SOHC.co.uk Forums > CB750
750 K2 Strip & Rebuild - help please
Lobo:
Have struck gold in finding a very nearby SOHC Engine guru, who’s been building, modifying, racing the ‘750 unit (sidecars) since 1972.
And extremely generous with his time / knowledge; I traipse up-to his workshop with bits in hand, and rattle through things that would otherwise take me days … if I even knew to do them.
Anyways, yesterday, Lindsay cleaned out the four oil galleries within the crankshaft. (I was convinced this was a waste of time as I’d bathed it 48hrs in degreaser, and then pressure washed those galleries with a garden hose….)
First up he TIG welded blobs of steel onto the ball bearing plugs, before tapping them out with a cold chisel. This gave us access to the gallery downstream of the big end ports…. which were truly blocked with **it. (same workings as a centrifugal oil filter I guess). With a 6mm(?) drill bit this was all removed to a depth of about 1â€â€¦ ie the bit broke through more-or-less at the big-end oil ports. (The crap that came out was a plastercine like sludge)
Then a wash out, blow dry… replace with a new ball bearing using the original punch marks to secure.
(The top photo shows the gallery path depicted with the screwdriver. The con-rods have been reinstalled and so the big-end oil ports not visible in the pics)
May not impress the established engine builders here - but this was all new to me.
K2-K6:
"which were truly blocked with **it. (same workings as a centrifugal oil filter I guess). With a 6mm(?) drill bit this was all removed to a depth of about 1â€â€¦ ie the bit broke through more-or-less at the big-end oil ports. (The crap that came out was a plastercine like sludge)"
The fundamental problem is from oils of that original period being susceptible to tarring at high temperatures. Newer versions of "mineral " oil have improved in this aspect.
The real change though comes from "synthetic" oils that in reality don't do this. I've put those two in " " as the names just don't reveal the full story of how they differ, with most commentators demonising the newer without understanding what they hold prejudices against.
I've used synthetic oils in most engines I look after since the 1980's and principally because that sludge and degradation is not present.
For another thread really though.
Cleaning the oil ways though, I'd use steam to go through these with. Make a nozzle from copper brake pipe or similar, fit to household steamer and blow through anything you can't immediately access.
Sounds a very thorough engine guy you've used there Simon.
AshimotoK0:
--- Quote from: Lobo on March 26, 2022, 11:59:19 AM ---Have struck gold in finding a very nearby SOHC Engine guru, who’s been building, modifying, racing the ‘750 unit (sidecars) since 1972.
And extremely generous with his time / knowledge; I traipse up-to his workshop with bits in hand, and rattle through things that would otherwise take me days … if I even knew to do them.
Anyways, yesterday, Lindsay cleaned out the four oil galleries within the crankshaft. (I was convinced this was a waste of time as I’d bathed it 48hrs in degreaser, and then pressure washed those galleries with a garden hose….)
First up he TIG welded blobs of steel onto the ball bearing plugs, before tapping them out with a cold chisel. This gave us access to the gallery downstream of the big end ports…. which were truly blocked with **it. (same workings as a centrifugal oil filter I guess). With a 6mm(?) drill bit this was all removed to a depth of about 1â€â€¦ ie the bit broke through more-or-less at the big-end oil ports. (The crap that came out was a plastercine like sludge)
Then a wash out, blow dry… replace with a new ball bearing using the original punch marks to secure.
(The top photo shows the gallery path depicted with the screwdriver. The con-rods have been reinstalled and so the big-end oil ports not visible in the pics)
May not impress the established engine builders here - but this was all new to me.
--- End quote ---
That's interesting Simon ... probably one of the most interesting posts I have seen on here recently and I wonder what our resident engine Gurus on here think.
It sounds a bit like what mate Graham Curtis does on pressed-together Honda roller bearing cranks. There are inaccessible grooves that clog up with thick highly compacted muck, that can only be removed by pressing apart the crank and physically removing the compacted debris. He claims that the particles preferentially build up in those internal areas rather than being centrifuged out in the oil filter 'cup' mounted on the end of the crankshaft. Also by removing all of the debris, more oil is flung up into the underside of the piston area (as intended) and lubricates the small end better.
K2-K6:
Ash, it's mirrored in any specific site within engines that take the oil above critical temperature limits. Probably in the range of exceeding 155 / 175 C for localised conditions. This will often be propogated when the oil becomes stationary after turning the engine off and the heat can no longer be extracted by continuous oil flow.
Ultimately, in these small passages it gives compromise to the desired flow as carbon builds, then making it even more susceptible to over heat.
Generally the oil is targeted to run at 90C continuous, and would largely be out of range for mineral oils above about 135C.
Oddjob:
Your wrong Simon, that impressed me. Never heard of anyone doing that before, great info and might explain those unexplained "why has my engine seized" questions when there is no rational explanation, a big clump of that stuff coming loose would ruin most engines, who knows what it contains.
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