I have found that to operate smoothly and at legal power levels th HW35 does not like much preload. I have just repaired yet another one and fitted an OX no 4 spring supplied by the owner. I cut off 3 coils, heated and flattened the cut end and reground the spring (always do this as the spring will be distorting under load and can break off a coil). The rifle then was smooth and quiet but only putting 16 grn pellets out at 500 fps which is a little low. I put 3 standard thickness washers (did not measure exactly at the time) into the piston and this upped it to 570 fps. The trigger block was easy to screw back. Nice firing cycle with no twang. I used BOSCH gear grease on the spring (very sticky and does not throw off, used on Bosch angle grinder gearboxes).
Baz
BE AN INDEPENDENT THINKER, DON'T FOLLOW THE CROWD