Monday 12 October 2020

Machining the Crankcases – Part 1

In addition to making the castings, the foundry also provides a heat treatment service. This is extremely important for structural aluminium castings and makes a huge difference to the strength of the structure. See here, for example, which gives details of the key strength metrics for the different heat treatments. These castings have been heat treated to the TF condition (also known as “full heat treatment”) and this nearly doubles their UTS compared to the “as-cast” state.

The shrinkage of the castings compared to the original patterns is around 1.3% and, as I allowed plenty of material on the main dimensions, shrinkage is not a problem here.

I picked up the castings about 4 weeks after dropping them off and, in addition to the set that I would need for this project, also made a spare set …just in case I fancied going into production at some later date.


It is important to determine the sequence the various machining operations are to be undertaken. Here, I decided on the following:

1)    For each crankcase half, bore the internal main diameter for the flywheels and the bearings and face off to size. A lip in one half and recess in the other would provide positive location to ensure both crankcase halves are aligned on assembly.

 2)    Face off the timing-side surface to provide a datum.

 3)    Make a jig to hold the crankcases vertically in the milling machine and bore the holes through the lugs

 4)    Make a jig to hold the crankcases horizontally and machine the top faces (for the cylinder bases), the holes for the part of each cylinder that protrudes into the crankcase and the threads for the cylinder retaining studs

 5)    Machine the timing-side to accept the studs, bearing and housing for the gear/sprockets for the drive to the overhead camshafts.

 6)    Machine all final details – oil drilling to the main bearings, oil collection volume at the bottom of the crankcases, oil drain-plug threaded hole etc.

 7)    There would still be more machining at a later date to accept the inner timing case but this would need to wait until the castings had been made for the inner and outer timing cases.

The first step was to mount the crankcases onto the faceplate. This required drilling and tapping temporary threads into some of the lugs and making spacers to ensure that each crankcase half is supported in the right position.

The picture below shows the drive-side crankcase mounted on the faceplate on the lathe prior to machining. One of the spacers at the top of the picture can clearly be seen as can the minimal clearance between the crankcase and the lathe bed; this assembly only just fits into the gap on the lathe and the combined faceplate and casting required 2 people to lift it off the bench and mount it onto the nose of the lathe.

One characteristic of machining fully heat treated LM25 is that the material is noticeably harder than, for example, soft aluminium bar. This is not a bad characteristic, providing carbide or sharp HSS tools are used, and results in cleanly machined surfaces and nice sharp threads.

It is important that all the machining operations for this setup are carried out before dismounting the crankcase from the faceplate to ensure concentricity. The picture below shows the crankcase mid-way through machining.

It took quite a long time to complete the machining, not only from the necessity of accuracy but also the out-of-balance rotational forces (which are proportional to speed squared) that could potentially shift the position of the heavy crankcase casting on the faceplate, with disastrous consequences; this required running the lathe at a very low speed.

The picture below shows the completed internal machining of the timing-side crankcase half.

The main bearings (1 1/8” x 2 ½”  x 5/8”) were obtained prior to machining the crankcases to check dimensions. I always buy good quality branded bearings and I have used bearings with seals during the build because many of the subsequent machining operations required accurate setup and the easiest way to do this is with the bearings in place and a ground shaft through the centre. The seals, which prevent swarf getting into the bearings, are removed when machining is completed.

Having completed the main internal boring of the crankcases, the next step was to set them up in the milling machine to bore the holes through the lugs.


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