Tony
The important thing to bear in mind is thermal mass. For work where the heat from the iron is going to be dissipated rapidly (as in a motor casing) then the bit needs to be sufficiently “chunky” to provide an adequate reservoir. If there is insufficient heat to melt the solder (or it takes a very long time) then the inside of the motor will get a roasting. For a chunky bit, a higher wattage is required, as advocated by Stavros, Peter and several others.
When dealing with electronic components, a thermostatted iron is best and a smaller bit required to avoid damaging other components in the vicinity.
Another important factor is the coating of the bit. In the olden days (I am very old!), bits were of plain copper and the solder etched this away surprisingly quickly. The picture below shows my 120watt un-thermostatted Weller which had a shiny plating when new. It gets very hot and after relatively little use the tip is being eaten into. (But it does solder capacitors onto a motor casing in about two seconds flat!) On the other hand, the lower iron in the picture is my 25watt thermostatted iron, which has an iron coated tip, shows no sign of pitting after at least 25 years of use.
Hope this helps.
Mike
