As RK notes, "Just about any clubmaker will tell you to keep the shaft bore clear of any or all epoxy." I am curious from this discussion thread on building LD clubs what effect the epoxy getting into the shaft bore has on shafts breaking? I am not building clubs for any big hitters, and mostly work on things for myself and a few friends. I don't use excessive epoxy, but I hadn't really considered that getting some inside the shaft bore would somehow weaken the shaft.
I don't know whether you're looking for an explanation or just confirmation of the effect. But it certainly does exist, and not just for big hitters. But just getting a film of epoxy on the inner bore is not a problem. It becomes a problem when the bore fills with a "plug" of epoxy to a certain height, or perhaps even let a THICK coat form on the inner wall -- but the coat has to be a significant fraction the shaft thickness itself to be a problem. Here's the explanation:
Anyplace there is a sudden change in the cross-section of the shaft, that is a candidate for "stress concentration". That is, all the stress trying to bend and twist the shaft has to rearrange itself as it makes the transition from one side of the change to the other. In rearranging itself, the stress will tend to concentrate -- that is, build up to large levels -- in the "corners" of the change in cross-section.
If you let the shaft bore fill with epoxy, you are creating a possible stress concentration at the top of the epoxy. That's because the epoxy plug becomes a structural part of the shaft, creating a discontinuity at the top of the plug. As long as this top is well inside the hosel, the shaft is supported and will not break. But if the top of the plug gets near or above the top of the hosel, the resulting stress concentration could be enough to break the shaft.
Hope this answers your question! DaveT
