SOHC.co.uk Forums > CB750
Oil pressure issue / motor rebuild
Chris400F:
Given the note you found that you posted earlier in this thread about a matte surface bonding with well over double the bonding force of a smooth surface, and the fact that the blasting removed enough metal to give the problems you have had, if you keep the cases would the first step be to get the mating surfaces skimmed to get them flat and smooth?
MrDavo:
What Trigger says is food for thought, as he knows more about these motors than most. Will the couple of thou that uses up my crank oil gap have the same effect on a the housing of a large ball bearing or a rubber seal though? There are no signs of trouble with the gearbox and final drive bearings, no witness marks or roughness. I do take your point though.
Anyway, until something else turns up, these cases are what I've got.
Yes Chris, I'd like to get the mating surfaces lightly skimmed, but then again every thou makes the problem worse.
AshimotoK0:
--- Quote from: Trigger on October 05, 2016, 01:21:36 PM ---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.
--- End quote ---
Agreed but depends if blaster has blasted centre bearings more than outer ones (smaller area too), which I suspect, as its only when he torques centre bolts down it nipped up or was I wrong on that ? Best thing is to clamp it all together with no crank etc and measure the bearing bores for ovality IMHO
MrDavo:
There is very little disturbance to the bearing housings as far as I can see, they must have been gentle there. The trouble is the metal removed from the centre join as far as I can judge, there is no trace of the original machined finish. :(
Ive sent Quasar this email earlier today, we'll see what they think, they are only 9 miles away so it won't hurt to get things accurately measured, at least.
--- Quote ---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
--- End quote ---
Chris400F:
--- Quote from: MrDavo on October 05, 2016, 02:58:13 PM ---Yes Chris, I'd like to get the mating surfaces lightly skimmed, but then again every thou makes the problem worse.
--- End quote ---
Agreed, and it would be more work and more money.
But can you be satisfied that the surfaces as they are are still 'true'? As there is no way of knowing exactly how much metal the blasting removed it is quite possible more has gone from some places than others. Maybe get an opinion from Quasar, to be sure this wouldn't be a problem.
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