I have now received a new set of wheels of the correct diameter (8mm) and I have given them a quick spray of primer (one set black for my second Terrier and this set grey for painting green and black for my Southern livery Terrier).
The next job on the chassis was to drill out the 0.3mm holes for fitting the brakes and the 'Simpson Spring' axle pickup wipers. Two of the brake support holes require drilling into the side of a glass fibre PCB spacer. Typically, my drill bit broke doing the last hole. I had to unsolder the spacer, remove the broken drill bit, put the chassis back into the jig and resolder the spacer. Then I had to redrill the holes with a fresh drill bit.
Now I was ready to start fitting the gears to the accurately turned 'muffs' and fit them in the chassis temporarily with some long pieces of axle steel. The muffs needed a little triming to length, and drilling and reaming to give a tight push fit on the axle steel. I also drilled across the centres of the muffs to let air escape and to insert superglue if required when the wheels are finally fitted. The gears were deburred with some fine emery paper and then fitted to the chassis. They would not turn. The main worm gear was fouling the axle muff. I took the axle muff, inserted a length of axle steel and mounted it in the jaws of a mini-drill. I then used a good small file to reprofile the muff to give room for the worm gear to rotate. Once refitted, everything turned freely. The turned muffs do appear to be better than the old ones. I might return to the '09 Diesel Shunter' I started building previously that suffered from non-concentric gears, to see if they improve things.