Crosman 160 Exhaust Valve stem gasket

[haze10]

Need some advise on how to change the gasket on my Crosman 160 exhaust valve stem. I am referring to the brass piece with the steel stem that has the gasket embedded by the steel stem, that stops the air flow through the exhaust valve body until the hammer hits. I watched a video on youtube with a guy changing it on a mod 150 but he didn't go into detail. He heated the brass portion with a torch, and then took a drift to drive out the stem. I need to know if the gasket is a simple o-ring or if it made from some specialty material like soft silicon. Can if an o-ring, can a standard one be used, do you have to sandpaper the curvature of the side to be flat. Is there some superior material to use. Also, do you do anything to the exhaust valve body seat. Would take a special tool that I don't have, but I could lighting touch it up with 1000 grit sandpaper if that is recommended.

All advice appreciated.

 

[nervoustrig]

Pretty sure the seal material is in a tube / cylinder shape given the way the model QB78 / QB79 poppets are constructed. They're pretty much a carbon copy of the model 160. The material is polyurethane.

For longevity, a hard plastic is better...Delrin / acetal, PET-P, or simply nylon. Sounds like you are a DIY kind of guy and making one is well within the realm of the tinkerer. I and many other have successfully made replacements with primitive tools like a drill and Dremel. No need for the brass shell. Just make a plastic cylinder of the same dimensions, center drill it for ~0.005" interference fit to the stem and press in the stem. Some guidance if you're interested:

Any one ever heard of the Dynamic PCP air rifle? - Airguns & Guns Forum

Any one ever heard of the Dynamic PCP air rifle?, Air guns, Airgun Forum

www.gatewaytoairguns.org

 

[haze10]

I could make one out of nylon but I didn't think it would actually seal against the seat. I wanted to make it air tight. If you use nylon are you glueing some type of soft gasket material to the surface?

 

[nervoustrig]

Soft poppets are by far the exception, not the rule. They certainly are forgiving of poor surface quality but also degrade over time and require a tremendous amount of hammer strike to knock open. If those limitations are acceptable or desirable, stick with it.

A plastic poppet is more common, and indeed the surface seating against the valve needs to be of high quality (square, flat, smooth) to ensure an airtight seal. A poppet made on a lathe can frequently be used as-is, assuming the operator knows what he’s doing, using a sharp cutter, etc. Meanwhile one made with primitive tools will need the sealing face dressed somehow. The simple sanding jig shown in the link is one way of doing it.