Remove the front cylinder cover if not already done, use a short length dowel of about the bore size or suitable piece of wood and  gently drive the piston back with a small hammer. Has it got an iron piston ring or an O ring ? If an O ring it may try to drag the ring between the piston and bore causing the whole thing to jamb as soon as it moves, this will require more care and very slow progress.  Noel.

Using the technique Noel suggests should work but you may need tap the piston back and forth a little at a time, not just in one direction, to free it enough to move in one direction.

Obviously don’t hit the piston-rod. Use a smaller dowel on that side, and acting on each diametral “side” of the piston alternately.

Replace both O-rings, as that on the free side is likely to be badly ageing too.

Sitting here mulling ! What is the piston made of, iron ? Is it the piston that has seized or is it the O ring that’s causing the problem ? Other thoughts, If there is sufficient bore clearance a thin strip of ALI drinks can (or shim brass) down beside the piston may get the O ring to move.  A fluid that will reduce the O ring to soup, Acetone or cellulose paint thinner ? With both cylinder covers off gentle violence should get it free, bronze has a high rate of expansion so heat will help, 2 – 300c drop the lump in a deep fat frier, or a pan of cooking oil on the stove – DO NOT use engine oil. There are small glaze breakers/ bore hones available quite cheaply that will clean up any small marks on the bore.Good luck Noel.

Gunmetal block and a gunmetal piston ? Noel.

Like that expression Noel “gentle violence”

Mark, I take it the rear cylinder is off, it’s not stuck on the packing gland ? Bronze piston and cylinder, what is holding it ? Just the O ring ?  Noel.

No-one has mentioned an ultrasonic bath. Is that inappropriate here? Borrowing the use may be possible or indeed they are handy and not that expensive these days.

Too late to help, but I wandered the web last night looking for chemicals that would dissolve an O-ring whilst not attacking metal.   Disaster!  O-rings are much tougher than I expected, and the chemicals already suggested cause them to swell, not disintegrate.   I fear the liquids recommended have made things worse!

Also seems that techniques that work well on metal are unlikely to work on an O-ring.   They resist chemicals, and them being elastic means heating and cooling won’t un-seize the joint.  They’re also remarkably tough – hard to break once fitted.

My thoughts:

1. Hammering – driving the piston out with a series of moderate sharp shocks.  Maybe adapt a impact driver.  Risks damaging the cylinder.
2. Pushing the piston out between the jaws of a big vice, hefty Arbour Press, or a Hydraulic Press. Risks damaging the cylinder.
3. Drill and bore out the piston until the O-ring groove is exposed from the inside.   Doesn’t risk damaging the cylinder, but a new piston has to be made.

Nasty!

Dave

There are numerous materials used in the making of O rings that resist various fluids,  silcone was favoured for piston ring as it stood the heat. Let us all be clear, is the problem that it is the O ring that is the reason the piston won’t move ? OR is it the piston it’s self that is seized ?

Does the OP have a lathe and could make a new piston if need be ? Remove the valve gear, valve rod, slide valve etc and heat the piston with a blow lamp. Have the cylinder well held in a fixture that will allow you to hit the piston with the dowel rod mentioned earlier, wrap the cylinder block in a damp/wet rag,eventually the O ring will be softened, or burn out. Once it starts to fail it will go in a rush. No idea where you are, in the UK ? I have a press and it should be an easy job. Noel.

Be  careful heating the cylinder to high heat if the O ring is of an unknown material. If it is a material that contains any fluorine compounds the fumes could/will be nasty, even fatal.

John