Hi chap's in 1993 I bought a 77k in .177 with a 25mm piston . Back then most people were fitting OX kits including me Anyway a year later I sold it to a mate and he has had it since then . The other week I managed to buy it back after 19 years and I stripped it . The ox was still in there so degreased etc and plan to fit a standard spring when I get 1 . But all them years ago I cant remember if i opened up the transfer port when I fitted the OX kit , just wondering if anyone knows the standard size so I can check it ATB Mike