You've seen this picture before but its here again whilst the actual fault is described. Firstly, remember this boiler is forty years old and has worked hard so it could be considered to be well into its old age. We also do things a bit differently these days.

The actual fault was that the boiler was it leaking past some of the rivets that went through all three plates. When they were drilled and drifted out it was found that the holes in all three plates didn't line up completely. Nowadays we would have bolted the bits together and then drilled and reamed the holes out thus ensuring that they were all the same size and lined up.

This boiler was built in the dying days of industrial steam and its looks as though they expected the rivets to deform and fill the voids caused by the offsets in the plates. If you look closely at the removed rivets you can see the effect of them deforming in the steps in the shank of the rivet.

Why now? This is only conjecture but for the first 30 years of the boilers life the feed water was treated with chemicals that tended to coat the inside of the boilers and would have sealed up the very small gaps were talking about here. For the last ten years or so we have been using a different set of chemicals which have a very searching and cleaning nature. They have previously revealed leaks in the more accessible places as they've cleaned them out and it could be that they have finally penetrated this far into the construction

The fix has been to remove the old rivets as can be seen in progress here.

The holes were then drilled through so as to be the same size and concentric though each of the layers.

The dome was then completely removed so that all the joint faces could be cleaned up so they would mate properly.

The flanges you can see are:-

Lower - where the regulator operating rod enters the boiler through a gland

Upper - the connection for the manifold shut off valve

The actual re-fixing is done with a fancy bolt described as a carrot bolt. I haven't managed to find a photo of one before it was fitted to a description will have to do, and its a 2nd hand one at that. The bolt has a tapered section and is fitted with a copper washer. This is fed into the hole from the inside and then tightened. The idea is that the copper washer is extruded into the hole to create a seal. All you see is the nut on the outside.

Apparently the Mk1 didn't work as advertised because of the three layers of material and the washer only sealing the first two.

The Mk2 saw all the surfaces coated with an industrial sealant before being mated. This had to be `cooked' so the boiler was actually lit up whilst the dome tops were still off to bring it up to temperature without raising steam.