hw77 piston washer,1mm difference ?

[Chandlers]

recently gave me trusty old hw77k an overhaul,looked on chambers website & noticed early 77's had 25mm piston seals & later ones 26mm. tried to order relevant spares but couldn't as main man's out of action so got my bits from another well known gunsmith who when questioned by yours truly about early & late hw77s piston seal size differences told me they were all the same so i took his word and purchased the gear.however when comparing to old washer it did look 1mm bigger and seemed to be tighter in comp tube will this make a difference to performance? or am i turning into an airgun nerd after all whats a 1mm between friends?

[Prodigal]

I was given the same info, 77's are 25mm and the later 97's are 26mm. Why the 77 wasn't "upgraded" for a bit more volume I don't know Maybe it's to do with weight, 97 is a little heavier.
Theory is the piston should slide down the compression tube under it's own weight, though I've never achieved it!

EDIT: If anyone's got any info how to "size" a seal to better than standard I'd be interested to find out!!

[Murphy]

NERDS

[Prodigal]

CHEEKY!

[Chandlers]

murphy,how soon is soon?

[ralphfcooke]

There ARE two different sized pistons/washers for the HW77. The early ones are 25mm and the later 26. If your gun is designed for 25mm then you won't fit the later piston washer without a LOT of effort, conversely, if you try fitting the early washer to a later gun it will fall straight down inside without touching the sides.
I suspect you have the correct washer, but, being new, it's a bit of a tight fit; this will improve over the first few tins of pellets.