I received an answer from Thor this morning. Here's the how to -
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I have attached a scan of an old leaflet which tells how to replace the faces on the copper/rawhide hammer which is the next size up from the one you have, but the principles are the same.
The text from the leaflet is below:
"You ask the best method of removing and replacing copper faces. This is not easy but may I advise using as an example a No. 2 Copper Hammer (04-312) with face diameter of 1 ½".
During the manufacturing process faces are placed in the socket of the malleable iron casting which is oversize by approximately .025" to enable the face to sit firmly. With faces fitted the casting is then put under a 50 ton hydraulic press which swages the malleable iron around the faces, thus ensuring that they are securely fitted. In this operation the socket opening is closed in, and after swaging could be 1 ½", -×005" or -×007".
You may appreciate therefore that it is not easy to replace worn faces and I attach a scan of a simple leaflet giving details of general fitting procedures. While many customers do replace rawhide faces, copper can be particularly difficult for there are grooves in the copper socket into which the copper spreads.
The use of an opening and closing tool is essential to change the copper face and I can send you a drawing of the tool or make one for you. A different tool is needed for each size of hammer and for example the number 2 size is £35.00, although anyone with tool room facilities can easily make one themselves.
Our copper is 99% pure and, being soft, will after considerable use, mushroom over the hammer head and in this state often lasts almost indefinitely.
Many customers find it more economical to use the copper end to destruction and then replace the complete hammer. Some large users use an air/pneumatic chisel to force out the copper face but this can only be done before there is any head distortion.
The time taken to replace copper faces is generally not cost effective.
I hope this brief summary is of assistance."
In the past I have actually managed to replace the rawhide faces with limited success without using the opening and closing tool by lubricating the face with washing up liquid, putting a hose clip around the face at the top and bottom to stop it spreading and squeezing it in with a vice, moving the lower clip up a little at a time as the face goes in.
An even better way, if you have a suitable piece of pipe and a press, is to put the face in a tightly fitting pipe, again lubricated and use something as a piston to force the face in with an hydraulic press.
If you do go the opening and closing tool route, there is a possibility that the casting will crack in the process and only the minimum opening up and closing in of the casting should be applied.
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Even if I calculate my time at $10hr, I think I'm better off pitching it and buying new.