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.

[electric sheep]

Size it with 400/600grit paper.

[I. J.]

Ive got it as 1446049 pre this serial number is small head size.
Give B.A.R. a ring and ask politely to confirm this number.

HTH
Ian

[Herx77]

You can size the HW piston seal by using 600grade wet &dry assuming you have the right size seal, cutting back the face to suit. I've thought the weakness of the Weihrauchs was the piston seal, probably why they have such a long spring as standard.The standard seal has a gap behind it which allows angular movement which can give a juddering action down the compression tube; this is not a smooth flowing progressive action. Due to this and the initial difficulty of accelerating the piston without it running away I now fit a hybrid piston seal made by steve pope, the constant fit part allows rapid accelleration and the parachute part controls the rate, overall a major improvement on the standard unit. Herx77