Marvel oil has a good reputation as a penetrating oil-I'd be inclined to take the cylinder off the engine, remove the head and stand the cylinder upside down with some marvel oil inside-and leave it for a bit-a day or so. Then-after draining the oil out again-repeat the 'tap the contra piston with a drift' process with the cylinder upside down on a hard surface....a steel block or something similar-or the flat portion of an engineers vice, if you have one big enough. A wooden surface will probably have too much give, and absorb the hammer blows rather than the force transferring to the contra piston. Again use a solid piece of rod-ideally brass, or at a pinch alloy to deliver the blows from a hammer-imagine you're using a nail punch to push the head of a nail below the surface of a piece of wooden joinery, prior to filling the hole with putty-except in this case the 'punch' is a suitable length of metal rod that's a loose fit inside the cylinder-and you;re trying to push the contra piston further 'down' from it's existing position-which since the cylinder is upside down, will be 'up' when you have it all back together. Refer to my hand sketch above-the central top of the cylinder projects about 1/16" higher than the wider flange with the screw holes in it-so when upside down, the cylinder is resting on this central raised projection-so if you can get the contra piston to be flush with this raised rim (the top edges of the contra flush with the cylinder top rim, when viewed from the top outside of the cylinder, you should be very close to the running setting. As soon as you remove the head you be able to see how far down inside the cylinder the contra is currently located-(see my sketch again-the contra is drawn in red)-I'd be surprised if it was much more than about 1/16" down from the top, as the Hunter compression screw isn't that long, and even if screwed down as far as it can physically go, it cannot have forced the contra down any further than the extent of the screw travel-which of course is still way further than the engine can be turned over, as there comes a point where the piston will hit the contra as the former approaches top dead centre.

ChrisM
'ffkiwi'