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MG MGB Technical - Lots of Crankcase Pressure

The stage 2 engine in my 76b has a good deal of crankcase pressure. The engine has less than 10k on the rebuild and is fully broken in. Did a leak down test and everything appears normal, but, when the oil fill cap is removed, there is a large amount of pressue. The car developed a massive rear main seal leak (exxon valdez type)- i'm thinkin that the high pressure is the cause.....anybody have a better guess?

rn
RN Lipow

Excessive pressure in the crankcase would definatly blow out the seal. You need to check to see if your crankcase is vented. Most cars have a vent in the valve cover and a vent on the side of the engine where the pushrods are. If you have these two vents then you should have no problems. It is possible that the vent in the side of the engine is blocked. If I remember correctly, there is a check valve on the side vent, this valve may be stuck closed.
Robert

it is vented. that's what's stumped me.

rn
RN Lipow

You seem to rule out excessive piston blow by. How is you vent system set-up? Stock?
Andrew Blackley

tappet cover to the hs6 carbs, rocker cover to the charcoal canister (and it's not blocked)...i just can't figure out where the pressure is coming from, it doesn't burn oil, has great acceleration and now, it's getting 10 miles to the quart of mobil1

perhaps i should pull the engine, inspect the seal and examine it with an eye to adding a speedy seal



rn
RN Lipow

RN. Your leak down test would seem to indicate that the bores are sealing properly and that there should be no excessive blow by. However, is this really true? It is my understanding, only from reading, not from personal experience, that the 2L conversions can have some cylinder side wall flex when the firing stroke takes place. Could this be happening and leading to pressurizing the crankcase beyond what the PCV system is capable of handling? These engines are fairly uncommon. I suspect the engine builder, if still available, would be the best one to contact about possible problems. Peter Burgess and Chris Betson, as professional engine builders, would probably also have some good insights.

Frankly, I do not see this as a "speedi sleeve" type thing. The speedi sleeve is added to make up for a groove worn in the rear sealing surface of the crankshaft. A less expensive alternative to welding and maching the surface to build up the groove which wears due to the seal pressing on it. From all you have written in the past, this seems to be a quality rebuild. As I remember it, when you had problems with the original engine, the rebuilder sent you a second engine, under warranty, at their expense. Thus, I would tend to focus on it being a problem beyond that which would normally be addressed during the rebuild and concentrate on either the PCV system not being capable of handling the crankcase pressurization, or there being an excessive crankcase pressurization problem beyond the capablilityes of a properly functioning PCV system. I am not, however, how this can be checked out in situ, although on an engine test bed I can see how it would be possible. Les
Les Bengtson

Could be you just don't have enough breathing for the engine. Many cars have the same problem. Ideally you should have a 'vacuum' source remove the blowby like connecting the hose to an air filter or into the intake. K&N breathers are notoriously bad for most engines and can cause severe rear main seal leaks in many engines. Running more efficient breathing = more HP, which is why the SCCA won't allow more crankcase venting in some classes. A friend built a vintage race car and was dumping about a quart of oil through the rear seal per race/practice. I thought about it and realized his breathers were way too small. The next time he raced after replacing the oil catch cans his engine stopped blowing oil and he was faster too.
M Landskroner

RN,

Is the pressure there at idle?

Does it increase or decrease as you rev the engine?

Static leak down tests may not be conclusive - if there is excessive side clearance on the rings they will seal when stationary and leak when moving.

If I remember this is the 1950 st 2 engine from EMS?

I would check that the front side cover is clear by leaving the oil filler on and pulling the large rubber pipe off the front cover - is there pressure coming from the cover?

It is NOT neessary to sleeve the rear oil seal surface, either fit a new seal - black viton one rather than the red sort - so it runs in the groove or just have the surface skimmed - no need for welding etc. The wear is typically 2 or 3 thou which is well inside the seal's tolerance.
Chris Betson

Les and all,

EMS went out of business in November 2001.
Chris Betson

thanks all for your responses...i'll check tommorow.

yes chris, this is the 1950 st 2 engine from EMS, and i knew they went out.

rn
RN Lipow

mystery solved. removed the cylinder head today and....it's the head gasket and it's clearly failed. the piston rings look fine and the cylinder bore is ok, bearing show little or no wear. i did notice that the exhaust valves have a bit of crud though.

going to check for the flatness of the block and the head, have it reassembled and away i go.

thanks for your comments.

rn
RN Lipow

oh yeah, the rear main seal is intact but was clearly the site of the source of Mobil Oil Company profit as it was leakin like the proverbial sieve.


rn
RN Lipow

This thread was discussed between 22/04/2003 and 25/04/2003

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