My problem is finally solved.
My friend was able to make my parts and I reassembled the pistol over the weekend.
When he offered to machine the pieces on his full size lathe he didn’t realized how small they were. He is an engineer and not a machinist but he likes to tackle different projects and he thought that it would be a good practice for him.
He fabricated the parts from my drawings but unfortunately I made some design mistakes that would be corrected if another set should be made at a later time.
I was fixated on replicating the original parts but, at the same time, keeping the option to have an “easy” disassembly of the Stem/Seat assembly.
My first approach was to have a thread but being that small I didn’t want my friend to struggle with that so I opted for a sliding fit with a cross pin.
The brass seat needs to have some flats to let the CO2 flows to the face seal where it will be stopped. I made a quick fixture to hold the small brass piece so I can make the flats on a belt sander.
My indexing was not perfect as I did it by eye and I made the perpendicularity correction by hand with a file. My measurements on flats are within the specs I needed.
The following picture shows the newly machined parts beside the original Stem/Seat assembly. There is a bit of shatter marks on the firing pin side but as it was already at the right size trying to smooth the finish would bring it undersized. I decided to just rub sandpaper on it to attenuate any sharpness and put silicone grease on the shaft (thinking that it will be kept captive in the tiny grooves and hoping that it would help to seal).
The valve block on the 451 is a large piece of brass and it is difficult to see inside so guessing where it is sealing is exactly that, guess work. I was assuming that the larger diameter after the firing pin would rest on a shoulder in the block when the face seal was against the valve seat in the block. I should have thought about it more carefully as to try to achieve 2 simultaneous contact points would make practically impossible tolerances.
Being in a hurry to try my parts, this assumption got me in trouble. This train of thought made me sure that the shaft would have nowhere to go, being sandwiched between the spring pushing on the back of the small brass cup and the contact surface of the larger diameter on the firing pin. The cross pin was then not necessary.
Wrong…
… but I discovered it only after reassembly and pressurization. When the face of the plastic seal is sitting against the seat of the valve the larger diameter on the stem is not touching the shoulder in the passage. There is probably enough room to allow for the plastic seal to wear and, in this case, the shoulder is contacted before damage could happen to the valve seat in the block.
Based on my assumption I used red Loctite to hole the shaft in place and as a sealer. Not being a product that I normally use, I applied it and let it cure for about an hour. This is not long enough, I know it now.
After pressurization, I had a leak that could be felt through the barrel so it was effectively coming from the seat seal area but firing the gun was doing funny things. The hissing sound was noticeably less when the hammer was resting on the firing pin than when it was cocked. This mean the pin was moving and the leak was around the sliding fit in the brass cup. Effectively, after disassembly, the Loctite was all around the plastic seal in the cup. I had to take everything apart and clean everything again to make sure that no particles had travelled in the valve body.
This time I reassembled the Stem, Seal and Brass Cup with Loctite and put it aside for more than 36 hours.
In the meanwhile, it is not because the main hissing was coming from around the stem that I didn’t have other potential sources of leakage. To minimize the possibilities I checked the valve seat in the large block. It seems that I was seeing some tiny marks as shown in the picture below.
Not looking forward to another assembly/disassembly cycle in case of failure, I decided to make a tool to polish the valve seat. I had to shave the diameter of one end section of a ½” plastic rod in order to reach deep enough in the hole. With a punch I made disks of 400 and 600 grits that were attached to the tip with double faced tape.
I rubbed it by hand until it looks smooth as shown in the next picture.
Finally, to be sure that the stem would not go anywhere I installed a cross pin that was also positioned and sealed with Loctite. I should have checked my picture before reassembly because the region I wanted to show is a bit blurry.
This time it is a success.
It was pressurized and stayed silent so I fired the gun and the blowback worked perfectly so I moved in my garage and proceed to shoot 30 pellets then I stopped and left the residual pressure for several hours.
When my friend came for our shooting session, I let him shoot the pistol; there was still enough pressure for another 2 shots. At this point, I removed the CO2 cartridge and reinstalled a fresh one. We got 34 good shots before it stopped recocking (which is about the same that my other 451).
I stored it overnight with pressure in it and when I shot it this morning it was still holding. I will continue to do that tonight and tomorrow.
I am feeling confident that it is now fully functional.
As for the changes in the design, I would make the cup with a blind hole, leave a precision sliding fit on the stem and keep the cross pin. Here only the small cross pin would need to have Loctite to keep it in place and seal it. The opposite side to the cup side in the brass part being intact; there would be no chance of leak.
I am also exploring a couple of other approaches with rapid prototype printers. I would like to know if the plastic from those machines might be a candidate for sealing material.
For the first, I have designed a 3 pieces set to be tried on the original stem that I couldn’t take apart. The intent would be to have two small half tubes placed around the stem under the lip and a larger tube that will slide around them and lock everything in place.
For the second, I remembered that stem replacements for some Crosman models are only a steel pin with a larger plastic head that is also the face seal. In those the brass cup doesn’t exist so I was thinking to do the same thing. I will have another stem machined with a thread at the end where the cup should be and, from the rapid prototyper, I will get a plastic replacement with a shape that would combined the brass cup with the seal in place.
I may show the result at another time as it is still a work in progress.
R-Gun Pete