Crosman exhaust valve repair

I recently got a new project rifle, a Crosman 766. I bought this rifle to see if I could make it into a bulk fill CO2 rifle. 

That Blog should be next....

Once I got the rifle apart I found this, a crusty exhaust valve with a crumbling seal. I have a couple spare exhaust valves but the stems would need to be shortened and they don't have the brass body. I'm not sure if the brass makes the valve any better but having the extra metal in the valve makes me feel better. 

Here is the exhaust valve as it came out of the 766, It's sitting in the pliers for display purposes. 

I used a couple cheap Chinese pellets to protect the stem while I clamped it in the vice. I should really invest in some soft jaws for my vice. With the valve secured in the vice I used my butane torch to heat up the body. 

The stem is a press fit in the body and the heat expands the body enough that it can be pried off. I tried to be careful but still managed to put a couple dings in the body. The small dings wont really cause a problem since they don't affect the  sealing of the valve.

I cleaned the crud and corrosion from the stem and body. With everything clean I measured for the replacement seal, .310" OD the hole for the stem is .14" and it needs to be .062" thick. 

I had some options for seal material, Delrin, PTFE or HDPE (High Density Polyethylene) I went with HDPE for my first attempt since it is a bit softer than Delrin or PTFE. Turned to the correct diameter and the hole bored I parted off a  new seal.

The seal was a good fit in the body. 

I used my arbor press to re-install the stem in the body. 

The rebuilt valve.... 

I didn't really expect this to work on my first attempt, much to my surprise once the valve was put back together and installed in the 766 it held air. Now I can get on with trying to get the 766 to run on CO2.

Homebrew .177 build, Part 2

Related blog:
Part 1

The first round on this build went well, I have a working air gun. Now it's time to work on the details and make this thing more comfortable to pump and stable to shoot.

I'll add some weight to the stock and a pump handle will give me a more stable rest along with making it much easier to pump.

I had a mystery stock in my stash of goodies, I had used it as a temporary stock on a previous build.
A trip to the band saw and I have the basic shape of my filler block.

Some work with an orbital sander and the block is a nice snug fit in the 1399 stock.

My pump handle is 2 pieces of Oak separated by a 1/4" thick piece of Poplar.

After adding some shape to it, I think it looks pretty good.

I finally found a use for these Industry Brand pellets.

I filled a cavity in the butt of the stock with the pellets and epoxy. During some testing it was a bit muzzle heavy, this will help balance things out. 

Some dark walnut stain and shellac and the wood is sealed up. 

Painting the pump tube will have to wait for warmer weather, I'll finish this up in the spring. 

I built a new piston to replace the one I robbed from my .22 cal build. 

The plug for the end of the pump tube and a barrel band.

Once I got the rifle reassembled I did some shooting and noticed it wasn't grouping as well as I was expecting. 

I thought I'd try some different pellets to see if one stood out as a good pellet, No joy....  

I tried a few different things to see how they affected accuracy. The 953 liked the RWS Meisterkugeln pellets so I used them for my first tests, 5 shot groups with a floating barrel, the barrel band installed then with the barrel band on I added the barrel weight from a Daisy 853. The best group was with the barrel band and weight installed shooting at 4 pumps. I saw a few encouraging groups but this thing was throwing fliers about every 5th or 6th shot.

Then I remembered a problem I had seen before with the Crosman steel breech. pellets don't always make it past the transfer port, I pulled the barrel and breech to check for this problem.

Sure enough about 1/3 of the transfer port was blocked by the skirt of the pellet. The skirt can collapse when the air from the valve hits it causing problems with accuracy.

My first attempt to solve this problem was to build a bolt with an extended probe. This got the skirt past the transfer port but created another issue. The pellet wobbled on the probe and wasn't going into the breech straight, this caused the pellet to be deformed as it was pushed into the barrel at an angle. I could feel this happen as I slid the bolt forward, the same thing was happening with the Crosman bolt but not as bad.

My cure for this problem was to build a bolt with a hollow probe. The hollow probe pushes on the edge of the skirt, keeping the pellet straight as it is pushed into the barrel. The other bonus of this design is it doesn't matter how deep the skirt is, the pellet is always pushed past the transfer port.  

I saw a big improvement in groups, two ten shot groups and no fliers. It doesn't like CPHP....

Two 10 shot groups over the Chrony with 7.0 grain RWS Meisterkugeln pellets. 

While shooting over the Chrony this thing was showing its potential. 717 fps at 4 pumps! 5 pumps gives just under 800 fps and the valve starts retaining air. With some work I think 10 pumps would put out close to 1000 fps. I'm more than happy with 800 fps at 6 pumps so I'm not going to try for 1000 fps.

UPDATE:

I wanted to do some work to the pump linkage to see if I could make it better. During the testing the brass bushing at the muzzle end of the linkage started to fail, the split pin was chewing up the bushing. I made up a new bushing and turned up a new pin from drill rod. I was surprised at how much smoother the pumping was with the new pin. This got me to thinking, would replacing the bushings with bearings make it even smoother? A quick search online found bearings that would fit. (R166-ZZ) 3/16"X 3/8"X 1/8"

The muzzle end of the linkage is 1/4" thick so it will take two bearings, the piston end is 1/8" thick so it will get one bearing. There isn't enough metal to fit a bearing in the middle so it will stay a bushing.

The bushing is 1/4" OD so I will have to bore the linkage out to 3/8" for the bearing to fit. 

The bore wasn't as tight a fit as I wanted so I staked the edge of the hole with a center punch. 

I checked the pump force before changing to bearings, at 6 pumps it took 29 lbs to pump the rifle. With the bearings installed it took two lbs less force to pump it. While trying out the new mod I noticed a ping noise while pumping then the pump linkage started falling away from the pump tube after firing. A quick check and I found some play in the linkage at the piston end of the linkage. When I took the linkage apart the bearing came apart. 

Hoping the bearing was bad when I installed it I replaced the bearing with a fresh one and tried again, but got the same results. These bearings just aren't going to take the forces that the pump linkage generates. So it is back to brass bushings. I turned some new bushings to match the OD of the bearings. 

I'm happy with the way the linkage ended up.  

It's back in shooting form again and I have learned something new.

Thanks for following along.