Over the years, I have acquired many Mk1 OHC Velocette
bikes, all as “projects”, and many other assorted bits and pieces. And so, rather
than attempt to make the upper-part of the engine, ie the cylinder/head/cambox
an easier solution would be to use Velocette parts, if at all possible, and
graft these onto AJS crankcases.
However, AJS crankcases of the period are either very
basic - the SV engine uses bushes rather than bearings for the crankshaft and
are easily obtainable, or quite sophisticated - the OHC engines have strong
crankcases and support a 3-bearing crankshaft, but impossible to obtain
secondhand.
Nevertheless, AJS engines evolved fast during their days at
Woverhampton and by the time the marque was sold to the Collier Brothers
in 1931, the 350 Big Port (which, by this time, had been officially designated
this moniker by AJS) had been developed into the SB6 which had a 3-bearing
crankshaft not dissimilar to the K7 of 1928. By a stroke of luck, a good friend
of mine (JT) just happened to have a pair of SB6 crankcases in his shed and so
the first mock-up of an AJS crankcase/Velocette barrel combination was tried.
It transpires that if a pair of SB6 crankcases are spaced
apart by about 0.4” then the spacing of the cylinder base retaining studs across
the engine matches that of a Velocette Mk1 OHC cylinder barrel. And if the
cylinder base flange holes of the barrel are elongated by ~ 0.020” in the longitudinal
direction then the barrel will slide on to the retaining studs easily.
The first step, therefore, was to make a suitable spacer
to move the crankcases apart properly (rather than a bunch of washers that had
been used for a ”what-if”). Luckily, these crankcases have a locating flange
when they were originally machined. The advantage of this from a production
perspective is that if the crankcase mating surface, the flange and the main
bearing machining is carried out in one operation but independently on the drive-side
and timing-side crankcase halves, then one crankcase half can be matched to any
other crankcase half and the main bearings will be in alignment. It is
therefore possible to machine a spacer that is also flanged, with male and
female sections on appropriate sides, and that will then maintain the concentricity
of the drive-side and timing-side main bearings with the spacer installed.
The plan, therefore, was to use a large piece of 7075T6
aluminium as a spacer by machining the final thickness and the male/female
parts of the locating flange in the lathe and to then send the assembly to a
local company to water-jet cut the outside profile of the crankcase.
A sizeable piece of aluminium plate was therefore mounted on the faceplate of the lathe (for reference, the faceplate is 15” diameter)
and machined.
And after water jet cutting to the outside profile of the
crankcases….
The spacer and one of the crankcase halves was then
mounted in the milling machine and the holes for the crankcase studs were machined and the
excess material in the cylinder bore was removed.
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