Spent a very enjoyable and informative morning watching the first of my bellhousings being sand cast. Having decided to produce a limited run of "customer cars" built using the same tools, techniques and the same meticulous level of detail as the first car, I needed to have a batch of bellhousings made. I drew up these items in CAD referring to original data as well as measurements taken from one of the very few originals. I had to modify the model to suit the later SOHC 5.3 and 6-litre blocks as well as my modified Quaife "ZF" 5-speed transaxles, but they do have a similar cosmetic appearance to the original prototypes. Items such as these sadly aren't available "off-the-shelf" - mostly because of the unique way of mounting the engine in the monocoque and the need for attachment points for a framework of tubes attaching the drivetrain to the ends of the sills. CNC machining from a solid billet would have been an option but sand-casting is closer to the process originally used as well as being more cost-effective for multiple items.
I started with a CAD model
The next stage in the process was to make a series of moulds and patterns. Traditionally, this would be done in wood by skilled pattern-makers but nowadays this has been superseded by digital CNC machining methods. The following pictures show the digital casting patterns produced from my CAD data:
These patterns were then produced by CNC machining resin blocks. We were now ready for casting ...
I was joined at the Foundry by Peter Wilson (author and one of few surviving members of the original project team) and Nigel Boycott (engine builder and the person entrusted with the assembly of my SOHC engines). The following video shows what we witnessed:
A few days later, the sand was removed from the castings to reveal the following:
The risers etc were then machined off and the final casting was ready for x-raying, dimensional checks and heat treatment. LM25 alloy was used and heat-treated to TB7 specification which gives strength coupled with a degree of flexibility to accommodate the loads this item will be subjected to.
The bellhousings are now ready for final machining. It was fascinating to see the technologies used - very much a case of "old meets new". Sand-casting is, after all, the second oldest profession ;)
The following pictures show the finished machined item in situ.
