Nicely made, very easy to fit and once I had the adjustable piston set correctly it made a world of difference. I had the chance to chrony a couple of shots and 10 pumps now gives me 7.5ftlb combined with the bigger transfer port and 18" barrel. The light faded before I could test fully but 15 pumps seems to be about 9ftlb. It's helped make my 1377 into a nice carbine.
Just be sure to keep that piston O-ring lubed.

after installing an american made flattop piston, I just wanted to compare it to the gmac pitson, and we have a winner! the gmac piston boosted the power of my 1377 up to 186m/sec with h&n baracuda with 10 pumps!!! wich means combined with my longer barrel, it has 12.5 joules of power . 20 pumps gives 16.50 joules with the same pellet (218,9 meter/sec.)

the american piston kit gave 161 meter per second with 10 pumps and the same baracuda pellet.

So far so good! Along with some porting,12inch barrel, maxed out transfer port,the power hike is excellent. 25 pumps gives 10ftlbs, 40 gives over11, but for just ambling along 10 gives 6.6
So I've altered everything back to keep it legal (cough,cough) and a max of 8 pump keeps it under 6. Working well so far!!!