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Briffer

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Since the blower was just rebuilt lets blame that.
I'm assuming crank case pressure was normal before the blower rebuild.
The blower gears are lubricated by engine oil, the oil drains back to the crank case.
There is not supposed to be any "boost" pressure from the blower getting into that part of the blower where the oil just gravity drains back into crankcase, but the fugitive air pressure follows that oil drain and over pressures the crank case. Something is wrong inside that blower letting the pressurized air get to areas it isn't supposed to go. Oil seals, end plates, something not sealing correctly. Take it back to the rebuilder.
 
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Why was the blower rebuilt? What engine? I agree that the end plates are likely not sealed. Probably got pinched going back together.
 
Welcome!

Since the blower was just rebuilt lets blame that.
I'm assuming crank case pressure was normal before the blower rebuild.
The blower gears are lubricated by engine oil, the oil drains back to the crank case.
There is not supposed to be any "boost" pressure from the blower getting into that part of the blower where the oil just gravity drains back into crankcase, but the fugitive air pressure follows that oil drain and over pressures the crank case. Something is wrong inside that blower letting the pressurized air get to areas it isn't supposed to go. Oil seals, end plates, something not sealing correctly. Take it back to the rebuilder.

Sorry the motor is a 6-71 The reason the Blower as overhaulded there was a oil coming from the air boxes the engineer recommended overhauling the Blower
 
Does this engine idle for long periods? The reason I ask is that oil coming from the air box drains can be considered normal. A very small amount of oil can be scraped from the cylinder walls and enter the air box through the liner ports. It can be hard to grasp because the air box is full of positive air pressure...right?

But just like in any powerful river or stream, there are what are known as "eddy" currents or eddies. This is where there is a bend in the river and a small amount of the flow can get caught in these eddies and, sometimes, flow backwards. The same thing is taking place in the air box.

During higher RPM operation, the air box is fully charged and not much eddy effect is present. But, when the engine is backed down to idle or if the engine is allowed to idle for long periods, then these eddy currents are present and oil can enter the box. This is known as "oil slobber". Slobber is a big reason for having the air box drains and why the drains MUST be kept clear at all times.

While undue amounts of oil in the air box COULD be attributed to leaking blower seals, I would hope that there was some discussion about oil slobber and a visual inspection of the blower rotors and seals before the work was authorized.
 
Did it slobber oil before, or did this just start or get worse before the blower was rebuilt?

How much oil comes out the air box drains? What oil are you using? Delo 100 40wt. ?

Does it smoke, what color ?

"Driptroits" is one of their popular nicknames.
 
And...........DETROIT DRIPPER.

I hear tell that back when these were the most popular truck and bus engine in the U.S. the centerline of every major artery of commerce had a nice oily trail from all the blower box drains.
 
Does this engine idle for long periods? The reason I ask is that oil coming from the air box drains can be considered normal. A very small amount of oil can be scraped from the cylinder walls and enter the air box through the liner ports. It can be hard to grasp because the air box is full of positive air pressure...right?

But just like in any powerful river or stream, there are what are known as "eddy" currents or eddies. This is where there is a bend in the river and a small amount of the flow can get caught in these eddies and, sometimes, flow backwards. The same thing is taking place in the air box.

During higher RPM operation, the air box is fully charged and not much eddy effect is present. But, when the engine is backed down to idle or if the engine is allowed to idle for long periods, then these eddy currents are present and oil can enter the box. This is known as "oil slobber". Slobber is a big reason for having the air box drains and why the drains MUST be kept clear at all times.

While undue amounts of oil in the air box COULD be attributed to leaking blower seals, I would hope that there was some discussion about oil slobber and a visual inspection of the blower rotors and seals before the work was authorized.

Hi thank you for replies I have been guilty of running the 6-71 in my boat at low rpm just cruising around, a lesson I have learnt to give it bursts of high RPM. The oil I use is Castrol Tection Monograde DD40.
After the Blower was rebuilt the oil from the air boxes has reduced a lot. I did see the oil seals that were removed from the Blower and they were in bad condition.
 
I have owned the boat two years and the motor has always slobbered.
Oil used is Castrol Tection Monograde DD40.
There is NO smoke at any RPM the exhaust is very clean.
 
Sounds normal. I don't think we get that oil here in the States but it sounds perfect.

The engineers who rebuilt the blower removed it sealed the sides in a press and applied 9lb pressure into the blower there was no air leeks.
Replaced the blower onto the motor with a new gasket and the excess pressure had gone.
Could a faulty gasket between the blower and engine block cause pressure entering the crankcase?
Thank you for your help,
Briffer.
 
I'm going to say yes, I think that's a possibility. If the mounting gasket had a flaw at or near the rotor bearing oil drain back holes then I suppose the crankcase could be pressurized in that way. Having solved the issue by simply removing and replacing the blower tends to confirm that something like that had to have been the cause.

Glad to see you got it sorted out.
 
I'm going to say yes, I think that's a possibility. If the mounting gasket had a flaw at or near the rotor bearing oil drain back holes then I suppose the crankcase could be pressurized in that way. Having solved the issue by simply removing and replacing the blower tends to confirm that something like that had to have been the cause.

Glad to see you got it sorted out.

Thanks for your advice and comments
Very helpful
 
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