SOHC.co.uk Forums > CB750
750 K2 Strip & Rebuild - help please
Bryanj:
Having done more than a few Triumphs, their sludge trap gets quite full and does restrict oil flow BUT they have no filter and the trap is in line with big end pins so gets max centrifugal force. The Honda is on a blind line so theory says it will only be a problem if there is so much it fills that line.
On an asside Fiat used small core plugs to block the oilways, i rebuilt one that had knocked an end where the plug had fallen out!
K2-K6:
The "Ash" content results from sulphated ash (phosphorus and sulphur) that's part of these original mineral oils and shows the oil has gone over temperature at that location. It's not abrasive, so low risk from that. This is what low SAPS oils are about now, if the oil gets into the combustion chamber it burns (over temperature same as this crank site) and forms ash which is detrimental to exhaust cleaning equipment.
The older engine types use these components in combination with their metallurgical makeup to help stop wear.
The sludging is only from oil going over it's critical temperature AT THAT SPECIFIC LOCATION because the component is too hot if the oil there becomes slow moving. It's also seen in turbo bearings (hence the addvice to keep running a turbocharged motor after extreme heat to prevent the oil becoming stationary. Also other strategies are now more common with flow pumps after switching off and watercooled casings etc, this to prevent localised sludging. Often seen on Rover V8 in rocker cavity as the aluminium skull caps over combustion chamber eject a lot of heat that way and particularly above the exhaust ports. Notice how Honda put airflow passages throughout the head to control that effect on these sohc engines.
I'd view the ball bearings as more gentle solution than threads to block the holes. Threads always carry more risk of propagating cracks from their form rather than a peaned ball arrangement.
I should be considered that the potential blocking with sludge can be serious in terms of oil distribution. If the crank pathways impede the volume of oil in any significant way the pump when meeting this resistance can blow the pressure relief valve open (this is to save it's own drive mechanism from potential hydraulic locking of oil pathway) too much of this can obviously starve the crankshaft and bearings, but just as serious is the interaction of the pressure check "jets" going to the head. These must be capable of containing 60psi as that's what they do in separating the pressurized crank from the open bath camshaft area. If the oil pump pressure relief opens, it's likely that no oil will be pushed into the head while that's happening. It'll also be worse when cold as the oil is thicker. Part of rebuilding would sensibly cover this off surely?
AshimotoK0:
I just had a quick look in Hondman's book on the CB750 and couldn't see any mention of removing balls/plugs or internal cleaning of the crankshaft ... surprised really and it's a very in depth book.
Unless I have missed it of course :-[
Oddjob:
--- Quote from: K2-K6 on March 30, 2022, 09:03:10 AM ---
I'd view the ball bearings as more gentle solution than threads to block the holes. Threads always carry more risk of propagating cracks from their form rather than a peaned ball arrangement.
I should be considered that the potential blocking with sludge can be serious in terms of oil distribution. If the crank pathways impede the volume of oil in any significant way the pump when meeting this resistance can blow the pressure relief valve open (this is to save it's own drive mechanism from potential hydraulic locking of oil pathway) too much of this can obviously starve the crankshaft and bearings, but just as serious is the interaction of the pressure check "jets" going to the head. These must be capable of containing 60psi as that's what they do in separating the pressurized crank from the open bath camshaft area. If the oil pump pressure relief opens, it's likely that no oil will be pushed into the head while that's happening. It'll also be worse when cold as the oil is thicker. Part of rebuilding would sensibly cover this off surely?
--- End quote ---
I'd agree with the blocking comment IF the part being blocked was in use, the area in question is a dead end or blind gallery as Bryan puts it, no oil passes through it. As soon as this area filled after the first fire up of the engine it most likely stayed there, it's ability to drain is severely compromised by the way the shells are made, conrods shells do not have the recess in them like crank shells so they'd effectively block the gallery as soon as the engine stops, when the engines running the oil pressure would be the thing pushing the oil through the very small gap between the shell and the crank journal, I suspect the oil is far too thick even when hot to get through the same gap without pressure. That's my opinion anyway. My findings were that below the blind gallery the oilways were free of all powder/sludge. When I ran a drill down them it came back clean except for the occasional sliver of metal where it caught the side of the gallery.
As for the peened ball question, surely that can't be oil tight, the very act of peening would distort the area around the ball causing small openings, maybe the fact they are quickly blocked by the sludge/powder is a good thing otherwise you'd surely have some loss of pressure. You wouldn't get that with threads especially with thread lock used.
Lobo:
Aren't carb galleries regularly plugged with peened balls? And if so, you’d have to assume the method as good engineering practice in terms of high performance / reliable sealing.
As a fan of ‘simple is best’ (been a LONG learning process that one 😂) I got to say I quite like the elegance of it all ….
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