Originally Posted by
20grit
Assuming the fit is not too tight, riding in the rain shouldn't be a problem. If the fit is tight and you were to allow the plug to be soaked, it could expand and potentially do some damage. Unlikely, but it could happen.
I would actually have issue with totally plugging the bottom of a steerer tube with something like a cork. That's a bit like a story told on here once about a sponge being found in a seat tube that had rusted completely through. You're blocking off water from entering going up. You're also blocking water from exiting in it's natural, gravity encouraged direction of down.
Yes, the plug does a very nice job at holding in any rainwater that trickles in around the stem, eventually causing the expander wedge to freeze up. I just had to drill the plug out of an old PX-10 to free up the bolt so I could get the stem out. If there's a practical advantage to the plug, I can't imagine what it is. The short one in my stem would have done nothing to hold a broken steerer together.