The Magnesium Engine Block In Your Porsche 911

The 911 engine has been around since 1964 and in various modifications of the basic design, was used all the way to the water cooled era. Its horizontal architecture layers various parts to build a complete engine. Wherever those parts have a sealing surface, is a chance for leaks to occur.

Starting in 1964 Porsche produced the engine blocks in cast aluminum. Though heavy, these were very stable and typically lasted the lifetime of the car, and beyond. These were the best for longevity and in fact Porsche brought the aluminum block back into production with the Carrera RS 3.0, Carrera 3.0, 76-77 930 Turbo, and of course the 911SC through the 3.2 Carrera of 1989.

Starting in 1968, the engineers came up with a different casting technique using magnesium, for the block. The first production cars were offered to the public then and with this architecture / design it continued through 1977.  The initial displacement was 2,0 liters and eventually the design worked up to the 2.7 liter .  As an aside, the Factory race department, and others were able to squeeze all the way to 3.0 liters using this block.

Magnesium is an interesting metal: Light in weight, excellent temperature transferring properties, easy to machine, stable….. to a point, and… porous. These last two items are the subject of this essay.

With the higher mileage 911’s still on the road, we have been observing the failure of these engine blocks. To start with in the mid 70’s , the higher operating temperatures that the lean burning USA smog legal motors were running  yielded massive oil leaks  and exhaust leaks from the cylinder head studs pulling loose from the block.

The ends of the studs were covered with magnesium that separated from the threaded holes. Porsche did cover some of these under warranty back in the day. And they changed the type of cylinder head stud, so the alloy matched the expansion rate of the rest of the motor. But they still failed. Now we are observing broken head studs from corrosion too.  Other studs that are in the block tend to pull from the threaded holes as well. Another problem of the magnesium block is the shift in case halves / warpage that can occur. Excessive off set wear of the bearings is observed when this happens.

Machine shops to the rescue, or how to properly rebuild a magnesium case engine: When rebuilding the magnesium cased engines, the block is sent to a reputable experienced machinist. The block is thoroughly cleaned and inspected for damage and flaws. Next, the block is fastened as if the crankshaft was installed. The crankshaft bore is measured to verify that it is straight and correct dimensions. If not, the machinist does his magic to make it so.

Afterwards the deck on each side, where the cylinder liners sit in the bores, is checked to make sure the bores and deck surfaces are square to the crankshaft center line.Before rectifying this, the head stud mounting holes are drilled, oversized steel inserts are installed. these inserts have a larger surface area than the original hole, so allow more grip for the head studs in the “fresh” metal.  There are other holes in the block that get the same treatment with larger steel inserts. Afterwards, the individual cylinder holes get machined for roundness and alignment with the crank. Finally, the block gets thoroughly cleaned and sent back to us for inspection and assembly.

Using the finest available sealants , quality seals, and gaskets. And following the correct assembly procedures, one would expect the rebuilt motors to be nice and dry/ free of leaks. USUALLY this is the case, but with the constant expansion and contraction of the motor in operation, we have been seeing some leaks due to Porosity of the blocks. We certainly share the frustration as it takes so many hours to assemble a motor, only to find out that it starts leaking.  The only remedy, when this occurs, is to apply sealant on the outside of the block and hope for the best.

Block failure: The latest design of the magnesium block was known as the 7R casting. Porsche included extra ribs on the casting and thickened other areas slightly,  in their effort to improve the design. These were produced from 1973 till the end in 1977. We consider these the best choice if an aluminum block is not available. Still, we have seen the oil leaks occur from time to time.

For the above outlined reasons, no repair facility can absolutely guarantee a leak free motor. Note that some of these leaks can be minor and somewhat rare. However high mileage engines, ones that have been raced, and ones that have been through years of California smog compliance will fail at a higher rate than others.

With the value of certain models, owners of these leak prone cars need to be aware of the “charm” of the older engines. There is no degradation in performance, but your garage floor may show where you’ve parked it.

(revised 8/5/17)