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MG MGB Technical - Blow by?

I recently replaced the engine in my 1970 B with a rebuilt 1974, large valve engine. The old engine was pouring oil out the rear seal, even though the seal had been replaced with clutch work and a camshaft change. It was getting progressively worse and finally, the clutch seemed to be slipping.

The rebuilt engine also pours oil from the rear seal, albeit at a lower rate and seems to be losing oil from the front seal as well. It now has about 8,000 miles on it so the rings should be broken in. It also has Viton valve seals.

Oil consumption is minimal on the dipstick but the amount that drips into the pan is excessive, even compared to the midget which only uses a scrolled crank and came with no rear seal.

The car is driven almost exclusively on the road. My theory is that the oil is blowing by due to load with OD engaged with sustained 75 MPH driving.

Just curious if there is any way to stem the flow. I guess oily everything is a fact of life when you drive a B.
Glenn Mallory

Sounds a bit like a breather problem, is everything plumbed up correctly ---
William Revit

In rebuilding the engine, was the rear crankshaft oil seal surface reground or sleeved?

Running the engine with the oil filler cap off will demonstrate whether there is significant blow by - you should be able to feel slight suction on the palm of your hand placed over the filler neck when you rev the engine.
Chris at Octarine Services

Weird that the problem is on both engines. I guess the common parts used included the front breather chest?

I think that is where the problem will lie.

Colin
Colin Parkinson

Crankcases should have two breather connections to the outside world so both would have to be blocked for slight pressurisation to push oil out, or there is significant blow-by exceeding what the breathers can relieve.

As Chris remove the oil filler cap and place a sheet of paper or card over the hole. If the breather is working correctly is should suck it down onto the filler with light pressure. If there is no crankcase suction it might flutter up and down a bit, but if there is significant pressurisation it will be blowing it off.

In 1970 North American engines got the charcoal canister which changed the engine breathing from using a ventilated oil filler cap. Instead there was a sealed cap and a port on the back of the rocker cover, plumbed to the canister, and carb suction (provided in 1969 in place of the PCV valve) pulled air through the canister to purge that as well as the crankcase. If the canister has been removed and the rocker cover port blocked of it needs a ventilated oil filler cap, or you can put a small filter on the rocker cover port and stay with the sealed cap. But even with that inlet blocked off that wouldn't account for excessive oil leaks as there should be a higher level of vacuum in the crankcase than normal.
paulh4

I will check for vacuum at the oil filler cap.

The car has the canister and plumbing intact (also air pump). It does run a Weber carburetor so the valve chest is vented to the air filter, not to the S.U carb bodies. The previous engine ran S.Us, Mikunis for a brief period, and finally the Weber for better heat tolerance.

There are no common parts from the original engine and the replacement except the carburetor and belt driven accessories.
Glenn Mallory

No through-flow ventilation with that system, so no vacuum at the oil filler, but you can still check for pressurisation.
paulh4

Glen
When you say Weber, is that a sidedraft or downdraft
William Revit

The Weber is a downdraft. It has proven heat tolerant compared to the S.Us, an advantage with alcohol laced fuel, running in +40°C heat at altitude with the A/C running.

There was no visible movement of the sheet of paper placed over the oil filler cap. Even so, the leaks are copious, apparently from front and rear seals. Tappet chest covers are dry.
Glenn Mallory

Maybe the seals are just worn out. When you replaced the seal, did you fit a speedi sleeve, as previously asked, and if so did you wrap some tape over the edge before fitting the seal? Not taping will damage the seal as it is put on.

Herb
H J Adler

No puffing of paper off the oil filler and dry tappet covers almost certainly means it doesn't have blow-by.
paulh4

Glen
If your car has the canister still plumbed up as you mentioned ,the system is designed to run at a slight vacumn---running the breather into the aircleaner won't produce enough suck
I'd be tempted to find a little in-line PCV valve off a small 4 cyl. car like a Mazda or similar and run the hose from the front tappet chest through the pcv direct to the inlet manifold (full vac) and make sure the oil filler cap is the non vented one
William Revit

The slight vacuum which was on all MGBs from Feb 64 was there irrespective of the charcoal cannister. The Weber removes that vacuum, which does help to prevent leaks from seals.

If the canister and all the plumbing is still fitted then use a non-vented oil filler cap as said. But if it has been removed and the port on the back of the rocker cover sealed then use a vented oil filler cap. An alternative is to put a small filter on the port and use a non-vented cap, if vented are more difficult to come by.
paulh4

The canister (and air pump) remain in place. There is not much positive suction through the Weber air filter. I have tried a Moss PCV set up, which uses a generic PCV valve for U.S. vehicles. This caused a problem with excessive oil consumption.

Oil consumption via external leaks is minimal, just messy. Most of it seems to be dripping from the front cross member, which while the previous engine preferred the rear oil seal.
Glenn Mallory

Even carb vacuum - which can't apply too much vacuum like the previous PCV valve could when faulty - can result in excessive oil consumption when the filter/separator in the front tappet chest cover has become clogged.

'Remanufactured' covers are available here at least which have been cut open, the mesh replaced, and welded back up again.
paulh4

With the Moss setup, although it used oil,did that stop the oil leaks while it was on there
William Revit

Yes, blocked separator will cause a problem with oil being dragged through rather than dropping out.
We retro fit a lot of PCV valves to earlier scroll engines (xpag, A series, B series) with good success in reducing leaks - typically a CD sized puddle reduced to a fingernail if the rings are in good order. Once they are tired then it's all too much.
Valve size is important, as per Willy's note get one for a small 4 Cyl engine.
Interestingly, on one installation there was no intake vent in filler cap or cover, and it worked just as well. Perhaps because the scroll clearance was double what it should be.
Paul Walbran

I was mistaken about the tappet chest covers. The front cover had been leaking behind the exhaust header such that the leak was not visible. I tightened the bolts on both covers and found them too loose. This seems to have stemmed the oil leaks on the LH side of the engine.

I also noticed that the back side of the harmonic balancer as well as the timing cover are soaked with oil. The timing cover seal is definitely leaking, which is why the cross member is soaked.

The pan on the floor is still collecting oil, perhaps less than before.
Glenn Mallory

This thread was discussed between 07/09/2019 and 02/11/2019

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