760 piston in a 13XX?

[Tom Tucker]

Will this fit in a 1322 with a flat-top valve?

[mav72]

On paper it should. Just try it.

[rangerfredbob]

yes, the 760 and 13XX are the same in piston and valve aside from the front grip screw in the 13XX valve, same length pump linkage as well. That piston would work in a Powermaster 66/Torrent/Recruit as well as they have the same valve and piston as a 760/13XX but a longer stroke via the pump linkage

In short, it's adjustable, so yeah it'll work...

[BillK]

Note:  There is a difference in the 760 valve and the 1300xx valve.  Pistons are the same but, valves are different.  Don't remember the difference,  Will look it up.

[mav72]

They are practically the same. The 13XX just has the grip hole drilled in it though.

[MI Yeti]

They are practically the same. The 13XX just has the grip hole drilled in it though.

The 760 and 13xx valves have more differences than just the threaded portion of the 13xx valve for the front grip frame screw. They use very different transfer ports and the front of the valve's cone is shaped differently on the two valves. My 13xx valve is a bit shorter than my 760 valve. The internals appear to be the same though.

I have attached a photo of both a 13xx valve and a 760 valve side by side. the 13xx valve is on the left, 760 valve on the right.  they both have the exhaust porting opened up but are other wise stock.

[cherokeesteve]

That is a noticeable difference in the shape of the valve tips. I just checked on Crosman’s site and the pump cup listed for both the 760 and 1377 are the same part number. I wonder if you put the valve half from a 1377 on to the port half of a 760 if it would improve performance or pump efficiency at all and if the overall valve length of each is the same?

[Tom Tucker]

Well, I went ahead and ordered one.  I'm not too impressed with my delrin piston.  The adjustability is great to have, but it's not that efficient.  I'm not sure if it's the wide o-ring groove or the plastic itself deforming a bit, but it doesn't have the "top end" of my non-adjustable brass piston [with a fresh pump arm].  The first 7 pumps are very close, but the brass piston will do 530fps at 10 pumps, while the delrin piston only gets me about 495.  More tinkering and testing to follow 

[Tom Tucker]

Apparently, I accidentally edited my last post instead of replying to it.  Basically, I stuffed teflon tape under the o-ring of my delrin piston and FPS was up to about 545-549 for the first several shots and then the tape must have migrated and the FPS dropped back down to around 495fps.  Delrin itself is an OK piston material capable of building very good pressure below 15 pumps, but this one that I got off ebay is not a good example of one - I went ahead a gave the old-school steel/aluminum/brass 760 piston (right) a good polish and swapped it in.  Seems to be pumping a lot smoother now that it's been polished.

It's hard to see in the picture but the o-ring in the delrin piston on the left is not held tightly in its groove.  I was hoping that when it was installed in the tube, it would compress down and spread out, but it just doesn't.  The groove is too wide, but not wide enough for me to really shim it with anything.  It's also not a radius-ed groove like the one in the valve, or in some of the better FTP's.  The o-ring compresses towards the muzzle end and air fills the groove.   The groove itself is especially huge because it uses a 1/8" o-ring instead of the more common 3/32" so there is a lot of room for compressed air to get trapped.  I've tried different ways of wrapping the inside of the groove with teflon tape, but it always gave just a very brief improvement.

The 760 piston ended up winning out for the ebay bargain FTP of choice with about 510fps at 10 pumps, built-in oil ring, and a beefy seal that should outlive most o-ring seals.