ffkiwi

[QUOTE=beatsal;12726397]Thanks. Just to clarify:
Enlarge the prop hole by drilling with a bit one size up till it eventually fits?

YES. The important thing is to keep the hole square in both axes while drilling, you do not want to introduce any out of squareness

A toothbrush may not fit inside the cylinder for cleaning.

I never implied it would-you use it for general scrubbing off dirt, grime etc from various surfaces-and you will be able to get it inside the crankcase via the backplate opening. You might recall that I have not suggested that you completely strip the engine-the purpose was the exercise is to get it unseized and in a state where it will run, not strip and clean every part...

Is the a connection between piston and contra-piston?

There is no physical connection between the piston and contra piston. The contra piston is a tightly fitting plug the closes the top of the cylinder and has a small range of vertical movement within the upper cylinder region. It does not move up and down of its own accord while the engine is operating. It may be moved by the operator turning the compression screw during the starting and adjusting for smooth running process. The inside face-that is the one that faces or is 'contra' to the piston forms the combustion chamber-it may be flat, have a concave shallow cone profile or a shallow concave bowl shape depending on the brand of engine-in your case the Hunter has a flat contrapiston face. There is very little clearance between the contra piston face and the piston crown at running settings-a matter of perhaps 0.010 inches or 0.25mm-certainly less clearance than you would find in a glowplug engine of similar size. This is why the risk of hydraulic lock is always present if you flood the engine-and you will need to recognise it when/if it occurs, lest you damage the engine.

ChrisM
'ffkiwi'