Crosman Model 451 Seal Repair – Part 1

At the end of the hammer saga, I unexpectedly came across another Crosman 451 that I was able to purchase in Canada. This new acquisition allowed me to put to the test the CNC machined hammer and sear that I made out of brass. You can read about the “Hammer Saga” in several previous articles (you’ll find a link to Part 1 at the end of this article).

The glitch with the new purchase was that the pistol had a leak that got worse when I tested the brass hammer and sear.

Having confirmed that the hammer replacement worked well, it was time to dig deeper and make the pistol fully operational again.

For this article, my first picture starts with the airgun disassembled to the level where I stopped in the “Hammer Saga”. The disassembly from the beginning can be reviewed in those articles.


Removal of two screws from the left receiver releases all the other components. Normally the barrel should stay attached to the valve body and I was not planning to remove it since a special gauge is required for reassembly. Unfortunately the set screw was loose and the barrel came out. This could be seen in the following photograph.


My initial plan was only to concentrate on the resealing aspect but as the barrel was out I decided to dismantle the turret magazine. Those parts are easy to remove except for the turret pivot. The pivot is cross pinned but even with the pin removed it is a really tight fit and I didn’t want to use force to push it out. I decided to leave it alone because if I succeeded in taking it out it might prove very difficult to put it back together with the proper alignment. This pivot is the transfer port, this means that it is connected with the valve cavity and the snug fit is probably what is preventing leakage. Furthermore, since the firing pin is sliding through it, the alignment is critical. These were reasons enough against tampering with it.


Reassembly of the turret components is not very difficult and the “squarish” short free end of the spring attached to the plate is to be hooked into a small hole on the front of the brass valve block.

For the barrel, since the gauge that the service station was using is not available, the procedure is fairly straightforward. Once the turret is reassembled it will become the gauge. The barrel has a curved cut that must match the curvature of the turret so it is just a question of rotating it until it feels aligned (there is also a mark made by the set screw and it could be used as a guide). The gap is a question of feel too. Start by pushing the barrel against the turret until it interferes with its rotation, from that position it is moved slightly forward until the magazine can be rotated without rubbing at any of the six stops. If the gap is too large, it can make a difference on the velocity of the pellets. The trick is to find a position where the barrel will not rub but still fairly close to the surface of the turret. When satisfied the set screw is tightened.

The next picture gives the relative position of the components.


To remove the internal components of the valve, the first step is to push out the cross pin. The cylindrical piece, at the end of the bent tubing, is held inside the block by the o-ring pressure. It could be carefully wiggled out and after that all the other pieces are easy to take apart. For the CO2 seal and piercing pin region, it is necessary to make a tool. Essentially the width of that tool is the same as the diameter of the slotted nut and the middle is filed off to accommodate the protrusion of the pin. Mine is made from aluminum and it is strong enough to unscrew the nut. You have to remember that it should only be finger tight when you put it back together.


Basically, with the exception of the blowback system (I didn’t touch it because it was working properly), there are only 3 seals. The thick donut shaped seal for the CO2 cartridge is a standard Crosman part that is still available. The o-ring that goes in the groove at the end of the cylinder is probably also a standard part. Both of them were still flexible and didn’t need to be replaced. The cause of the leak was the face seal that is in the brass cup on the stem of the firing pin.

The face seal was hard and dry; it was also compacted to the point that it was flush with the cup instead of sticking out. I had to pick it out piece by piece since it was crumbling.


Similar to the hammer approach I was looking primarily for a homemade solution. Furthermore, that solution shouldn’t require dismantling the Stem/Seat assembly. In the past I have been successful in making face seal replacement with the plastic of coffee can covers.


The example below was for a Crosman 622 pump action rifle. This type is easier to fix because the stem is only one diameter except for the flared shoulder. The plastic being a bit flexible it is fairly easy to coax it to go underneath the lip.


In this case of the Crosman 451, the cavity was fairly deep so I placed one plastic layer, one leather layer and two other plastic layers. The problem is that the shaft is larger at the middle so the seal parts had to be stretched.

Normally a slight tap with a mallet would imprint the valve seat in the plastic. In this case I should have paid closer attention to the result. I was focussing on taking pictures and it is only after reassembly and failure to hold pressure that I looked carefully at it. In the picture below it is visible that the seating groove is not centered as it should have been. In hindsight, I now know what happened. I normally don’t use leather but because here the space was large I thought that the leather soaked with oil would help to seal. The fact is that layer being soft created instability and when I tapped it with the mallet the brass cup didn’t sit square.


Since it didn’t work I had to remove the packing and think of something else. We can see the different layers in the following picture.


Looking more closely I saw that there was a spring pin, I thought that if I remove it I would be able to separate the cup from the stem and make a nylon cylinder that could be drilled and placed in the cup before reinserting the stem.



Again, nothing is simple; after the pin was removed the stem didn’t want to get out. At first I thought there might have been a thread but it was not moving at all, so I figured that it might be a press fit.

I heated the brass cup with a heat gun and try to punch out the stem without any positive result. Being stuck, I posted a request for help on my airgun forum. In the meanwhile, I discovered that the cup didn’t seem perpendicular to the axis of the stem. I couldn’t figure how it could have happened since I use a piece of wood with a small hole for the stem.  The brass cup was sitting square on the face of the wood piece when I tried to punch out the shaft, so this is not what could have bent the stem.

Anyway I tried to straighten it. This pin (because it is fairly long) is not very stiff and was bending all over the place. I got it worse to the point that it didn’t want to slide in the valve body anymore.

As a last resort (I couldn’t make it worse), I placed it in the chuck of my press drill (it was wobbling like crazy) and use a small wood block to push it toward the centre. Slowly I was able to make it practically straight and square so it was now sliding freely in the valve body.

Then I realized when I had messed up. During my tryout with my plastic disks approach, the bending probably occurred when I did the tapping against the valve seat in the brass body. The stem is deep in the valve body and I used a 3/8″ dowel (smaller than the cavity) and I was possibly off centre when I smacked it. Looking back at the picture with the yellow seal it is visible that the groove is not concentric.

Since the stem and the cup couldn’t be taken apart the solution would be to pour some substance into the cavity. I used Goop which is some sort of rubbery adhesive. After leaving it to dry overnight, I reassembled the pistol and pressurized it only to experienced, again, failure. The material is way too soft and gets deformed.

At this point I decided to make a CAD model and to prepare a set of drawings for my friend who offered to make new parts for me. This time, the intent was to have an assembly that could be taken apart when required.


The initial plan was to have a thread at the tip of the stem and in the brass cup but after talking with my friend I changed my mind and opted for a sliding fit.


R-Gun Pete

Related pages

Crosman Model 451 Seal Repair – Part 2

Crosman 451 Hammer saga – Part 1

Crosman Model 451 review


Technical Articles


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