There's a nice article in the current (May) Trains magazine on the 1472 day check for a Grand Canyon Railroad Mikado. It appears to cover much of the same steps documented here, just a bit quicker (they have a paid staff). The article may give folks some insights into what is yet to come for 1630.
I suspect that Nigel could write a similar article when 1630 is steaming again.
I have followed with interest this series on patching the firebox for #1630. I'm sure others have too.
One question: Since the patch will be riveted, what is the purpose of tapping the screw threads in the patch?
Actually the thing being threaded is a patch screw. Once fitted this looks like a rivet from the outside but is blind (does not go thru to the other side).
There are a few of these on the corner. The curve means that rivets tightly spaced in the inner sheet would be quite widely spaced on the outside. So the patch screws fit between the rivets and attach the outer sheet to the mud ring between the rivets.
One of these was damaged and is being replaced.
I have trouble imagining why there is so much variation in tube length. Are the tube sheets that badly bent? I have never been up close with one, but I would think that tube sheets are pretty thick hunks of steel. Why aren't they parallel to each other?
I certainly would not consider them badly bent.
Bear in mind that the tube length, and therefore the spacing between tube sheets, is 17 feet. The boiler diameter is around 5 feet. The tube length needs to be accurate to around 1/16th inch when we roll and bead the ends.
The variation between the longest and shortest looks as if it will be a little over 1/2 inch.
All sorts of factors are involved. The tube sheets are not perfectly square to the barrel (hardly a surprise. We are only talking a 1/8 to 1/4 variant from perfectly square on a 5 foot sheet). The original forming process followed by years of usage causes a certain amount of distortion of the sheets. Again you are talking fractions of an inch across 5 foot sheets of metal subject to high temperature and pressure over many years.
So overall it is not surprising or that the sheets are not perfectly true and equidistant across their whole area.