"How To" by John Bowring
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This is the 1st of a series of “How to” articles. Click here for the 2nd

How to make an oil tight joint between the cylinder block (made of cast iron) and the engine crankcase (made of aluminium).

For all of you who do not have a PhD in physics the expansion ratio between aluminium and cast iron is somewhat at odds. Imagine a length of aluminium bar any diameter you wish, but exactly 12” long, cold. When liquid it will be 12 3/16” long. Cast iron in similar configuration will be 1/32” longer. These different rates of expansion cause no end of conflict. The iron block is fastened to the aluminium crankcase by 8 studs 5/16”f when both are cold (room temperature). As the engine warms up to an operating temperature of just under 100°C (at which water boils) the crankcase is in serious tension on its top deck as the rest of the crankcase is free to expand as its temperature rises.

Putting all this to one side, temporarily, the physical manner in which the two are joined has to be looked at.

Firstly, when new, both surfaces are flat and smooth and the studs are a snug fit in their respective tapered holes. The secret of an oil tight joint is the correct gasket, one that allows the crankcase to move (expand) whilst still being held firmly to the block. No jointing compound is needed when surfaces are new or have been machined to as new condition. Making the crankcase too “as now” has many taps for the engine re-builder. Firstly, if too much is removed from the top of the crankcase the piston will protrude beyond the top of the block and could be in danger of hitting the cylinder head, not only that but the mesh of the generator gear will also be altered, in the worst cases stopping the engine from turning over when the generator is fitted in position.

How much then do you remove from the top of the crankcase?

Machining 0.010” to 0.012” from the top of the crankcase should be sufficient to restore the top surface to achieve an oil tight joint; it will not however clean up the whole of the top surface. At the front end of the crankcase on the camshaft side there will be left what in engineering terms is called a “witness”, in the shape of two sides of a square prescribed by a circle on the diagonal. At the rear on the same side a similar witness, this time around the hole for the oil pump spindle cover.

Has the crankcase been machined before?

The two ways to tell are –

Examine the engine number (driver side rear). If the numbers are bold and deep chances are it is still original. Should the numbers be thin and hard to read, go to the top front of the crankcase and look for a plus or minus stamp together with a number, these numbers refer to the gear centres, camshaft to generator to allow the purchase of the correct timing gear when replacing it. Should you have a combination of no numbers at the front and thin hard to read engine number then some one has been at it before you.

All is not lost. Help is at hand. Club Spares have, or are having made, a 0.010” stainless steel spacer to be used in conjunction with 2 gaskets, no jointing compound, to make up for the missing top deck material. All these allow you to take another .010” off the top of your crankcase. This is the Rolls Royce Repair, it will never leak again (Sorry, I lie).

Next time the Mickey Mouse repairs.

John Bowring Next