Monday 22 June 2020

The AJcette Project ….how it started ….part 2

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.

At the same time, a slitting saw was used to remove a thin “slice” from the timing case surface that would later be used for part of the casting pattern for a new inner timing case.

Note the number of machined “holes” in the timing-side crankcase. There are holes for inlet and exhaust camshaft bushes and the cam followers/tappets in the OHV engine. None of these are needed in the OHC engine and need to be filled before work can be started on making a new OHC drive.  

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