Crosman 1377 FT piston and valve questions

[mata777]

Just finished installing a non adjustable ft piston and valve (aluminum) from a well known supplier. I was hoping for a realistic 580 fps with 7.9gr pellets/10 pumps/stock barrel but it’s doing an average of 505 according to my Caldwell chrono (spread is tight). Before the work it was doing an average of 491. I deburred all the slots/holes and lightly lubed the piston/valve orings. It’s my first pumper that I’ve ever worked on, I’m not sure if I did something wrong or if these things have a break in period. Bolt oring looks like new and the transfer port gasket looks new as well (I measured the transfer port and it’s .140 which is in spec if I’m correct). Any help would be well appreciated.

[Tack Driver 10]

A healthy 1377 should produce about 550 FPS with 7.9 CPHPs.
With a FT setup add about 100 FPS give or take.

I cobbled this 1377 based gun together from spare parts and it gets 725 FPS with 10 pumps with CPHPs.
Has an adjustable cup piston, valve mods, 18" Crosman barrel, DAQ LH shorty breech, trail baffle suppressor.
Pretty accurate and very quiet too.

[tkerrigan]

Are you sure the piston assy is non adjustable? I've never seen one.  You should feel the handle cam over from about 3/4" from closing to get a full capacity pump by adjusting the piston stem length.  Try pumping an extra 5 pumps over what you do now and see if that increases the speed.  Regards, Tom

[CraigH]

There are one or two non-adjustable FTP's available, I don't recall who makes them.    Don't see the point of a non-adjustable version though.   

[Tack Driver 10]

Are you sure the piston assy is non adjustable? I've never seen one.  You should feel the handle cam over from about 3/4" from closing to get a full capacity pump by adjusting the piston stem length.  Try pumping an extra 5 pumps over what you do now and see if that increases the speed.  Regards, Tom

This one is from Alchemy AirWerks. There are several others just search Ebay for Crosman piston.
https://www.ebay.com/itm/ALCHEMY-AIRWERKS-FLAT-TOP-HP-VALVE-BRASS-FLAT-TOP-PISTON-for-Crosman-1322-etc/302092469771?hash=item46561d460b:g:DzgAAOSwzaJX7ztb
They look like this:

[tkerrigan]

I think I would be making an adjustable rod for that piston.  I have a 1322 with long steel breech, 24" barrel, flat piston and valve, disco hammer spring, usual things done in valve to make more volume that does 735fps with 14.3gr premiers at 25 pumps.  By the way, it does 566fps at 10 pumps