Just did mine
Just replaced my riser as it was clogged at the sea water injection site. The one removed had been there for 14 years and was galvanized pipe. All were in good shape except the last section, but spent $15 to replace all the pipe. Used galvanized pipe (as I removed), so will post in the future if there are any problems. I considered brass pipe, but was told that in sea water it will "dezincify" and weaken, so stuck to that which lasted 14 years. Hope it and I last that long in the future. The flange was fastened with stainless bolts and came out of the warm engine easily. The gasket was cemented in with what appeared to be a red silicone material and came right off too. All sections of the pipes were screwed together and welded. I am told that heating and contraction can cause the pipes to unscrew. They are at the welders now for the same treatment. Pipes were well wrapped with insulating material and wrapped with stainless wire. Looks like the boatyard (or whoever) did it right. Cost down here for pipe and welding will be under $25. My labor is free (and it was an easy job, once I figured out the problem...I started with the impeller, then the thermostat, the hoses, etc. I expected the riser to be an awful job, it wasn't)