To clarify on Gen tube fitting to a BP SD, firstly the case needs counter boring, not countersinking. There is a differnce, the counterbore is 4mm x 0.7 mm deep and is flat bottomed, hence the term " counterbored" others may call this "spot faced "rather than counterbored. Either way it is not countersunk.
As we are aware the SD has a triple lock crown fitted, this means there are three seal areas on the crown. they are as follows 1) the O-ring inside the crown itself fitted around the clutch stem which seals to the top of the case tube when screwed down. 2) the two O-rings within the case tube itself, which seal on the clutch stem and the 3rd being the O-ring just below the external threading on the case tube.
Now here is where the confusion arises, as there is a fourth seal, which is the one we're discussing above, the one directly between the case tube and case. This is classed as a case part and not part of the "triple lock" crown system as such
The depth and diameter of the counterbore is actually quite critical, if it's not deep enough you will crush the O-ring and split it rendering it ineffective, if it's too deep it won't compress the O-Ring and effect the seal.
The counterbored hole in the case at 4mm dia keeps the seal in place laterally and as the case tube screws in and compresses the seal it does it two dimensionally both width ways and depth way, so as we tighten the case tube into the case the O-ring expands in diameter sealing against the outer wall of the counterbore and also compresses to form a seal between the bottom of the case tube and the bottom of the counterbore
And for this reason a countersunk hole is not effective
Hope this helps
