Project Gamo Expo-26

Well this is my $110 "bargain" recently acquired via Trademe.  Being the age it is I was kind of hoping it might be a better quality instrument than the current batch, but oh no, it's just as junky as the rest!  This thing may shoot OK, I didn't bother trying the iron sights with my rubbish eyesight.  I pushed on and stripped it down.

The moulded plastic breech block and barrel hinge were bad enough, but the plastic spring guide inside definitely confirmed its pedigree.    However, it's a project so lets see what we can do with it.  There's virtually no chance that cylinder mounted scope is going to work with the flexi-breech and I'm not that keen to make a new one out of steel, so a red-dot on the rear sight mount it will be.

The piston is similar to the Air Chief but a bit better made.  It could do with a new seal - anyone got one lying around they'd like to move on?

The wooden stock is nice (marked but still nicer than the AC plastic job) and the trigger mechanism is a  bit more sophisticated than I'm used to.  There is some rust spots on the top of the cylinder but won't affect its performance.

The spring is toast so I have a new one coming from my mates at AliExpress.  I'll have to shorten it a bit, but that's part of the challenge.  I've made up a new spring guide from some junk steel and a piece of stainless tube I had lying around, along with a locating washer for the seal end.  The guide is quite a bit longer and will be a hell of a lot stronger than the pathetic plastic swizzle stick the factory put in there

That part (breech receiver?) is moulded plastic (polymer) with an O-ring to seal the cylinder and secured with two steel pins.  The barrel is also encased in plastic which incorporates the breech, although the breech seal is in the receiver portion and the steel barrel end closes against it.  Horrid setup!

I need a new seal - any suggestions?