I'm certainly no boiler engineer, but I can share a few images that may help here.
First off, the images in that PDF file properly illustrate the various methods of securing flues into a flue sheet, including modern locomotives like the 765.
This is a flue sheet. This is the rear flue sheet in NKP 765. This shot has been taken while standing in the firebox. The boiler itself is just beyond that steel sheet with all the holes in it. In this image, the sheet has been secured with several bolts along the top. Those bolts will not remain there! They are just holding the sheet in place for the riveters. This sheet will be riveted into the throat sheet all around the circumference of the sheet.
The rear flue sheet is now riveted in place and NKP 765 crewman Mark St. Aubin is carefully placing new flues into the rear flue sheet. The flues have been inserted into the boiler from the front, through the front flue sheet and on towards the rear of the boiler. Mark is standing in the boiler, and we are looking towards the rear of the locomotive. The firebox is just beyond that sheet with all the holes in it. Just above Mark's head you can see all the radial staybolts that support the firebox.
NKP 765 crewman Mike Hall is checking the placement of newly installed flues in the rear flue sheet in the 765. Mike is standing in the firebox. The water space of the boiler is on the other side of that sheet. Note that the flues are just laying in the tubes and are not yet secured in any way. You can also see a slight gap exists between each tube and the hole in flue sheet. A thin copper ferrule will take up that space. Mike is making sure that the tube is positioned properly so the exact right length of tube is sticking out beyond the sheet. This is necessary to prepare them for the next step in the process.
Almost done! This image was taken from inside the firebox. The image shows the flues in the 765's rear flue sheet have all been rolled into the sheet. Note how the circumference of each tube has been rolled over and back against the flue sheet itself. These tubes will now be seal welded around the circumference of each tube to finish the job.
All these images and many more, plus a full explanation of the 765's overhaul, is in my 192-page book about the 765. Order it here.
All profits generated from sales of this book go to the Fort Wayne Railroad Historical Society, owners of the 765.
