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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
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