SOHC.co.uk Forums > CB750
Oil pressure issue / motor rebuild
royhall:
Oil light on steady with associated knock from the engine, that's a knackered bottom end for me. Take it off the road now before you do irreparable damage to the crank. Sorry mate.
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Bryanj:
If its knocking its already irrepairable mates
MrDavo:
I suspect you're right, the crank is probably toast, but I'm not looking for a new one until I see for myself. I'm not going to run it until then, my 1200 Harley Sportster did a crankpin, then the metalflake oil did for all 4 cams as they seized in their bearings and span them in the cases. It took a year to fix that mess, if I'd stopped as soon as I heard it going 'potato, clank, potato, clank' I could've avoided a lot of the secondary carnage, but I went through a 'denial' phase first, until it became too loud to ignore.
The odd thing is that she runs very smooth and quiet until it gets hot and the oil thins, you'd think shot mains would knock all the time.
I do need to get to the bottom of 'why' first - if, say, the oil feed line is collapsing internally under the braiding, it could repeat the process even after I fix it, I will be having a close look when I get it apart, I may even replace it anyway as a precaution. Those two little episodes where the oil light came on as it suddenly lost pressure, then got it back, both with cold oil, just up the road from my house, are very significant I think, as well as very unusual. I shall also be having a very close look at the oil I drain out. I have an impressive collection of glitter that I washed (using white spirit) out of the Harley oil tank.
Not great, but not the end of the world either, I've fixed much worse.
royhall:
A knock is usually a big end caused by the change of direction of the Conrod. Mains are usually a low rumble noise. As Brian says you can't regrind these cranks but if the damage is tiny they can be micro polished. Good luck with it hope it goes well.
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Trigger:
--- Quote from: royhall on July 14, 2016, 07:58:24 PM ---A knock is usually a big end caused by the change of direction of the Conrod. Mains are usually a low rumble noise. As Brian says you can't regrind these cranks but if the damage is tiny they can be micro polished. Good luck with it hope it goes well.
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--- End quote ---
Can be reground to STD size after metal spraying. You can't get oversize shells.
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