Darryl Kirk in Western Australia is getting on well with his Norton 750 Interstate project, but upon lifting the head he was disappointed to find that the barrel is rough and already out to its maximum plus 40 oversize. He can’t find a replacement and his local rebore specialist is not keen to sleeve the cylinder, as these have a reputation for weakness. “I’m hoping the bore will clean up with a hone,” says Darryl, “but if not, have you any suggestions?”
Sleeving can be risky, because manufacturers don’t always make castings thicker than needed for more than a couple of rebores, so the cylinder can end up thin enough to break free from the base flange. Ashley at Andover Norton tells me that their new cylinders are the stronger, 850-type though-bolted barrels which fit early 750s with a conversion kit, and suggested I speak to Norton specialist Norman White about sleeving. Norman confirmed that he’s relined several 750 barrels without problems. The liner must be stepped at the top for security and it’s worth trimming off thin liner remains which could split or flake off.
After racing both his own and Norton’s Commandos, he finds 750 cylinders are fine provided excessive compression wasn’t used – anything over 10.5:1 lost the torque that kept the bike ahead anyway. Norman added that standard 750 barrels can be converted to through-bolts for added strength, but it’s tricky and probably unnecessary on a road bike.
Andover Norton’s replacement barrels feature stronger through-bolt fixing