if you,ve got an HW 97/77,thats dieseling this is the way to stop it,get your steel shim make sure you cut it 5mm longer than it needs to be,so you can cut little v,s out of it like a crown, then it will when these v,s are bent inwards form a hole enough for the piston cocking rod ,to pass through, then should form a lip for the spring to keep in place until a few shots seats it into its housing, this shim /stops any unwanted moly getting back into the compression tube on firing, and keeps it where it should be,use this info as part of your tune,as just changing the factory lubes in the HW range of guns makes a big difference, even a simple wipe off the old/ wipe on the new,,molybdenum grease/lithium grease from a car auto spare shop very cheap////and good silicone oil (bisley) for poly seals(plastic type seals)knock your self out!!!