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 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 being taken from
the cylinder drawing. 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, spot the positions through onto the cylinder, Photograph 34, then remove
and drill (2.1mm) and tap the cylinder 7BA.
As mentioned 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 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 check once more.