I seem to recall that the Alfin barrels on my Velocettes were made in a similar fashion with the alloy outer 'bonded' onto a steel liner. They were known to distort but I never had any problems on any of mine. It did cause a bit of head scratching by the old boy that rebored my '47 Cammy Velo barrel though. An expansion test of the unknown piston was achieved with a plumbers blowtorch and a ruler! I renewed the small end in situ and a degree of guesswork was needed on the clearance to gugdeon , still must have been about right as I put several thousand miles on it before I sold it and it's still going strong with the new owner.
Back to brake drums, some are balanced which would be removed by the machining so would need rebalancing, the ones on the Healey are renowned for balance issues that cause significant vibration if not done.
