1377 piston mods questions and info.

[TheBullsEye]

Okay so recently i came across a post where someone merged a 13xx and a 66 i believe it was well i had a beaten up 1377 and a good 760 around so i said hey why not try it so the last week or so i made my aluminum piston into a flat top and am in the process of making and adjustable pump cup.( got to vist the hardware store) I am planning on useing the pump tube from the 1377 the arm frm the 760 (in better condition) the grip frm the 1377 the piston frm the 1377 the barrel from the 760  not sure which hammer to use yet i will probably test both ... I had to modify the barrel in order for it to work i had to drill the port hole and counter sink it. I cut off the xtra fitting material from the 760 barrel that held its port in place  then i chamfered the outside to fit the plastic breech of the 1377  then the inside was coned out like the original 1377 barrel so a bit of work done but not put back together yet for testing due to the need to find a way to make an adjustable piston.

Now to a few questions would it do me any good to make the hole on the flat top bigger or would that make it worse ?
On the port between the barrel and the pump tube theres an oring on the pump tube but is there one on the barrel side or should there be one ? Also how do you figure the lengeth of the piston so it stops right and the flat top?

[TheBullsEye]

Can some one post pictures for me i have it all finished

[bgmcgee]

My opinion is it would hurt performance to make the inlet hole on tbe valve bigger. The breech seal is only on the breech valve side but some use a tiny oring in the barrel side if there is a sealing issue. Adjust your pump arm so when it contacts the valve face you have between 1\2 ans 3\4" gap before the pump handle closes. It's easier to do with the orings off of the piston. Hope this helps.

[DavidS]

You do know that the 760 is smooth-bore, correct?

Though most of what you want to do is pretty strait forward.

though as for the piston:
turn down the raw meterial (aluminum is most common) to the correct size, cut a cavity for the pump cup, and angle the area where it comes out at about 5 degrees (puts more pressure on the pump cup), cut that end off, drill out the center and tap it for what ever all thread you have, then turn down some for the other end, cut the slot for the pump linkage,  and cut it down, again drill and tab for your all thread, make sure that the end peices are not to long, tread the allthread into the slotted end, weld into place, add a nut to the all thread (going to be the set nut), thread on the cup end, install the cup, adjust correctly, and lock it down with the lock nut.   Piece of cake.

[TheBullsEye]

surpriseingly my 760 had grooves in it. I double checked my self. David what did you all use to turn down your aluminum i have acess to a drill press grinding wheel dremels drills but not much more so id have to jerry rig some stuff and red neck it a bit. my trip to the hardware store was unsuccessful wht i had planned on adapting was too big and couldnt be turn down easy for use..

[DavidS]

surpriseingly my 760 had grooves in it. I double checked my self. David what did you all use to turn down your aluminum i have acess to a drill press grinding wheel dremels drills but not much more so id have to jerry rig some stuff and red neck it a bit. my trip to the hardware store was unsuccessful wht i had planned on adapting was too big and couldnt be turn down easy for use..

Hopefully some one else can help you with that part.   I have a lathe, so no issue.   Sorry, wish I could help.

[TheBullsEye]

Mabe i can modify the plastic one to work ik u can stuff and shim them mabe il just try doing tht

[DavidS]

Mabe i can modify the plastic one to work ik u can stuff and shim them mabe il just try doing tht

Yes you can stuff the pockets in the piston with pressure fit eight inch steel rods length wise.   Then it is a matter of shimming the pump cup a little with washers, and an O-Ring, getting exact sizes requires some work.    You would also want to shave the nose of the valve so it fits the pump cup better, thus wasting less air space.

[rangerfredbob]

In the late '90's the 760's were rifled, I have 2 that are, one the barrel is shot so it's pretty much just a parts gun...

[TheBullsEye]

Makes sense tht it wud be a 90s gun  it was my grandpas and i got it when he passed

[TheBullsEye]

Go a new stock on my 1377 looks good i think had to add an oring to the barrel side too due to looseing air now i just got make a few minor tweaks and hopefully get it chronographed sometime soon !

[stonykill]

 I like it. A true FrankenAG.