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I would also bin those crank cases and seek a replacement. If the bearing surfaces are damaged, then all bearing housing will be beyond spec, this will include bearing and oil seal housings.
Hi, I have a 1971 CB750 Honda four cylinder classic bike.I bought the bike already restored, but with damaged main bearings. I have changed the crank for another, known to be in very good condition, measured the journals with a micrometer and fitted the correct shells. When I tourque up the crankcases, the crank locks up, just as the final torque setting is reached. I believe that the problem is that previous media blasting of the cases has removed a few thou of alloy from the crankcase centre joint, meaning that when the cases are tightened up the crank journals are slightly out of round, nipping up the required oil clearance. I have the Honda spec measurements for the bearing shell housings we are talking about a range between 39.000 and 39.008 mm I.D. for the shells I have. If I brought the cases to you, (I am in Stalybridge) could you measure the crank journals' internal diameter for this, and if possible line bore the journals back to spec?What sort of cost are we looking at?The alternative is to use another set of cases, but I would like to save the originals if I can.Thanks, Dave
Yes Chris, I'd like to get the mating surfaces lightly skimmed, but then again every thou makes the problem worse.
It was only a suggestion Dave.. personally, I would get replacement cases but if you want to preserve numbers then that's possibly not the route to go. I assumed it was only the centre 'pillars' that were nipping up the crank so small bits of 1 thou copper foil may have just confirmed that the problem is mainly on those and not the outers, but you could probably ascertain that anyhow from a decent bore gauge measurement, wiith the crank out and the cases bolted up.I checked the cases the crank I sold you came out of as below and apart from one outer half, all the shells are still there if you wanted to try used, not particularly worn SH shells to check whatever method you come up with is satisfactory without trashing 100+ quids worth of new ones. (Attachment Link)
I'll see what the engineers say in reply, cases have been linebored in similar circumstances, but one worry would be the deck height of the cylinders, you would effectively move the pistons nearer to the valves! - in fact you already have by taking metal of the crankcase face.Ash, you don't think Freddie's unstamped cases would be any good? He doesn't say what year they are, though I take his claim that they'd fit all years with a big pinch of salt, can anyone shed light on this?