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MG MGB Technical - Engine Breathing : Pressure

I have the early MGB G/GA engine in my Magnette and I have the reverse problem to Neil's in the "Engine Breathing" thread - too much pressure.

In common with the MGA and 3-main MGB engines the Magnette breather is an open draft tube on the side plate with a very small baffle plate behind it. (See Pic.) Therre is also a plain rubber tube from the rocker cover to the oil bath air cleaner i.e. there is virtually no suction involved.

Last weekend, on my first trip with the newly remanufactured 3-main fitted I lost a whole sump-full of oil blasted out of the draft tube in about 200 miles! Luckily I saw the oil pressure falling and was able to prevent a disaster.

Basically I believe the crankcase pressure and hence flow is too great but the engine supplier wants me to use the 18V type baffled sideplate they supplied on the engine. This is stuffed with metal turnings and is of the type only fitted to later engines which had a proper rear crankshaft seal.

I am worried that if I use this sideplate it would increase the crankcase pressure lead to oil loss through the rear main shaft scroll unless some form of suction is also used.

Any experiences of this out there? Any recommendations??


Andrew Dear

If you should choose to use the later baffled side plate I would suggest that it would be imperative to use the valve assembly (13H 5191) which is fitted to the inlet manifold and connected to the side plate with hose to provide the suction necessary. I think you should also have the engine checked to ensure that the piston rings are sealing correctly. That was an awful lot of oil to lose. Many engines of the fifty"s and sixty"s had open breather pipes, some connected to the air filter intakes, and as such very little suction, and ran with only modest oil consumption.
jim soutar

You're most likely experiencing too much crankcase pressure due to the fact that your rings haven't had a chance to seat yet. Once they have, most of the problem should subside. The fitment of the early mushroom PCV valve is a good idea and will maintain negative pressure in your crankcase. RAY
rjm RAY

Jim, RAY,

Thanks for your comments. I agree it would be advisable to use some decent suction, and the PCV would do the job. However, I am not sure how it could be mounted.

The pipe from the later sideplate comes up underneath the cross brace on the Z Magnette (See Pic - not my car!) and the intake air plenum and oil bath air cleaner make it difficult to connect a pipe to the induction manifold.

I am going to keep running in the short term using the later sideplate, and watch carefully for any oil passing the crankshaft scroll. There is a tell-tale channel down the rear engine plate that should show if oil is passing the scroll.

The engine has done about 600 miles now so the rings should soon have bedded in - then I will try the open draft tube again and see what happens.


Andrew Dear

If you have excessive blow-by, and you connect it to the inlet, all you accomplish is burning a lot of oil instead of blowing it out.
You MIGHT reduce leakage past the scroll if the applied vac can overcome the blow-by. However, a newly recon engine should not have leakage past the scroll.

FR Millmore

There should be no problem using the later side plate as suggested by the re-conditioner. In fact I would use it to prove to the supplier it makes no difference and his rework of the engine is sh**e.
The later "stuffed" side plate, when clean, only provides a greater surface area for the oil vapor to condense on and should not increase the crank pressure by any significant amount. Another possibility is to fit a one modified as suggested on Paul Hunt's web site ( and in all cases, I would use a catch tank. At least then you can return the oil to the engine when you stop rather than lose it :0)

BTW I know it's an "old" engine but that's an awful lot of oil to lose via the vent from blow by.. Are you sure it is coming out of the vent? If it is, I would be inclined to put a letter on file with the supplier registering your dissatisfaction with the product supplied. At least then you have some evidence should you need to pursue them in the courts.

Where are you in Clack's? I work in Kincardine and would be more than happy to help.

M McAndrew

Blowing out a sump-full in 200 miles is a huge amount and engine side covers is rather tinkering round the edges, I'd say. If there is that much pressure something is very wrong, and you should be able to feel it at the oil filler cap if not have oil spraying out of there and the draught tube even at at a standstill.

Using a PCV valve should normlly put the crankcase under negative pressure which should help reduce losses from the rear scroll contaminating the clutch, if nothing else. But with that much blow-by (if that is what it is) the PCV may not do much at all.
PaulH Solihull

Thanks for your comments.

Hi Mike, I am based in Menstrie so not far away. I have been in touch with the engine supplier and at his request I am using the later side plate. Since fitting it I have done about 140 miles and lost no more oil but the blow-by is still excessive. I do not have access to a proper leakdown tester but have contrived a method which tells me that most of the leakage is on cylinders 2 & 3. The engine supplier clearly believes that the rings on 2 & 3 have not yet bedded in properly and that they eventually will, - I am not so sure, I am afraid there are broken rings.

The engine has done 600 miles now and the compressions on 2 & 3 have fallen about 10 psi below 1 & 4

Paul, The pressure from the crankcase can easily be detected at the rocker cover vent. The attached photo shows a small piece of cardboard being lifted by the force while the engine was at a fast tickover (~1000rpm). There is a similar flow at the draught tube.

I have agreed to drive the car with the later side plate for a few more days to see whether the situation improves or get worse.

Thanks again for your comments


Andrew Dear

Bedding the rings in involves forcing them against the bores, you need to generate some pressure to do this. Now you have few miles on the car you can open it up a bit and see if this seats them. With the possibilty of backsliding from his responsibilities you might like to ask for guidance from the rebuilder in writing first.
BTW, my car has a wire wool filled side plate and an open draught breather and it works fine. Even so the pistone will be coming out this winter as it should be able to use the mushroom valve.
Stan Best

Hi Stan,

I had advice by telephone from the rebuilder regarding running in. He told me to drive it quite hard from the get go, with alternate bursts of hard acceleration immediately followed by shutting the throttle to suck oil back up into the bores. Exactly as you suggest.

I took the car out today for a trip down to the Borders, but had to turn back home after only 25 miles as I was getting serious clutch slip. It would even slip when accelerating from 50 to 60 in 5th gear!

There is oil running down the engine backplate and also from the grommet on the clutch arm, so I guess my fears have been realised.

Now I shall find out how good their warranty is!

Andrew Dear

Hi Andrew,
I can feel your disappointment. Just when you want to get out in the car, you've got to get the engine out again... not good.
Hope all goes well with the supplier and let me know if you need a hand to pull the motor in double quick time.

M McAndrew

Taking the car out and forcing the issue was the best thing you could have done. A rebuild is not what you or the vendor want but its the way forward. He will need to hone the bores again after replacing rings or pistons as required. It is possible that the oilways/sump are being prssurised via another route. A crack in the cylinder head can do this or head gasket problems. The rocker cover tappet chests crankcase etc are all at the same pressure.
Stan Best

Mike, Stan,

Thanks for your concern and advice.

I had a call from the rebuilder first thing this morning after copying him yesterday with the pictures and info by email. He has been very reasonable throughout this and just wanted to be sure that the rings were NOT going to bed in before committing himself. The clutch slip has now forced the issue and we have agreed that the engine is being returned.

Its a real pain to have to pull the engine again but there is clearly no alternative!

Mike, thanks for the offer, but I have help on hand.


P.S. I spotted another anomaly as I was adjusting the valve clearances (see attached). I have already had to replace the rocker shaft because of a bent rocker arm but had not spotted this before! Quality Control was on holiday for this job I think.


Andrew Dear


I've been reading your posts on here and on the Magnette section with interest. I was considering sending my BGT engine to the suppliers you mentioned so that we could swap out the 1500 unit that is in my Magnette. On reading your posts I think I'll stick with my known contacts who may be slightly more expensive but could work out cheaper in the long run.

I hope they offer to replace the ruined clutch plate for you if the oil has been pushed out the scroll end onto it.

Andrew McGee

Hi Andrew,

To give them their due, the warranty guy has been very easy to deal with, and says they will do whatever it takes to sort it. I have also been laid back about it as I do not believe that getting uptight is helpful. After all they did not wish for this to happen any more than I did!

At this stage there has been no mention of anything by way of compensation as they still have to establish for themselves exactly what is wrong. All I have received so far is an apology.

Like you, I was planning to get my 3-main engine rebuilt locally, then this one came up on Ebay at a lower price so I traded it in. Might have been worth the extra cost for keeping it local!


Andrew Dear

Well, the engine is out and the bellhousing opened. Everything inside is covered with a film of oil. It is clearly engine oil as it is a different colour from the synthetic gearbox oil.

Perhaps the most significant observation is the attached photo which shows engine oil pooling in the recesses of the engine backplate behind the flywheel

The clutch driven plate is very greasy - no wonder it was slipping.

The return crate for the engine arrived from the rebuilders today so things are moving quite quickly. It will be interesting to see what they find.


Andrew Dear

Well, it is better than an indeterminate confubulation! Moreso since somebody else gets to pay.

FR Millmore

This thread was discussed between 10/05/2012 and 16/05/2012

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