Twin Overhead Cam Head Drawings.
following is an overview of 27 drawings, most of which are signed by Harold
Hunter describing his twin overhead cam conversion for the Austin Healey
100. None of the drawings include a title block describing part number,
revision date, material of construction etc. usually found on engineering
drawings, in fact only one drawing is dated (May 23rd 1955). This makes
the task of allocating order to the design process quite difficult as many
of the drawings appear to be "thinking on paper" and involve variations
in design detail of the same components. The drawings do not fully describe
the prototypes that can be seen under Hunter Head Images on my index page,
this comes as no great surprise as low budget development work quite often
involves the make it now and draw it later principle. In fact to quote
Geoffrey Healey discussing the development of the Healey 100 from his book
The Healey Story "many of the parts were made by Roger (Menadue) without
drawings and were then drawn". While some drawings deal with machining
of components, the drawings that deal with the castings appear to have
been produced to provide the necessary information to produce the patterns.
Only basic machining dimensions are shown and tolerances are rare. Hunter
described the four heads produced as prototypes, and looking at the drawings
it becomes clear that only enough was done to make it run. Having spent
the last thirty years involved in engineering, through manufacturing, foundries,
design and prototype construction, in my opinion, (others may disagree)
what Hunter produced was a successful design, ready to be fine tuned into
a production model. It is unfortunate that this didn't happen considering
the time, effort and expense that must have gone into producing the
prototypes, and a shame that his work was left to judged by them.
part of my ongoing interest in the Hunter design I intend to faithfully
reproduce the drawings in digital format (autocad) as time permits.
The drawings describe a
twin overhead cam cylinder head and cam drive arrangement to suit the Austin
Healey 100 engine. The design incorporates a 60 degree included angle between
the cam boxes which are cast integral with the head. The valve gear arrangement
is similar to that seen on Jaguar engines of the period, having an adjustment
shim sitting directly on top of the valve stem. The shim in turn sits beneath
a tappet which in turn sits directly below the cam (see image).
These tappets appear to be running
directly in the head. However many aspects of the design are not shown
in the drawings including the camshaft bearings ( if there were any ) so
it is dangerous to make too many assumptions! It should also be noted that
materials of construction are rarely mentioned an exception being one of
two camshaft drawings which calls for "Atlas SPS245".
The two overhead cam shafts
are driven by the existing camshaft via two duplex chains of 3/8 pitch.
The first runs on a 23 tooth sprocket (Healey crank sprocket) attached
to the existing cam and sprocket by an adapter. In some of the arrangement
drawings there appears to be a fairly crude attempt at incorporating a
timing adjustment in the adapter design by means of tapered seats for the
sprockets. I should mention here that the drawings depict 5 variations
in this primary drive and chain cover arrangement between the existing
cam and overhead inlet cam. The variations mainly involve the chain cover
design, either a single cover over the chain from the existing cam to the
overhead inlet cam or the two piece design seen in the photo's, the latter
probably to allow for changes in either head height or variations in the
block height. Many of these drive and cover variation drawings are only
partly dimensioned, perhaps indicating my thinking on paper scenario.
In some drawings the exhaust cam is shown extended through the front chain
cover and fitted with a pulley to drive the water pump and generator. In
these designs the crankshaft fan belt pulley is removed with the cast timing
chain cover blanked off over the end of the crankshaft. None of the drawings
show the adapter for the distributor drive off the exhaust cam as seen
in the photo's of the head. Some drawings also show the cams supported
in bushes in the front chain cover. Two 23 tooth sprockets are fitted to
the front of the overhead inlet cam, the front sprocket being the primary
drive from the existing cam, the rear driving the exhaust cam also fitted
with a 23 tooth sprocket. Other than the rubber ring tensioner fitted to
the existing cam sprocket, no means of dealing with adjustment of the two
additional chains is described or it appears allowed for.
The combustion chambers
are of hemispherical design at 1 15/16 radius from a point 7/8 below the
head gasket face, giving a diameter at the gasket face of 3 7/16 inches.
The distance from the cylinder head gasket face to the centre of the cams
is 6 1/8 inches. The spark plug holes are shown in one drawing at 10 degrees
angle toward the front of the head and at 15 degrees in another. It appears
that 3/4 inch reach plugs were allowed for. While the valves are
shown at the centre of the combustion chambers the spark plugs are toward
the front. Four inlet ports are shown on the left side of the head at 1
1/2 diameter at both the manifold face and valve seat. The inlet ports
sweep toward the rear of the head from the manifold face. There are also
four exhaust ports on the right side at 1 3/4 diameter at the manifold
face and 1 7/16 diameter at the valve seat. The exhaust ports are at ninety
degrees from the centre line of the head. Valve seats are shown cast
in place, with the head material surrounding the seats on three sides.
While the design is based on the 100/4 head stud pattern, one extra stud
has been added between cylinders two and three 2 5/16 inches from the centre
line of the cylinders on the inlet (left) side. This stud emerges through
the top of the head between the cam boxes as do all the right hand side
studs, whereas the rest of the left hand side studs emerge in small cutouts
between the inlet ports low in the side of the head. Oil supply to the
valve gear is shown as a hole emerging in the bottom of one cam box, how
this supply is directed to the cam bearings is not shown. A 1 inch oil
return hole is shown in the front of the head for oil to return to the
sump through the chain cover system. Water return is also shown as a 1
inch hole in the front of the head. The location of both these holes varies
in the two versions of the cylinder head drawings.
There are two drawings detailing
two versions of the camshafts. Both drawings depict the camshafts as have
three bearing journals of 1 1/4 diameter and a 1 1/2 diameter shoulder
each side of the centre journal to deal with end float. While cam lift
is specified on both drawings, duration is not specified.
In one drawing the cam lobes
are shown at 5/8 wide with a lift of .406 inches. No centre hole is shown
for lubrication and material is not shown. The exhaust cam is extended
through the front cover in this version.
The other camshaft drawing shows
the exhaust cam dimensioned to accept only the sprocket, and as such does
not fit with any of the designs or the photos of the head produced. In
this drawing the cam lobes are shown as 3/4 wide, with a lift of .3805
inches. A 3/8 diameter centre hole is shown with cross drilling for bearing
lubrication, and a 1/32 hole in the heal of each cam lobe, to direct oil
to the tappets. Material is listed on this drawing as "Atlas SPS245".
The pistons are shown as
domed with an overall height of 2 1/4 inches from the centre of the 7/8
diameter gudgeon pin to the top of the crown. The dome is shown as 1/4
inches high. Piston diameter is 3 7/16 inches and piston length is 3 9/16
excluding the dome. Dimensions are given for three piston rings.
In closing I would like to that the friends that have
provided information on Hunter and helped me to assemble the information
that I have so far.
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