Back off the compression screw a full turn, now try and turn the engine over. Hopefully it will move back upwards. the contra functions a bit like the plunger seal in a syringe-it is not free moving of its own accord-it does not reciprocate when the engine is running. Its position in the cylinder is determined solely by the extent to which the compression screw is screwed in-the end of the screw stops it moving any further upwards-if you unscrew the screw a bit-the contra can move up inside the cylinder until its upwards movement is halted by the screw. If turn screw the compression screw in, the contra is forced lower in the cylinder. That's it-its surely not a difficult concept to grasp is it? If you screw the compression screw down-you can go too far and push the contra far enough DOWN inside the cylinder that the ascending piston will hit it as it rises towards the top of its stroke-and then it is impossible to turn the engine over TDC.

Moving the contra back up the cylinder can be achieved either by using compression to do the job-which is what happens in normal usage, or-in the case of a tight unit, by removing the cylinder setting it on a fiirm surface upside down and tapping the contra piston towards the top of the cylinder using a piece of down or a brass or alloy drift. In your case try the heating trick first-prop off, put it back in the oven again, with the compression screw backed off at least one full turn, let the engine get hot, , take it out, prop back on and see if you can turn it over top dead centre. Hopefull the contra will now move up in the bore till its movement is halted by the end of the compression screw. If the heating process does not achieve this, then you have no option but to take the cylinder off-by undoing the four screws at its base-and then proceed as I've suggested above. I hope I don't need to tell you to remove the compression screw before attempting to knock the contra piston up the bore? And only do one or two taps to move it. Then put everything back together and see if you can turn it over now. If you do have to go down the cylinder removal route, take the opportunity to clear out any gummed up oil from the underside of the contra piston with a cotton bud or tissue wad.

from your description it sounds as though ignorant fingers have played with the compression screw and wound it down so the contra is currently too far down inside the cylinder.

ChrisM
'ffkiwi'