Jens,


It seems you have done your homework very well.

The TT GP .07 engine is ABN, and I am not sure if it is lapped, since honing, or sanding by hand, i.e. not by a machine, can also produce lines across the normal movement of the pistons.


MVVS ABC engines, before they went fully to CNC, used to match pistons and sleeves and to do the final sizing of the sleeve, with nothing more than a finger and some fine emery/carborundum paper/cloth.

Some of these very tight, older sets are now sought after...

This TT 07 engine, however' is too small to use a finger...


As to barrel grinding the piston; some manufacturers replace this with a final process that removes a 'phase', a few microns from the top 1.5-2 mm of the piston, so its contact area with the sleeve is below the crown.

I have seen this on the OS FX.


The MVVS .49 is clearly barrel-ground. The shiny contact area is 1.5-2 mm below the piston's crown and there is no 'phase' machined into the piston's top.
This attests to the barrel-profile, although I have not a micrometer to show you the actual measurements.