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I think your problem with the tubes not sealing was because you expanded the tube into the tube plate thus preventing any solder penetration of the joint have a read of Alex farmers book he recommends a rough rat tail file to give a bit of clearance in the joint , which is vital to get a good sealThree roller tube expanders are normally used in steel boilers with copper tubes no other sealing was done
The hard fact is no Even for a small boiler the heat required is considerable at least a 25mm /1 inch burner head would get it done , don’t forget it has to be hot and even , tube plate and foundation rings are the worst because you cannot easily inspect your work on the inside , so it needs to be heated though the job to get a good penetration of the jointStuart