used a hand reamer to ream away the puncture pin seat and to taper the exit hole slightly. I then polished both the entrance and exit holes. Here is a link with pictures of an unmodified, disassembled valve.

The disassembled and modified RWS 850 valve

When I put the valve back together, I used threadlocker to lock the threads and seal it. I tried both blue threadlocker gel and regular blue threadlocker from Permatex, but neither worked. It would leak air when pressure was applied. Thinking back to when I took the valve apart, I remembered I had seen something that looked like dried white glue on the threads. White glue is similar to wood glue, but wood glue is supposed to be more flexible when dry, so I tried wood glue. That worked. In fact, I've tried to open the valve again and haven't been able to.

Trying the modified valve
The valve was back together, it wasn't leaking, so time to try it out! I put together the same setup as last time, loaded the gun and fired. Psssssssss.... where was that sound coming from? I turned off the air and checked. It was coming from inside the valve! I unloaded the gun, shot the pressure down, removed the valve and took a look at it. There was a small piece of spiral shaped aluminum stuck in the valve! Apparently the extra air flow had caused the Cooper-T adapter to let go of some aluminum debris left from the machining process!

Once the debris was cleared, I set up to record velocities starting at 1000 PSI down to 850 PSI at 50 PSI increments. I shot two shots at 1000 PSI and turned off the air so I could adjust down. Psssssssss... what now!!! The on/off on the regulator had failed!!! I frantically played with everything to try to stop it! Finally it stopped after losing 1000 PSI of pressure. At this point I had the output pressure set to 850 PSI and wasn't going to get a nice table at 50 PSI increments! Oh well, I might as well record the speed at 850 PSI!

Here is the "table" using .22 caliber JSB Exact Jumbo pellets.
1000 PSI: 685 fps
850 PSI: 660 fps

That works out to 15.29 foot pounds @ 660 fps and 16.47 foot pounds @ 685 fps. The advertised weight of 15.8 grains was assumed for the pellets. The same setup with an unmodified valve recorded a speed of 619 fps @ 1050 PSI, so that is a nice step up in speed! If you are interested in speed/energy calculations, there is an article and calculator at the Pyramyd Air web site.

Things Learned
-Plan ahead when travelling with guns.
-Machined parts may have debris in them.
-Using a lot of wood glue worked to lock and seal the valve closed. It worked too good for taking it apart again! I filled the threads with glue, screwed it together, wiped off the excess, unscrewed it, wiped off the excess and screwed it back together.
-There are potential problems with an on/off air source.
-There is restricted air flow in the standard RWS 850 valve. There are a couple of possibilities for where this exists. One possibility is that the valve volume may not be large enough. When I enlarged the entrance to the valve, the extra pressure and air flow could have compensated for this. Another possibility is that the valve and/or hammer springs need to be changed to allow all the air in the valve to be used. It could also be a combination of the two.

There are more parts to come about this conversion, but next time I will probably discuss the possible applications for air power and some problems with it.