Friday 9 October 2020

The Crankcases – Part 2: Completion of the Patterns

There are 3 outstanding tasks to complete the patterns:

1)    Make and attach all the detailed parts – lugs, ribs, external oilways and the timing case

 2)    Use body filler to merge in smoothly all of the attachments and fill all unnecessary holes

 3)    Smooth and paint the final patterns ready for the foundry

The first parts to attach were the main lugs for holding the crankcases together on assembly and supporting the engine in the frame. The lugs, 2 for each side and 8 around the periphery, were machined from aluminium with a 1 degree draft angle and then attached to the main crankcase body with JB Weld. To ensure they were aligned properly and at the same radius, a simple jig consisting of 2 pieces of aluminium strip with a hole at each end was used to position each pair of lugs. The aluminium strips were then simply rotated about the central dummy shaft with each pair of lugs in turn sandwiched between the holes.

Rather than using JB Weld, which is a strong adhesive but not so easy to rub down and contour to a smooth finish, multiple applications of P38 body filler were used to fill in the gaps and to fill all unnecessary holes, including those that were used to hold the round bar on the faceplate for the initial machining.


The final step was to add the ribs that support the drive-side main bearing boss, the external oil feed and the timing case. All of these additions are made out of wood and are attached and blended using the same methods as the lugs.

After final rubbing down, both crankcase pattern halves are sprayed. It is customary practice to use different colour paints to indicate the purpose of different surfaces on the patterns, for example, whether or not a surface is to be machined. This is an aid to the foundry to indicate where the sprue and riser should be (or not be) attached. The foundry that I use, Newpro, has so much experience and the quality of their work is so good that I have found it unnecessary to use a paint code and I just provide the patterns painted black.

It’s interesting to compare the crankcases of the original AJS V-Twin with the patterns here. The picture below is a photograph of the drive-side crankcase of original bike that I took on a  visit to the National Motorcycle Museum.

The only significant difference is that the original crankcases have provision for independent oil feeds to the base of each cylinder barrel. This aspect of the design has not been carried over to my engine as I will be using an oil pump with much higher delivery and piston/cylinder lubrication is not anticipated to be a problem. Unfortunately the bikes are squeezed in so tightly that it was not possible to photograph the other side easily.

And finally, a T-bar is made to screw into inserts that have been fixed into the main bearing centres to facilitate withdrawal of each pattern from the mould at the foundry. The patterns are quite heavy and this helps the foundryman to avoid knocking off bits of sand and damaging the mould.

  ….and then off to the foundry. The patterns were delivered for casting in August 2019 on the way to Heysham to catch the ferry for the Manx GP.

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