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Technical Information (MGB 9)

MGB Engine Oil Usage

Oil Leakage (visual)
External oil leaks can be divided into two groups and usually occur at the following locations:

Engine Oil Pressure Leaks:

Rear Engine Main Oil Seal
Oil Pressure Gauge Flexible Line
Oil Cooler & Oil Cooler Hoses
Block to Oil Filter Housing Hose (non-oil cooler)
Oil Filter Housing Base "O" Ring
Various Copper Washer & Brass Plug Fitting Locations

Crankcase Pressurization Oil Leaks:

Side Plate Cover Gaskets
Front Timing Cover Oil Seal & Gasket
Oil Pan Gasket
Valve Cover Gasket

Oil Leakage (visual upon examination)

Internal oil leaks, although not very common, can occur usually into the cooling system, although a more common problem is for the coolant to enter the oil pan. The usual cause, in either instant, is either a cracked cylinder head or a head gasket failure. An extreme situation that I have come across is oil actually in the charcoal canister (1970 on) due to excessive crankcase pressurization.


Oil Consumption

This may or may not be seen as a "blueish gray" smoke at the exhaust tailpipe and will depend upon the actual amount of oil being consumed.

PCV Valve System (1965 thru 1968)

This system consists of a diaphragm control valve connected by hoses between the inlet manifold and the engine front side plate cover. This cover incorporates an oil separator to prevent oil being drawn, along with "blow-by" gases leaving the crankcase. As gases are withdrawn by inlet manifold pressure, a vented/filtered oil filler cap provides a supply of fresh air through a restricted orifice (9/64" dia.). The PCV valve diaphragm varies the opening to the inlet manifold according to the depression or crankcase pressure acting upon it. With a decrease in manifold depression or an increase in crankcase pressure, the diaphragm opens the valve allowing these crankcase gases to be drawn into the inlet manifold. During conditions of high manifold depression, such as idle, low engine speeds or loads, the diaphragm closes the valve and restricts the flow into the inlet manifold thus preventing a "leaning off" of the air/fuel mixture.


First and foremost, the front engine side plate cover location is a very bad area from which to draw crankcase gases. Although this cover incorporates an oil separator in its design (wire gauze), oil being drawn along with crankcase gases into the inlet manifold via the PCV valve, was a always a problem. Add to this the fact that gases being drawn from the crankcase are always influenced by inlet manifold depression, it is easy to see that this leads to oil "puddling" within the PCV valve.

Carburetor Control System (1969 onwards)

With the introduction of this system, the front side plate
cover outlet was now connected to the "Y" fitting from which hoses are routed to each individual carburetor. These carburetor connections are positioned between the piston and the throttle disc. Gases are drawn from the crankcase via the depression in this chamber. Increased throttle disc openings along with increased depression in the chamber allows a greater volume of gases to be drawn into the inlet manifold. The oil filter cap, as mentioned in the previous paragraph, is retained and works on the same principle. In 1970 a non- vented oil cap was introduced with breathing now taking place through a charcoal canister. This system stayed in effect until 1974, and was retained for the Zenith Stromberg carburetor equipped vehicles (1975 onwards). A redesigned front side plate cover was introduced on 18V engines to provide better oil separation and drain-back.

With both system designs, oil consumption (via the induction system) can be greatly influenced by excessive "blow-by".

Oil Consumption (cont'd).....

Worn valve guides and valve stems will contribute considerable to excessive oil consumption. New silicon valve guides should be installed and "hone" clearance fitted to their respective new valves. Positive type valve stem oil seals should be the only type fitted. If you are running duel valve springs, you will need to have the valve guides step machined to accommodate the teflon type seal; this will ensure clearance between the inner valve spring and seal. However, if you use factory P/N 12B2104 seal, this machining will not be necessary. Never use the factory type "O" ring P/N AEK113 alone, although this seal could be fitted in conjunction with 12B2104 seal. Always install valve stem oil seals to both intake and exhaust valves.

Cyl/bore & Piston Rings

We can divide this subject into two specific problems, first.. Oil Migration (combustion chamber)
Excessive oil film thickness deposited on the cylinder walls will allow the piston rings to hydroplane on this oil film, which allows oil to migrate into the combustion chamber to be burned and can be caused by one or more of the following:

  • Oil pan overfill
  • Wrong cyl/bore cross hatch honing method (too flat causing oil "puddling")
  • Inadequate piston oil control ring wiping, due to low tension and probably due to excessive oil rail end gaps.
  • Piston skirt fit in cyl/bore (excessive clearance) resulting in piston skirt not effectively shearing the oil film, thereby allowing oil to be scraped to the combustion chamber on the upward stroke.
    Worn or tapered cyl/bores.
  • Cyl/bore distortion.
  • "Windage" due to wave motion of oil in the oil pan being whipped up and deposited onto the cylinder walls as in the case of oil pan overfill.
  • Piston tilt due to piston skirt design protruding below cyl/bore which results in the piston ring's inability to provide proper contact with cylinder walls.


Excessive "blow-by" combustion gases are allowed to escape past the piston rings on the power stroke, creating excessive crankcase pressure build up which in turn forces gases, oil vapors and oil through the breather system to be burned in the combustion chamber via the induction system. This excessive crankcase pressure will also actually force oil and oil vapors past the oil control and compression rings on the induction stroke especially under high manifold compression conditions. The amount of oil migration to the combustion chamber under these "blow-by" conditions is open to debate. Several experts I have spoken to gave contrary opinions on this problem.

"Blow-by" conditions can be caused by the following:

  • Piston rings not seating.
  • Glazed, worn or tapered cyl/bores
  • Cyl/bore distortion
  • Broken piston rings
  • Excessive ring-end gaps
  • Misaligned connecting rods

Thermal Bore Distortion

Generally, this will not be a problem when reboring up to and including .060". However, it could present a problem on "large bore" engines. This cyl/bore distortion is greatly exaggerated under running conditions, therefore, if you are going to proceed with "large bore" configurations, steps must be taken in the reboring process to minimize this distortion. A "torque plate" must be fitted to the engine block and torqued down to the cyl/head appropriate ft/lb specification. This will "stress" the engine block and is the best we can do to simulate actual running conditions. This "torque plate" must also remain in position for the final honing finish.

Cyl/bore Refinishing

Cylinders must be plateau honed finished and must follow the correct cross hatch pattern; two flat of a cross hatch angle will result in oil "puddling" and excessive oil consumption. Too steep of a cross hatch angle promotes oil migration down the cylinder, resulting in a too thin oil film which can cause piston ring and cylinder scuffing.

Cyl/bore Cleaning

The single most critical factor of any cylinder refinishing job is the final cleaning of that particular cyl/bore after the honing process. Proper cylinder cleaning consists of a thorough scrubbing of the block with hot soapy water, taking care to clean the surface under the cylinder facing the crankcase. Rinse with hot water, air dry and oil lightly to prevent rust.

Piston Ring Recommendations

Top compression ring (Molybdenum) should be barrel shaped. This barrel shape condition is what you would normally find on a regular seated ring. This shape is the current manufacturing style and its fine line contact with the cylinder wall promotes rapid piston ring seating.

Intermediate compression ring should be a reverse torsional taper face ring. This ring must seal compression and combustion gases and also assist the oil control ring in scraping excess oil down the cylinder wall. The ring's taper face also offers line contact with the cyl/wall and rapid seating. This ring design also prevents oil from getting around and behind ring assembly.

TOTAL SEAL intermediate compression ring - this is a regular compression ring that has been machined to accept a narrower width ring which rides on the underside of the main ring. Although more costly, we highly recommend this setup.

Oil Control Ring

This should be a low tension assembly with a good drainage expander ring held in position by two chrome scraper rails.

The piston rings, which are supplied with British Automotive's JE custom made forged pistons, are checked for end gap accuracy in presized sleeves and are guaranteed to be within specifications, however, you should check for the correct end gap in their respective cyl/bores and for any reason should these end gaps be greater than that allowed, that particular cylinder(s) in question would have been in all probability over bored.

Piston Ring Assembly Procedures

Never spiral compression rings into piston ring grooves. Always use the appropriate tool. Oil control rings can be spiralled into position. Oil should be applied to the piston rings and cyl/walls. From past experience, this is the only method I will ever use. I departed from this procedure several times using dry and semi-dry techniques, as recommended by "so-called" experts, with costly results.

Fuel Wash Down

This is extremely important. Under no circumstances allow excessively rich fuel mixtures or flooding conditions to occur. Overhaul carburetor(s) and choke mechanism where necessary. If you are using Weber, DeLorto or Mikuni carburetors, place them on one side and use the original SU carburetors until the piston rings have seated. NOTE: DO NOT RUN RICH MIXTURES.

Oil Consumption

Oil consumption can vary under different driving conditions. I consider the following figures to be a generalization:

Under 300 miles per quart - very poor
350-500 miles - poor
500-600 miles - fair
600-750 miles - good
750-1000 miles - very good
1000 miles & over - excellent

However, we are concerned here with oil consumption and its relationship to the engine rebuilding process. There is absolutely no guarantee of very good to excellent oil consumption mileage figures unless good engine rebuilding procedures are undertaken and piston ring seating recommendations strictly adhered to.