Friend Oyvind:
You only bump up the ends of a lap...........no matter whether one is physically bumping up the lap, like me, or, like you, tightening a screw to do it...
The major lapping of the bore must be done with long, non-deteriorated laps.....
But, if you use one of these long laps too long, you will start to round off the tops of the lands....
Therefore, at some point in the lapping process, one must reduce the length of the lap, to about 1-1/4" in length, then start the bumping-up process......
The bumping up process, is actually a "finishing" process, to make sure the lands maintain their shape......
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Lapping is an art......................
I don't see how any human being, could possible do it day in and day out, and keep up the pristine quality, without going nuts....
This is why all RFBR schmidts, who half way expect to turn out consistent winners, must be perfectionists in the art of lapping...
Your friend, Bill Calfee