Crosman 760 First Variant with Bolt O-Ring, Grooved Receiver

[egillman]

One of annoying features of the 760 early variants is the air that escapes from around the bolt.  For my latest 760 rebuild, I decided to modify the bolt to hold an o-ring.  To do this mod, you need some size 003 o-rings, a drill, and a small file. First, I marked a line around the long end of the bolt exactly 1/2 inch from the tip.  Then, I chucked the bolt in my drill so that I could see the line.  I taped a small bolt counterweigt to the handle end of the bolt so it would spin smoother.  While the bolt was spinning, I used a small v-shaped file to make a groove at the 1/2" line, to a depth of about 1/2 the thickness of the o-ring.  I then took the bolt out of my drill and slightly tapered the end using my bench grinder.  This allows for better air flow to the pellet, and it also makes sliding on the small o-rings easier.  If the bolt with o-ring does not slide into the barrel, you might have to make the groove a little deeper.  A drop of oil also helps. 

[TooJung2Die]

It would be great if you posted a photo or two of your 760 bolt mod. I repaired a couple of old auto-cock 760's and the puff of air from around the bolt is noticeable. 760's are great. They're so quiet and easy to pump.

[egillman]

I am taking it apart this weekend and will get some photos then. I will also check to see if the new bolt resulted in any fps improvement.

[ezman604]

Good deal. I ordered a replacement OEM bolt to experiment with. Haven't had the time to cut the O-ring slot yet. Also thought about maching a Delrin sleeve and glue it onto the bolt to help seal it. Will try both mods.

[egillman]

Here's a photo of the bolt.  After using it to shoot about 20 pellets, I noticed that the o-ring was showing some signs of wear.  I am interested in how the delrin idea works out.

[egillman]

I just finished putting this 760 back together.  It has the bolt modified for an o-ring, and the valve "spacer" was drilled out to give about 20% greater volume.  I also added a second variant receiver with scope grooves. I will check the new, improved 760 fps tomorrow.

[egillman]

From what I have read, urethane is the way to go for o-rings that need good wear resistance.  I have been unable to locate any in the -003 size (3/16" OD).  Anybody know of a source?

[longhunter]

try Mcmaster-Carr. they usually have the stuff you need.

[BillK]

Tim at MAC 1 airguns has 90 dureometer (sp?) urethane O rings in stock.