Canadian Airgun Forum

Part 1: Introduction

After finishing the .177 pellet mold project that kept me busy since I reopened my shop in the garage at the end of April 2023, I decided to revisit an old one.

It all started in the summer of 2016 when I found a pristine Crosman 451 working flawlessly. Knowing that the sintered metal hammer was its weak point I wanted to make a replacement in steel but information was very scarce and it was nearly impossible to find the Factory Service Manual for it.

Around the same time, I was also in the process of buying a vintage rifle from Mr. Marvin and, after checking with him, he told me that he has the Service Manual and he graciously scanned it for me.



The Hammer Saga is documented in these posts.
topic73597.html
topic74795.html

In October 2016 another Crosman 451 blowback CO2 pistol was purchased from Mr. Marvin and was also used to test several brass hammers (even one that was CNC machined). These articles (part 5 to part were not posted on CAF but can be found there:
https://worap.wordpress.com/2016/12/13/ ... ga-part-5/



Unfortunately, the pistol from Mr. Marvin had a slow leak that I wanted to fix. It was documented in the post linked below.
topic74196.html

After the repair described in the post, the pistol worked relatively well for a period of time until the blowback failed. Since the slow leak also reappeared, I had a chance to experiment with other valve stems and valve seals options covered in the posts. Some options were leaking more than others.



As for the blowback, even if it was indexing the pellets correctly, it was still not strong enough to recock the hammer.

R-Gun Pete

Part 2: Looking more closely at the Blow Back Piston.

In 2019, I decided to dig a bit deeper in unknown territory and disassembled the blowback components. After inspection, I couldn’t see anything wrong with them, so I lubricated each part and put them back together. Unfortunately it didn’t make any difference.



It was annoying. No parts available for this airgun and nothing I could do about it.

Fast forward to 2023.

Now that I have machining capability, it was time to revisit this project that was nagging me.

My first step was to redo the valve stem with closer tolerances, then I made a new “one-piece” valve head/seal in Delrin. After reassembly the situation was still the same: the slow leak started after a few shots and the blowback was strong enough to index the pellets but too weak to recock the hammer.

Internet searches, after all these years, didn’t discover anything new. Some other peoples have worked on their Crosman 451 but none have touched the blowback system and the most detailed instructions I could find were mine.

At this point, I had found the picture I took in 2019 which showed the plastic cup with a tapered side and wondered if it could be reinterpreted as a cylinder with a o-ring instead. A discussion with a friend made me consider that it might be an option.

After disassembly and some measurements, I realized that there was not a lot of meat to do what I was thinking. I decided to stick to basic and to make something along the line of the original and machined a tapered cup from a half-inch Delrin rod. First, the locking pin burrs in the hole were sanded smooth and I was careful to keep the cylinder diameter as close as possible to the inside cavity of the brass body for a nice sliding fit, then the taper was machined leaving a thin ring of the cylindrical surface. A 3/8 countersink was used to remove some material inside the cup and the cavity was enlarged with the side of a 60 degrees cutter. A 1/8 hole was also drilled through.

A new steel pin was machined to the same size as the original (.185) and the tip reduced to .130 to provide a press fit with the Delrin cup. A steel washer sits on the pin shoulder and protect the base of the tapered cup.



A drop of Pellgun oil was applied to the surface of the cavity in the brass body and a bit of silicone grease was smeared on the Delrin cup before it was carefully inserted in the hole. After a few rotations and back and forth movements I had the feeling that it was sealing properly. Furthermore, since the cavity in the cup was smaller than the original, I was hoping that with the same expansion of gas the piston would be driven back a bit more forcefully.

Pressurization produced the expected slow leak but I didn’t worry because I knew that I have time for between 24 to 30 shots. The first 6 pellets were loaded, the hammer was cocked by hand and the shot fired. The hammer didn’t recock!!! I pulled back the hammer a second time and fired. This time, Bingo!!! The pistol cycled properly and was ready for the third, fourth, fifth and sixth shots… one after the others.

I continued reloading and shooting until the cartridge was exhausted then loaded a fresh one. The pistol worked flawlessly through the second CO2 cartridge but the annoying leak was still there. For me, in the several sealing jobs I did, it was hit and miss with the Delrin valve seals I made, in some case it was working immediately and in some others I needed to heat and smack the head to make it seals properly.

In this case, I was not eager to reopen the airgun again so I opted for heating the valve seat area from the outside with a heat gun when the pistol was pressurized with a third CO2 cartridge. As I could still hearing the slow leak I shot the Crosman 451 several times and it seemed that the noise was decreasing so I heated it a bit more and after a couple of shots the noise stopped.

To confirm that it was holding its pressure, I stored it away for half a day. Later when I got it out to shoot, it was not leaking and I was able to record on my I-Phone that the blowback was working perfectly.



Mission accomplished.

R-Gun Pete

R-Gun Pete,

I remember the original threads on this project. Nice to see you have gone back to the 451.

Congrats on the success, simply great work.

ColdAir

Hi ColdAir.

Thanks.

I am glad to have gotten rid of an itch that was bothering me.

It was really annoying to have a blowback airgun that wasn't recocking the hammer.

I hope the info might be useful to other owners of the 451.

R-Gun Pete