Not sure on the Tayana, but on my Tashiba the hawsepipes looked like they were not through bolted. However, on very close examination one could see that they where and the heads probably spot welded and then ground flush and polished. It was necessary to drill them out for removal. Once they were out it was obvious. One other suggestion. When I got them all cleaned up I filled in voids (mine were wood blocks between outer and iner hull) with
epoxy and used plastic straws as guides for the new through bolts. After the
epoxy set, the straws could be removed, worked extreemly well. I then rebedded with new through bolts which were put in the same countersink holes in the pipe and cap.