SOHC.co.uk Forums > CB750
Oil pressure issue / motor rebuild
Bryanj:
Two things
(1) a large piece of plate glass from a glazing firm and some very fine compound will check if the case joints are flat
(2) Back when i did a lot of Ford engines if a big end knocked it was common practice for the rod end to need resizing as they ovaled
royhall:
--- Quote from: AshimotoK0 on October 05, 2016, 01:16:16 PM ---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)
--- End quote ---
I 2nd Ash with this, torque the empty cases and get an engineering firm to precision measure the case bores. You may only be able to measure the outer bores accurately enough but that will do. If they measure small or oval your cases are toast and anything you try will not be acceptably accurate unless your an old school engineer with amazing skills. Sounds to me like the case blasting has removed material from the mating faces, and thousanth of an inch on each case would be more than enough to cause your problems. Good luck with it.
MrDavo:
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?
Trigger:
--- Quote from: MrDavo on October 06, 2016, 12:33:33 AM ---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?
--- End quote ---
I have answered the question many times about these unstamped cases but, not on a forum. I can see that these case's are a late shape and would be suitable for a K3 to K6 .Possibly a F1 if I could see the crank case breather.
Bryanj:
If you want to spend some time before going to an engineer here is what I would do:
Fit all the knackered shells then replace 1 journal at a time with the new and torque up to see if the crank locks up checking all journals individually.
If only one journal locks up the crank try fitting the shells for smallest case and biggest pin then try again, if all lock it up i would look for different cases or a complete engine using the rest of the bought engine as spares.
As somebody else said the gearbox bearing clearances will be wrong as well and you will be clamping them too tight, and whist they are robust steel cages I don't think it's a good idea.
As too altering the piston/head clearance that is minimal considering you can skim both head and cylinders
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