It is normal that the cylinder bore diameter will be largest about 8 mm below the deck height, measured at right angles to the crankshaft axis. The wear will be greatest in the "major thrust direction", with very little in the opposite direction, so nearly all of the wear will be on one side of the bore: it will not be worn symmetrically on both sides. This is because when the crankshaft is just say, 20 degrees past top dead center, the combustible gas in the cylinder has largely burned and the pressure in the combustion chamber is very high, while the angle of the connecting rod causes a large sideways force on the piston, against the cylinder wall. This high rubbing force causes wear on the cylinder bore in that position.
A brand new standard bore should be 2.677" diameter. You report that yours is much smaller than this, along the line of the crankshaft axis (the direction in which there is no thrust and consequently very little wear). It is not possible that this dimension is less than 2.677", so it appears that your measurement is inaccurate. Did you use an inside micrometer? If so, did you check it with a gauge or against a known-accuracy outside micrometer? Are you sure that the screw-on end-cap is not loose? If it were loose, all of your readings would be erratic, not only in absolute terms, but also relative to each other.
Let's posit for a moment that your micrometer has an incorrect zero, and what you have found is that the bore is standard (2.677") along the crankshaft axis, and 0.006" more than this (2.683") at right angles to the crankshaft axis, measured about 8 mm down the bore from the top deck of the cylinder casting. That would mean that you have .006" of bore wear at the most worn point. The bore diameter at right angles to the crankshaft axis usually tapers down from that maximum as you move down the bore, and is usually not measurably worn down at the bottom, below the lowest point the piston rings travel to. The service limit for bore diameter is 2.684", so your wear is only 0.001" less than the service limit. If this is the true situation, it would then be possible to re-ring the engine and put it back to work still barely within Honda service limits. However it would last considerably longer if you rebored it to a larger diameter.
There are two ways a cylinder can be "rebored". The way it is usually done in service centers is to perform a sort of glorified honing operation, with the hone guided by the existing cylinder bore, rather than following the original centerline. This operation is relatively cheap, since it uses only the rudimentary equipment that most service centers have. The bore would only need to be increased by one piston size (0.25 mm, or about 0.010"). On the other hand if a genuine rebore was performed, it would require using a very rigid boring head in a milling machine that was in excellent condition. If the setting up of the boring job was performed very accurately, it would be just possible to clean up the bore if the boring bar reached out at least 0.006" further radially than would be called for by the standard bore dimension. That is, the bore diameter would have to be increased by twice 0.006", or 0.012". This is not a standard rebore dimension, so you would have to go up to 0.5 mm, or 0.020", oversize, so that you could use ordinary oversize piston and ring set. Oversize pistons and rings are available up to 1.0 mm, so this does not seem to present any particular difficulty.
The questions to consider are:
- how important is it to have the engine last a long time? Usually in home service, with regular scheduled oil changes, cylinder wear happens rather slowly. Honda engines only become as worn as yours now is, either through long service as contractors' mowers, or in home service if the oil has not been changed in a long time.
- if you want the engine to last a long time, do you actually need it to last as long again as it has lasted up to now? If you would be satisfied with, say, 60% of the time it has lasted up to now, you could adopt the glorified honing approach rather than a true rebore. This might be quite a bit cheaper than having a competent machinist set up the job accurately in a milling machine.
- if you need this machine to last like a new one, you need at least a true rebore - but it might be cheaper to get a new short block instead.
