The size and position of the rectangular aperture in the valve chest is not critical
but it would be good practice to make it reasonably central, both in width and height.
Because of this, take account of the requirement when machining the first side so
that after machining the second both sides are nominally the same thickness and therefore
the aperture central, to a rule dimension will be more than adequate. Similarly,
for the height.
For those not conversant with the above method of using a four jaw, some guidance
may be worthwhile. Its use should be limited to holding parts that use all, or almost
all, of the depth of the jaw, using it to just grip parts at the jaw's tip is definitely
The part should first be gripped using two opposing jaws followed by the other two.
However, when using the second set of jaws it is not that simple because if the first
is over tightened it will tend to twist the workpiece. This can be largely avoided
by moving the first jaw so that it just touches the part and then doing the same
with the second, followed by adding a little more grip on each jaw in turn.
Now, mark out and drill the four 7BA clearance holes, the position of which must
be taken from the drawing for the cylinder. With that done, position it onto the
valve chest cover, clamp in place and drill the cover, Photograph 33. Also clamp
the valve chest onto the cylinder and spot the positions through onto the cylinder,
Photograph 34, then remove and drill (2.1mm) and tap the cylinder 7BA.
As mentioned with regard to the cylinder you can omit the 1/4” x 32TPI tapped hole
if the engine is to be a display model only, otherwise drill and tap it now.
Mark the rear face of the valve chest cover around its edges with marking blue and
assemble with the valve chest using four of the 7BA studs provided, ensuring that
the cover is centrally placed. Using a sharp Stanley type knife, scribe around the
edge of the cover, remove and carefully file up to the lines produced, not beyond.
Final matching the cover to the valve chest will be done at the finishing stage.
Having recently made a filing machine I took the liberty of digressing from the method
I am expecting the viewer to use so as to put it through its paces, Photograph 35,
using a hand file will though be perfectly acceptable. If you are interested in the
machine, photographs are shown here.
Due to the casting process the bosses on either side of the casting are very slightly
tapered making it difficult to be assured of a secure hold in the chuck. As an aid
to achieving this, wind a piece of soft copper strip into a ring and then place this
onto the boss and grip it in the
chuck. Before fully tightening it, check the distance between the side of the flywheel
and the face of the chuck at various points around the circumference to ensure that
it is running true, within a few thou. With that done give the chuck a final tighten
and perform the check once more.