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Is it true that the shaft gets stiffer when you put it in a bore thru head. I.E. if I put a regular flex aldila on my 905 it will play more like a stiff shaft.

Normally you tip trim it differently when it's used in a bore through to prevent this from happening.

Your close...... it depends on your swing as to whether it goes from an R to an S, could be closer to firm. As I'm sure you know, an R with one company is another company's S or A. There are no standards in regards to shaft flex. The $4 OEM shafts that the big names put in there drivers tend to be on the softer side, so it can cover a larger customer base (no custom fitting). Upgrading to a true Aldila (NV I assume) is a good move, get away from the stock shafts and have in done right, not just "cut and glued.

Does it feel stiffer, yes it does. Reason is because the Bottom of Bore to Ground Measurement (BBGM) drops considerably (almost nothing). The closer the tip gets to the ground, the stiffer the shaft feels, and vice versa.

Hope that helps.....