Friday 10 July 2020

Making the Timing Case Castings: The Patterns – Part 2

Having the ability to make multiple copies of the original timing case castings in resin provides a straightforward means of making patterns for new castings that embody the features of the original engine. Nevertheless, there are differences that must be incorporated into the new patterns, in particular, the centre-to-centre distance of the overhead cam chain drive is about one inch longer and the mating surface to the crankcase is a quite different shape.

The process adopted here for making the patterns is:

     1)    By making a number of resin copies, these can be cut up and “stuck” to other pieces to develop the overall shape and dimensions. JB Weld is used for attaching pieces of resin together.

      2)    The chain drive is left is place with its dummy camshaft spindle to ensure that the inner timing case is in the correct place. The outer timing case is made to fit the inner when the latter has been completed.

      3)    The mating timing case-to-crankcase surface is made by merging the thin slice of aluminium casting taken from the original SB6 engine crankcase to the resin copy of the inner timing case.

      4)    A strip of metal is used along the long vertical side of the resin pattern to ensure it stays straight and does not warp.

      5)    Small pieces of wood are used to incorporate, for example, bosses for screws.  

      6)    P38 body filler is used when the main body of the pattern is complete to blend in curved surfaces and to fill pinholes and sections of the resin that did not cast so well.
      7)    When the resin + aluminium + JB Weld + wood + body filler is complete, the mating faces (ie, those to be machined) of both patterns is coated in epoxy adhesive and laid on a large piece of thick gasket paper. When the epoxy has set, the gasket paper is cut away with a scalpel. This adds an additional  ~1mm of thickness to the face and is repeated a few times to build up the height of the surface to ensure that there is sufficient material on the casting for machining.

The series of pictures below shows the patterns in stages of completion

And finally, the patterns are taken to the foundry and cast in LM25 aluminium. The castings are then heat treated by the foundry to the TF condition (maximum heat treatment) and this effectively doubles their strength.

On the topic of casting, it is worth a few words about shrinkage. If you google "aluminium sand casting shrinkage" then you will get the response "aluminium shrinks by over 6% during solidification". I do not know where this information comes from or how it is defined. Even if a value of 6% is taken as the volumetric shrinkage, this would correspond to a linear shrinkage of over 1.8%, which is too high.

I have seen practical linear shrinkage values quoted at around 5/32" per foot which equates to 1.3%; this agrees with my experience. Comparing the dimension of the long outer timing case between casting and pattern, shown below, gave measurements of 433.5mm and 439mm, which corresponds to a linear shrinkage of 1.3%. The smaller dimension across the timing case gave a shrinkage of 1.4%.

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