Classic Bike (UK) - - WORKSHOP RICK’S FIXES -

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

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