Sunday 25 October 2020

Machining the Crankcases – Part 3

As it is now possible to clamp securely the crankcase halves together the top surfaces can be machined. There are essentially 3 operations that are required on each face:

1)    Face off to give a flat surfaces at the desired distance from the crankshaft

 2)    Bore the hole for the cylinder

 3)    Drill and tap the holes/threads for the cylinder base studs

 All operations are carried out on each face before changing the setup to the second face.

When I started making the patterns for this project (February 2018) the intention was to use side-by-side connecting rods with the obvious consequence of machining the crankcases with offset bores. Very sadly, in June 2018, Max Nightingale of Alpha Bearings died suddenly at the age of 59. I had got to know Max very well over a period of many years and Alpha had refurbished many crankshafts for me and also designed and made the complete crankshafts plus connecting-rods for the AJcette and for 2 Velocette Mk1 KTTs. This was no longer an option for this V-Twin and I needed to find an alternative solution.

Whilst making a crankshaft is not a trivial project – the design, required tolerances, choice and availability of materials, heat-treatment etc… the one parts(s) that would be extremely time-consuming to make in a home workshop would be the connecting-rods. In the original design, the exact connecting-rod length was to be determined when the rest of the engine had been built as it depends on a number of  factors. The original plan was to fix the stroke (81mm) and compression ratio and to then determine the exact length of the connecting rod. This was now no longer an option and a revised plan was to exchange the 2 variables of stroke and connecting rod length in the design and to now fix the connecting-rod length and compression ratio and to then determine the stroke. This has the advantage that off-the-shelf connecting-rods could be used and the (slight) disadvantage that the exact swept volume of the engine would not be known until close to the end of the build; but as this bike was never intended to be an exact replica of the original AJS this doesn't really matter.

The initial design had a connecting-rod length of around 7 1/4" - 7 3/8" and a search was therefore made to find a pair of rods of around this length to retain the side-by-side arrangement. A good friend of mine turned up a pair of rods that were fitted to 1930s Ariel Red Hunters. These were in excellent condition and knowledgeable Arielistas assured me that these are sturdy rods and not known to break

Although these rods were slightly shorter than I would have wished at 7”, I was on the point of using them – and even found some suitable needle-roller bearings, but then started looking at Harley Davidson knife-and-fork rods, which of course would require in-line rather than staggered cylinders. It just so happened that the HD EVO has rods which have 7 7/16” between centres and are much closer to the con-rod length I was looking for. Brand-new rods + crankpin + bearing were available in the USA at a very reasonable price on ebay and these were ordered and delivered a couple of weeks later.

I was extremely pleased with these and a revised set of calculations for the crankcase machining was therefore made, based on this rod length and with in-line cylinders.

Now that I had resolved the conundrum of the staggered vs in-line arrangement I could get on with the machining. The next step was to make a suitable jig to support the crankcases on the milling machine table. The jig consisted of an angle plate, a very substantial piece of steel bar with a 1 1/8” hole bored through it at the top and welded to another piece of steel for the base that could be clamped to the milling machine table and a 1 1/8” ground bar that passed through the bearings and into a bored boss supported rigidly on the angle plate using a large bolt.

With this arrangement, once the bar had been setup with a dial gauge in the X - Y plane, the bar could be withdrawn through the bored hole on the left, the crankcases “inserted”, the bar then passed through the bearings into the locating boss clamped to the angle plate and the crankcases clamped to the surface plate.

There is now only one degree of freedom, namely rotation about the crankshaft axis, and this allowed the crankcases to be positioned for all machining operations on the top surfaces.

In addition to the usual micrometer and Vernier measuring instruments, I have found that a digital “level box” and digital protractor to be extremely useful aids in setting up machining operations on the milling machine. They are both shown below using the crankcase patterns to illustrate how they can be used to setup the castings.

The first operation is to clean up the faces with a fly-cutter

Although both surfaces have been faced-off,  the 2nd face has not yet been finished to size and approximately 0.050” has been left before final machining.

The main hole for the rear cylinder is bored and the 4 stud holes drilled and tapped before rotating the casting through 500 and repeating the operations for the front cylinder.

On completion, the crankcases are full of swarf

And after emptying the crankcases using my workshop vacuum cleaner, it was time to put in the cylinder base studs and see if everything actually fitted.

It did! And for the first time, I had something that was starting to resemble a V-Twin engine ….A moment to celebrate.

In case anyone is wondering why the rear cylinder has a somewhat ragged-looking exhaust port, this is because it is a KTP (twin port) cylinder head that has had the right-hand exhaust port largely removed. More will be said of this later.


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