Benjamin 312 RestoMod

[Dave S]

Very nice work, Ron! I clocked my 312 2 months ago. Tim (Mac-1) did his 12 pump on it 15 years ago and thought you might want to see the results for comparison.
These stats were done on a Pact2 XP Infrared  Chrony at 76 degrees F. 115 ft above Sea Level and 14.3 gr Crosman Superdomes. Looks like you may be on to something good!
 BTW, I always keep 2 or 3 pumps in all my Bens and Dans when stored.  dave

6 pumps 550 FPS
7 pumps 574.3
10 pumps 622.2
12 pumps 644.3

[Tack Driver 10]

Very nice work, Ron! I clocked my 312 2 months ago. Tim (Mac-1) did his 12 pump on it 15 years ago and thought you might want to see the results for comparison.
These stats were done on a Pact2 XP Infrared  Chrony at 76 degrees F. 115 ft above Sea Level and 14.3 gr Crosman Superdomes. Looks like you may be on to something good!
 BTW, I always keep 2 or 3 pumps in all my Bens and Dans when stored.  dave

6 pumps 550 FPS
7 pumps 574.3
10 pumps 622.2
12 pumps 644.3

Dave,

Thanks for the chrony results.
Hard to find performance stats on these old guns especially a Mac-1 tuned gun.
I think I can get about 650+ FPS with 8 pumps with a few more mods.
Trying to keep a balance between power vs pump effort.

[Dave S]

Very nice work, Ron! I clocked my 312 2 months ago. Tim (Mac-1) did his 12 pump on it 15 years ago and thought you might want to see the results for comparison.
These stats were done on a Pact2 XP Infrared  Chrony at 76 degrees F. 115 ft above Sea Level and 14.3 gr Crosman Superdomes. Looks like you may be on to something good!
 BTW, I always keep 2 or 3 pumps in all my Bens and Dans when stored.  dave

6 pumps 550 FPS
7 pumps 574.3
10 pumps 622.2
12 pumps 644.3

Dave,

Thanks for the chrony results.
Hard to find performance stats on these old guns especially a Mac-1 tuned gun.
I think I can get about 650+ FPS with 8 pumps with a few more mods.
Trying to keep a balance between power vs pump effort.

Ron, curious to see what yours does with 10 pumps. Keep up the good work....and keep us posted! dave

[Tack Driver 10]

I made a new piston seal.
I noticed I couldn't adjust the pump arm to cam over and clear the breather hole.
This seal extends the piston so it touches the valve face and clears the breather hole at the top of the stroke.
The gun is harder to pump and the pump arm doesn't spring open much between pumps as before.
A definite sign that a good amount of dead air has been eliminated.

Pictured is the gun with the pump arm open after 7 pumps.
Previously the pump arm would spring open past 45 degrees.

[Tack Driver 10]

Ran the gun over the chrony.
Wow, bumped up the power an average 80 FPS.

With 14.3 CPHPs   Previous results in ( ).
03 pumps 498 (426) (402) FPS 07.87 (05.76) (05.13) FPE
05 pumps 615 (528) (473) FPS 12.01 (08.85) (07.10) FPE
07 pumps 685 (602) (524) FPS 14.90 (11.51) (08.72) FPE

Need to trim the check valve and replace the heavy spring in the valve.
That should get more volume.

[Rob M]

those are some really good numbers.. any chance of pumping to 10 or is it just too hard ??

[Tack Driver 10]

those are some really good numbers.. any chance of pumping to 10 or is it just too hard ??

I don't want to attempt 10 pumps until I max out the volume in the valve and change the springs.
It was retaining some air but I don't think that would make much difference.

In it's current configuration I think 8 pumps is the max.
I can always detune the gun by adjusting the piston for more head space.
I had to do that when I built this MF-DHP gun on the Green Forum
https://www.tapatalk.com/groups/thegreencrosmanforum/the-mf-dhp-will-rock-your-world-do-you-feel-lucky-t48451.html

[desertplinker]

am I wrong or do you need to have the felt washer behind the pistol to lubricate?

[Tack Driver 10]

am I wrong or do you need to have the felt washer behind the pistol to lubricate?

Steve....With the stock steel piston a felt oiler should be used to minimize wear on the brass tube.
This piston is made from Delrin so no need for it, still has the oiler on the other end.

In the meantime took the gun apart to finish the mods on the valve.
Cut the Teflon check valve down and added some flutes to help it breath.
Using the original check spring with a blue Delrin spacer and a light SS valve spring.
Added a poly bumper to the back of the valve to prevent the hammer from impacting it.
Marked the valve seat with a sharpie and installed it with the port at 12:00 aligned with the barrel port.
Used a spare hammer spring that I cut 1 1/4" off so it's easy to cock.

Assembled the gun and it completely dumped 8 pumps.
Have to cock the gun before pumping due to the light valve spring.
The first few pumps are very easy until it starts to build pressure from 5-8 pumps.

[Tack Driver 10]

Ran the gun over the chrony.
Made one pass with 10 pumps and an 8 pump pass with JSB Monsters.

With 14.3 CPHPs   Previous results in ( ).
03 pumps 471 (498) (426) (402) FPS 07.04 (07.87) (05.76) (05.13) FPE
05 pumps 598 (615) (528) (473) FPS 11.01 (12.01) (08.85) (07.10) FPE
07 pumps 668 (685) (602) (524) FPS 14.17 (14.90) (11.51) (08.72) FPE
10 pumps 743 FPS 17.53 FPE (some RA)

With 25.4 JSB Monsters
08 pumps 545 FPS 16.75 FPE

With the added valve volume I was able to pump to 10 pumps but was hard.
Still made 25 FPS per pump from 7-10 pumps.
I will call this an 8 pump gun max, for everyday use 3-6 pumps.

[Dave S]

Ran the gun over the chrony.
Made one pass with 10 pumps and an 8 pump pass with JSB Monsters.

With 14.3 CPHPs   Previous results in ( ).
03 pumps 471 (498) (426) (402) FPS 07.04 (07.87) (05.76) (05.13) FPE
05 pumps 598 (615) (528) (473) FPS 11.01 (12.01) (08.85) (07.10) FPE
07 pumps 668 (685) (602) (524) FPS 14.17 (14.90) (11.51) (08.72) FPE
10 pumps 743 FPS 17.53 FPE (some RA)

With 25.4 JSB Monsters
08 pumps 545 FPS 16.75 FPE

With the added valve volume I was able to pump to 10 pumps but was hard.
Still made 25 FPS per pump from 7-10 pumps.
I will call this an 8 pump gun max, for everyday use 3-6 pumps.

Ron, those are incredible numbers! My Mac-1 Sheridan Blue streak clocked at 7.27.7 best I could get at 72 degrees, so I figure you're way in the ball park...and then some! BTW, I have to cock the Dan before pumping it up....coincidence...I think not! BTW, I talked to Tim Friday and told him what you're doing....and to check it out. I'll probably be in his shop Saturday.  Your use of polymers and tooling in this appliction is impressive to say the least! dave

[Psipumper]

Ron , zippy 312 for sure. You may be able to pick up some free power by filling the annular groove in the valve seat exit. Some of those 312’s had a solid ( no groove) along with a non rotate provision.

Does a lead seal go on each side to seal it?

Interesting. I rebuilt the family 312 when I was 12 years old. I ordered a new valve seat . It was the type you have. The original was solid. I still remember that all these years later. My brother may still have it.

[desertplinker]

I believe per Tim recommended always cock before pumping unless you were just putting a couple pumps of air in for storage. My memory may be faulty.

[desertplinker]

Ron
You got a 1969 342 coming your way.

[Tack Driver 10]

Ron , zippy 312 for sure. You may be able to pick up some free power by filling the annular groove in the valve seat exit. Some of those 312’s had a solid ( no groove) along with a non rotate provision.

Does a lead seal go on each side to seal it?

Interesting. I rebuilt the family 312 when I was 12 years old. I ordered a new valve seat . It was the type you have. The original was solid. I still remember that all these years later. My brother may still have it.

The valve body in this gun doesn't have the notch that aligns it with the valve seat.
I could make a poly ring to fill in the groove but aligning the ports in the gun would be difficult although not impossible.
I figured to just live with whatever loss there is.
I'd prefer to have the one hole valve seat type.

There was a lead seal on both ends of the valve seat but I put an o-ring in the front and no seal in the back.
Seems to get good power without it but I should install a seal to see if it makes a difference.

[Tack Driver 10]

Ran the gun over the chrony.
Made one pass with 10 pumps and an 8 pump pass with JSB Monsters.

With 14.3 CPHPs   Previous results in ( ).
03 pumps 471 (498) (426) (402) FPS 07.04 (07.87) (05.76) (05.13) FPE
05 pumps 598 (615) (528) (473) FPS 11.01 (12.01) (08.85) (07.10) FPE
07 pumps 668 (685) (602) (524) FPS 14.17 (14.90) (11.51) (08.72) FPE
10 pumps 743 FPS 17.53 FPE (some RA)

With 25.4 JSB Monsters
08 pumps 545 FPS 16.75 FPE

With the added valve volume I was able to pump to 10 pumps but was hard.
Still made 25 FPS per pump from 7-10 pumps.
I will call this an 8 pump gun max, for everyday use 3-6 pumps.

Ron, those are incredible numbers! My Mac-1 Sheridan Blue streak clocked at 7.27.7 best I could get at 72 degrees, so I figure you're way in the ball park...and then some! BTW, I have to cock the Dan before pumping it up....coincidence...I think not! BTW, I talked to Tim Friday and told him what you're doing....and to check it out. I'll probably be in his shop Saturday.  Your use of polymers and tooling in this appliction is impressive to say the least! dave

Although the gun makes good power there are several things holding it back.

The pump link is not very sturdy, a steroid pump link would fix that.
The barrel is only about 19", a 24" barrel would be better.
The valve and barrel port are fixed and not accessible to enlarge. 
There's a lot more power in that gun.

As an example:
The MF-DHP is a Disco tube based pumper with similar swept volume (although smaller) of this gun but made 960 FPS with 15 pumps.
It made 1034 FPS with 20 pumps and a 593 second shot with a little more valve volume.
https://www.tapatalk.com/groups/thegreencrosmanforum/the-mf-dhp-will-rock-your-world-do-you-feel-lucky-t48451-s10.html

[Tack Driver 10]

Ron
You got a 1969 342 coming your way.

Thanks, I'll keep an eye out for it.

[Techie]

Do you get air leakage around the breech/bolt?  My 1959 312 had no O-ring there and was leaking air until Duy made a new bolt with O-ring.

[Tack Driver 10]

Do you get air leakage around the breech/bolt?  My 1959 312 had no O-ring there and was leaking air until Duy made a new bolt with O-ring.

No o-ring on this bolt, it's probably leaking some but it looks pretty tight.
I made a custom bolt for a 317 that had a worn breech so might make one for this gun.
It also had a nice power adjuster.

https://www.gatewaytoairguns.org/GTA/index.php?topic=160425.msg155789207#msg155789207

[Tack Driver 10]

Ron , zippy 312 for sure. You may be able to pick up some free power by filling the annular groove in the valve seat exit. Some of those 312’s had a solid ( no groove) along with a non rotate provision.

Does a lead seal go on each side to seal it?

Interesting. I rebuilt the family 312 when I was 12 years old. I ordered a new valve seat . It was the type you have. The original was solid. I still remember that all these years later. My brother may still have it.

I had to

The valve body in this gun doesn't have the notch that aligns it with the valve seat.
I could make a poly ring to fill in the groove but aligning the ports in the gun would be difficult although not impossible.
I figured to just live with whatever loss there is.
I'd prefer to have the one hole valve seat type.

There was a lead seal on both ends of the valve seat but I put an o-ring in the front and no seal in the back.
Seems to get good power without it but I should install a seal to see if it makes a difference.

Had to try it.
Made a poly ring to fill in the groove around the valve seat.
First try I drilled a port @ .159" in the poly but the power actually dropped about 40 FPS.
I figured the port closed up when compressed in the gun.

Next, cut to the chase and cut some off the ring so there was some reserve space around the valve port.
Power came back up but basically the same as the stock setup.
Ran a limited chrony test.

With 14.3 CPHPs   Previous results in ( ).
07 pumps 667 (668) FPS 14.12 (14.17)FPE
08 pumps 698 FPS 15.47 FPE

With 15.9 JSB Strattons
08 pumps 669 FPS 15.80 FPE