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Re-bore tolerances

Submitted by simon_ratcliff on Sat, 19/11/2011
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Commando

Does any body out there know where I can get a cylinder block rebored and honed to a finished size of less than 0.0002'' ( two tenths of a thou') out of round/ ovality?This is the recommended sizing according to CP pistons, AE technical and others. I know it is achievable as I've recently re-fitted a cylinder block to a1979 T140D Bonneville, after flex-honing, with what appears to bea genuine 4,500 miles on thespeedoand tolerances measured with a Mercer bore gauge within 0.0002''.

Cheers, Simon.

Submitted by john_whiting on Sat, 19/11/2011

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Any reputable reconditioner will hold out of round to less than that.Its a matter of serviceable bearings in the machine,and no more than five or six strokes of the hone,to bring to finished size.Operators that bore several thou under,then hone for five minutes, have worn out equiptment.You should allways check a rebore for size,taper and roundness,and not trust the reconditioner to do a satisfactory job.Regards John.

Submitted by simon_ratcliff on Mon, 21/11/2011

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Thanks for the reply John but it's finding the person to do the rebore properly that I'm having a problem with, can you suggest anybody specifically?

Simon.

Submitted by john_whiting on Tue, 22/11/2011

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I can,but it wouldnt be any help to you.Ask around the bigger bike shops in your area,who does their work,and especially who do they get to do their race bike work.Jap bikes require a higher standard of accuracy than old Brits.Dont go to car workshops who will have to use adapters to fit bike cylinders to their borers. Larger machines rely on setting the block up on the main bearing bores,and require adapters for bike cylinders.Regards John.

Submitted by simon_ratcliff on Tue, 22/11/2011

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Why so secretive? I disagree that 'Jap bikes require a higher standard of accuracy than old Brits'. A piston, set of rings and cylinder liner fitted to an air cooled engine will require the same tolerances whateverthe country of manufacture may be. I have asked around and received unsatisfactory answers from the majority, butmay be having the block rebored by Dave Rushton Engines, Burslem, Stoke-on-Trent. Anybodyhad work done by him?

Submitted by john_whiting on Wed, 23/11/2011

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Im in Aus,and its probably too far to come,but I do know people who ve flown with Commando motors as carry on luggage. If you re still keen, give me a ring.Regards John.

Submitted by ashley_cutler on Sat, 26/11/2011

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Maybe you can try a few local precision engineering firms, if they can not achieve the tolerance, they may know a firm that can. Modern machines should be able to bore / grind a hole with very great accuracy these days, just a case of finding out who has them. Bike shops may be a help, but seeing as most are tied to manufacturers they will either not advise, or say that they just replace with new, due to cost of machining and re-coating.

Owslebury Crankshaft service in Hampshire do such work, and there used to be a place up near Liverpool that specialised in Motorcycles, but I'm uncertain if they are still there.

 


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