Saturday 3 October 2020

The Crankcases – Part 1: Start of the Patterns

When I started this project I realised that there would be no “off-the-shelf” crankcases that I could use. Whilst plenty of V-Twin engines have been made during the past 100+ years the appearance of the engine reflects the chain-driven overhead camshafts and is a very important feature of this bike. I could not see anything that had been made in the past that even vaguely resembled the design that I wanted.

Looking at various crankcases at autojumbles, I could not find any crankcases that would be a good starting point for a pattern. I was quite prepared to hack a set of clapped-out JAP cases, for example, but there was nothing that was really suitable. The decision was therefore made to start from the beginning and make the complete patterns.

Casting patterns are traditionally made out of wood but I am not really a woodworker. Yes, I can put up a shelf and a new handle on a garden fork and I made model aeroplanes using balsa wood 60 years ago but that doesn’t qualify me as a skilled woodworker. I also don’t possess much in way of woodworking tools - a couple of wood saws and planes, a Surform and a set of straight chisels. I therefore decided that I would make all the large parts of the patterns in aluminium and just use wood for all the small fiddly bits that can be time-consuming to make in metal.

Having made the layout drawings – pictures in the previous blog, I ordered a couple of pieces of aluminium bar – 10” in diameter that would be machined to form the main crankcase structure.

My Harrison L5A lathe is a decent size machine tool for a home workshop but it immediately became apparent that this project would stretch the limits of both my lathe and, particularly, my milling machine, a Tom Senior Major.

The faceplate in the above picture is 15” diameter.

The 4 studs clamping the bar to the faceplate have been positioned to allow machining of both the inside and outside diameters of the structure. The outside diameter is machined using a left-hand boring bar.

After generating a couple of dustbins of swarf, both halves of the crankcase patterns had been machined and could be assembled into a single unit. They are positively located with a lip and although this is not an important detail for the casting it does allow the subsequent positioning of other parts on the patterns to be carried out more accurately. There are also 1 degree tapers on all the machined surfaces to allow extraction of the pattern from the mould at the foundry.

The boss is for the 2 drive-side main bearings and the central hole and shaft is for positioning the lugs that would be attached later.

The next main part of this basic structure to be made was the flat section that would form the 50 degree angle base for the cylinders. Again, this was made of aluminium and then TIG welded to the round sections. The picture below shows the single piece on the milling machine and a short movie showing the machining. As with all the attached parts, these are machined with a 1 degree draft angle on the appropriate surfaces.

The 4 separate pieces that are required to make the cylinder base mating surface were then cut off and cleaned up ready for attaching to the round base

and these were then positioned  accurately on the milling machine table and attached using JB Weld on surfaces distant from where they were to be welded.

In the picture below, a further block has been added at the bottom that will form the oil drain compartment at the bottom of the crankcase for oil scavenging.

The basic crankcase structure is now complete and the next step is to start adding smaller pieces for lugs, ribs, the timing case etc.

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