I'm rebuilding my crosman 766 again...

[bustachip]

(Probe update)

I received the new bolt assembly and finally got some time to work on the o-ring probe. I first made a test piece using my old bolt assembly to get the fit and dimensions right, just for giggles I threaded it M4 x.7, it
worked, but screwed together tight because of the major O.D., but the main reason was to get the o-ring in
the right place, because there's not much sealing area in there...LOL.

Next I threaded the "new" bolt assembly 10-32 like Tack Driver used and it worked much better, the major
diameter is closer (since the plastic bolt probe doesn't get turned to the correct diameter). Also the tiny
003 o-ring worked great, and I made the new probe about .253" longer for a deeper pellet seating.

So before the 766 was shooting around 680 fps and the bolt didn't stay completely shut. Now with the o-ring
probe it's shooting 708 fps and the bolt stays shut, so it picked up around 28 fps. This was using the JSB
7.33's and 10 pumps, just as before.

Here's few picks of the "Tack Driver" inspired o-ring probe....


Tiny o-ring laying in front of the brass probe...


Finished o-ring probe.....



I'd say it's dumping about 98% of the air with 10 pumps, so I'm still going to try and find a slightly stiffer
hammer spring. I'm also going to try and seal the front of the barrel support, where the barrel goes in.
But all in all a big improvement in my 766 since I started the rebuild/reblue job.

Thanks to all that responded and big thanks to Tack Driver for his mods sharing. I will update this thread
when more work is done.

TMurray

BTW, my phase II tapered steel barrel 766 piston doesn't have any plastic on it, just a steel shaft with
a milled slot at the other end...
 
 
 

[Tack Driver 10]

Looks real good.

I used an 8-32 thread as 10-32 was too big for the 3/16" brass stock.

Like all things Crosman there's so many variations it's hard to keep track of them.
Below are some of the 766/2100 piston variations.

If you make an adjustable piston you'll make the biggest gains in velocity.
I have a 1377 that gets 725 FPS with 10 pumps using 7.9 grain CPHPs, 800 FPS with 15 pumps..
It uses a "Spin Tight" (patent pending...LOL) piston so the rubber cup threads into the piston.
Makes for a super tight fit that builds pressure like crazy.

[bustachip]

In your piston picture, mine is second from the left.

Does your "spin tight" use a factory pump cup and angled head valve, or is it a flat top valve with a custom
pump cup?

Also in the second picture what does the back half of the piston look like? And when adjusting, do you just
bottom the pump cup out on the valve with a small amount of clamping distance for the pump arm to
clamp shut at the bottom of the stroke?

I haven't measured my piston in the tube so I don't know how close it comes to the valve when pumping.

TMurray   

[Tack Driver 10]

In your piston picture, mine is second from the left.

Does your "spin tight" use a factory pump cup and angled head valve, or is it a flat top valve with a custom
pump cup?

Also in the second picture what does the back half of the piston look like? And when adjusting, do you just
bottom the pump cup out on the valve with a small amount of clamping distance for the pump arm to
clamp shut at the bottom of the stroke?

I haven't measured my piston in the tube so I don't know how close it comes to the valve when pumping.

TMurray   

The spin tight piston uses a stock pump cup.
The threads are 1/2" x 20 same as the threads between the valve.
Adjust it so the piston touches the valve with about 3/4" space between the pump arm and tube (aka cam over).

[bustachip]

Another update..lol

Today I made a small collar with 3mm set screws and a o-ring to seal the front of the barrel support
where the barrel goes in.....like this


I also made a hammer spring spacer to preload the spring (since I wasn't able to find a stiffer spring)
and it works good. Now my 766 is completely sealed, and shooting at 720 fps with 11 pumps using the
jsb 7.33's, I'm happy to report it's dumping 100% of the air with 10 pumps.
I'm going to take a break and shoot it a little...lol

TMurray 

[bustachip]

  Velocity update today....

still shooting the JSB 7.33's, I decided to try 15 pumps over the chrony and got 777.8 "high" fps and
775 "avg" fps. I bet with a BB it would be humming along pretty good...LOL.

That's 50+ fps over the 10 pumps before.

766  is shooting strong!

TMurray

[unionrdr]

The new, properly-fitting bolt probe has to be the reason. Another guy made a ring land on his original to fit a small O-ring and got another 50FPS. Why I've been wanting a small lathe to make them myself. But look stock on the outside...that's the heart of restomodding!

[Jon Foster]

Any chance we can get the broken images in this thread updated?

Jon.