If it hasn't been out since we built the boat (at WFBCo) thirty(?) years ago it will not give up without a fight. Expect to have to hammer all manner of long thin things - chisels, screwdrivers(!) - between the collar and the middle pipe to free it, or if you're lucky unbolt the collar and the collar lifts off with the middle pipe attached. Worst case is having to take an angle grinder with a cutting disk to cut the middle pipe in half. Messy and unpleasant. I think high temperature silicone wasn't invented back then, or we hadn't come across it.