Oil Tank Connections

[StevieWonder]

Hi, On my 1970 CB750 K0 I’m confused about two connections to the oil tank, these are the smaller connections at the top and back. Removing the tool tray reveals the top connection, which points up and is bent back over. The other is hidden below and points down. They are shown on a diagram of the lubrication system (attached), which includes a cut-away view of the oil tank, revealing that they both connect to a ‘mini-tank’ within the main oil tank. I’d be grateful for definitive advice on where these points should be connected to. I suspect the top is connected via rubber tube to the crankcase breather and that the lower may be for the rocker cover breather or to atmosphere.

[K2-K6]

The whole system has to be be vented to atmosphere in avoiding internal pressurised crankcase which potentially makes them leak.

Highest small tube on oil tank vents straight down to the floor.

Highest vent on engine also vents straight to floor, which is why both of them effectively have oil air separation facility to avoid oil mist exiting.

Lower one on oil tank,  I believe,  is connected to vent at rear of gearbox. You'll need confirmation on this as I'm not in front of anything to confirm currently.

Welcome to the forum by the way,  More bikes on the road always appreciated.

[StevieWonder]

Hi SOHC Jedi, Thanks very much for your prompt response, which makes good sense.

Vent from rear of gearbox to lower oil tank connection, passing through the mini-internal tank and then to atmosphere through the top connection, thereby separating oil from air.

Presently, the vent from the cam shaft cover is just a tube with no oil collector. Whilst I have only noted a slight mist from this tube, it has no form of oil/ air separation, venting directly to the road, which might play on my mind when heeled over on a fast bend. Do you know if there was any oil/ air separation for this pipe on the original K0? I’m a purist, but happy to fit an aftermarket filter to save my skin!

Good to be on the forum.

[K2-K6]

The vent assembly from top of engine is built into the cam cover.

That "wedge of cheese" shape secured with three screws effectively covers an arrangement in which the main casting covers the camshaft drive sprocket with internal gap left to vent safely, and seperate from direct exposure to oil splash.  So it's all done for you and built in as part of the design.

I've never seen one blow oil out,  steam on a very cold day,  but that's all.

[StevieWonder]

Hi SOHC Jedi, Thanks again for prompt and knowledgeable response.

Fitted a new 8mm dia. breather tube to rear of gearbox and connected it to the lower connection at the top of the oil tank. Then added another tube to the oil tank top connection and routed it to the side to avoid any possible oil on rear tyre. But just-in-case pushed about 5cm of sponge up the open end.

Routed the camshaft breather pipe to the opposite side, away from tyre, and pushed another bit of sponge up that open end.

Hope to give it a run soon and not find any oil in the sponges. I’ll let you know.