Smoothing a bore.

The success achievable with abrasive finishing, and the amount of resultant diametrical enlargement, will depend on the present state of the bore. How much material removal can be tolerated (how deep are the present scratches/imperfections)?

A possible problem with abrasive finishing is the risk of embedding abrasive into the soft brass. Consider a non-embedding abrasive for lapping?

You could consider 'Ballizing' (I think that was the name of the commercial process), in which a highly finished ball, very slightly larger dia than the existing bore, is pushed along the bore. In brass, it should work-harden the surface usefully. It will retain, or improve, circularity, but will not straighten a 'bent' bore. Parker Hale offered this process for gun barrels. Compacting the working surface of the barrel was said to prolong barrel life. Probably impossible to get a ball of the desired size - for a reasonable price.

A quick update.

I've had a go with Ramon's mandrel and wet & dry method and it works very well.

As expected, as the liners are a tight friction/force fit in the block, the bores had "shrunk" slightly when fitted. This was soon fixed.

I started with 800 grit and worked up to 2000. I did it on the watchmakers lathe under power at about 200-300 rpm. It does remove metal quite quickly like this (the paper loaded up in no time) but it is very controllable. I was able to get all 8 bores to have the same "feel" with the plug gauge.

I'm half way through making the pistons now.

Thank you to Ramon and everybody else for the help and suggestions.

Steve