Oil leaks and crankcase pressure

[K-ron]

Having recently rebuilt my Sabb 10 HP engine, I am now suffering from multiple oil leaks from lots of different gaskets, around the water pump, governor door, fuel injection pump and from behind the fly wheel. The rear oil seal is also leaking engine oil into the gearbox. I am thinking that there must be a problem that is pressurising the crankcase. I have just stripped the hand start assembly to gain access to a small plastic disc valve where the crankcase is ventilated, I am hoping that this valve is getting stuck and causing the problem. I have a couple of questions, if this valve is the cause and I rectify it, will the rear oil seal stop leaking or once they start to leak they will continue to do so until replaced? If this valve is not the problem what else could be causing the crankcase to get pressurised? Piston blow by seems unlikely because I fitted a new cylinder liner and piston rings. Your thoughts much appreciated.

Kieran

 

[essexboy]

Did you hone the liner after fitting, or did you check it was honed ?

 

[K-ron]

It was already honed.

 

[vyv_cox]

You can run a simple test by taking off the filler cap while the engine is running. If there is significant crankcase pressure you will feel it.

 

[Skylark]

In gearbox development testing it is not unusual to blow oil through a shaft lip seal due to too high an internal pressure. This is common when, for example, testing to establish the required oil quantity/level in a new design. Subsequently, the seal usually does its job providing that its max specified operating pressure is not exceeded again.

My experience of gaskets is that once they leak they are pretty useless and are best replaced.

 

[neil_s]

I expect that all your leaks will stop when you fix the crankcase pressure problem. Certainly that is what happened when I had a similar problem with my Bukh DV20. If the rear oil seal is a lip type, then the pressure is just lifting the lip against the spring and when you fix it, the spring will simply re-assert itself. I remember that the relief valve looked simple, but some serious cleaning was required to get it working again!

 

[K-ron]

Thanks Vyv/David.
Just tried turning the engine over by hand decompressed with one bolt from the door on the engine block removed air is sucked in and then blown out. Damnit! There must be a problem with the piston rings even though compression appears to be good, I can hand start it very easily since the rebuild. Could it be the ring gaps somehow getting aligned?

 

[K-ron]

Thanks Neil, replacing the rear oil seal is a total pain in the backside however it looks like I need to get the piston out now which is really frustrating.

Kieran

 

[earlybird]

Thanks Vyv/David.
Just tried turning the engine over by hand decompressed with one bolt from the door on the engine block removed air is sucked in and then blown out. Damnit! There must be a problem with the piston rings even though compression appears to be good, I can hand start it very easily since the rebuild. Could it be the ring gaps somehow getting aligned?

Don't rush into removing the piston if the compression etc. is good. A single cylinder engine will have crankcase pressure changes as the piston moves.
It is single cylinder isn't it?

 

[K-ron]

Thanks for that earlybird, yes it's a single cylinder, i did notice the blowing before but thought it was normal. The engine has been running really sweetly since the rebuild we have done about 100 hours since. I have cleaned up the plastic valve where the engine vents and changed both engine and gearbox oil. I will run it under load tomorrow and see if anything improves. If there is no improvement I am fresh out of ideas.

Kieran

 

[Bilgediver]

Don't rush into removing the piston if the compression etc. is good. A single cylinder engine will have crankcase pressure changes as the piston moves.
It is single cylinder isn't it?

I agree with this. Seems to me you have a breathing problem. Check the manual to see how the breathing is carried out. Sometimes even putting a head gasket in the wrong way round can block a passageway at the head joint.

Sabb engines were certificated for use as ships lifeboat engines so there might be some sort of internal system as on the single cylinder Volvo MD!

 

[RichardS]

If everything is running well Kieran, I would try improvising a breather using a pipe from the oil filler or dipstick tube (depending on whether it reaches down as far as the oil) and take the end of the pipe upwards to form a return "trap" and then into an old plastic coke bottle or something. The bottle needs to have a small opening to the air. If you run the engine for a few hours like this and the bottle is not filling with oil you then have the choice of working out why the proper vent is not working or is being overloaded or carrying on with the temporary breather until the winter.

In the "olden days" the pipe and bottle were a standard fitment on many cars and motorbikes. :ambivalence:

Richard

 

[PaulRainbow]

In the "olden days" the pipe and bottle were a standard fitment on many cars and motorbikes. :ambivalence:

Richard

Many didn't even bother with the bottle, just vented under the car.

 

[lw395]

Many engines, particularly singles, have 'timed' or valved breathers, the idea being to use the piston displacement to lower the pressure in the crankcase.
This not only helps keep the oil in, but also helps water in the oil or crankcase evaporate.

How is the Sabb supposed to breathe?

 

[K-ron]

Cheers Richard,Paul and lw,

The Sabb breathes with a small plastic disc valve located at the bottom (inside) the hand starting housing. The valve has no spring so presumably moves up and down as the pressure changes from the piston movement. When I removed the hand start housing a couple of days ago the valve was stuck in the open position. I have cleaned it up with some diesel as best I can, the access is tricky. So far so good although I haven't run the engine for very long since. The Sabb also has a pipe running from the bottom of the sump, out of the block and into a short piece of plastic hose, the idea being you can turn a lever that holds down the disc valve, pressurises the crankcase in order to remove all the oil. If I am still getting leaks I will unplug this plastic pipe and see if oil starts to flow without closing the valve. Also I will keep an eye on the gearbox oil and see if it remains clean and not turning black from engine oil finding its way past the rear oil seal. Hopefully just a few teething issues.