Fitting a PTFE washer, Please read carefully.

If the piston washer has a screw or nut fixing do not over tighten the retaining nut or screw, instead Locktite or similar thread locking product should be used. Aim to get the screw or nut slightly recessed into the face of the washer. This will allow for bedding in.
The washers are normally made a couple of thou under size and a perfect air seal is achieved after a short "running in" period (usually within 500 shots) when the washer has sized to the bore of the cylinder.

This method is preferable to dry firing which can cause over expansion and frictional problems.

Never dry fire a spring air gun.

If on rare occasions the washer is a tight fit, you can benefit by wrapping 400 wet and dry around the washer, and with a back and forth motion while rotating, take a little PTFE off until a smooth fit is obtained.
For extra swept volume, PTFE washers are in some cases thinner than the standard washers to obtain more swept volume. And on some models protruding studs or screws may need to be shortened accordingly.
With the new washer in place, the gun should be inspected to make sure a small clearance remains between the rear of cocking arm and the rear of the piston slot. This is rarely a problem.
Degrease the cylinder completely before assembly. Follow the lube instructions sheet which is supplied when any of the Robb lubes are purchased, or see the Airgun Tuning Manual.
Fibre backing washers, if fitted with the original washer, should be used if the piston is a poor fit in the cylinder.
Finally, PTFE washers require one drop of MS 40 Silicon oil smeared around the piston head before assembly. But do not reapply until the next overhaul.

The HW 35 used to be fitted with a leather washer which was held on by a large nut. The Later HW35s are fitted with a polypropylene washer, the same as the HW 80 and 90. This type just clips over a lip, after peeling the old one off.

The Original Diana model 5 and 6 pistols have a dovetail type fitting to hold the washer on which makes it harder to fit a PTFE one, but I have fitted lots of them by stretching them over after forcing them into the lower edge of the dovetail. Rounding the front sharp edge helps. Then push the washer back into the groove and roll to get it to fit in the cylinder. The washer will size itself to the bore when the gun is fired a few times.
The PTFE washer will last a lot longer than the standard ones that turn to soft cheese or break up to dust.

The last photo shows our PTFE washer on the left and the standard one on the right. Our design also reduces drag as well as the low friction material of PTFE.
Made to fit 99% of all airguns.
The second photo shows our washers with the steel insert. The third washer shows a Very Old BSA Meteor washer that replaces the old Leather type washer.
Always check the power to make sure the gun does not exceed your limit, after changing any parts.

The Original / Diana 45s and To1 50s have a rivet holding the washer on so this has to be removed and reused, but I prefer to make a screw and tap the piston 1BA so it is screwed on like the normal screw on type. I can convert yours into screw on type if required. The very early type had the piston rod come through the centre and then riveted on. But I can also modify those types if required. Guide price for the conversion is £25.00 plus the washer.
1 ba Screw for Original 45 variants to convert to screw after tapping 1ba has been added to the otpions. See last photo for Original 45 Converted to PTFE with screw.

BSA Cadet and Cadet Major Special tool has now been added to the option list. It costs £13.78