The cylinder block starts with a trued up work piece of 6061-T6. This was done on the lathe, but could have been done with a fly cutter on the mill. After using both, I get more accurate results in the mill, but not quite as nice a finish—probably could if I were more patient.
The first machining operation is from the side to create the internal cavity. I spot drilled the location of the block cover screw holes. In hindsight I would not have done this as I ended up match drilling these holes using the cover plate. My fear was that if I used the spot drilled holes and the cover was slightly out of position, it would interfere with the block to crankcase mounting.
If you look closely at the cavity in the block, you can see a .010″ step where the depth of cut exceeded the .75″ flute length on my cutter. This technique allows me to cut deeper than the flute length without hitting the shoulder of the cutter.
Next, all of the machining from the top was done in one setup. The large holes for the mounting of the cylinder sleeves were machined at the same time with the same tool to insure concentricity of the hole at the top and bottom of the block. The hole at the bottom is .004″ smaller to ease the installation of the sleeve and allow for a cutter to machine only one of the surfaces, top and bottom, at a time. In hindsight I would have left a bit of excess material on the top of the block and used a fly cutter to deck the top after installation of the cylinder sleeves.
A dime for scale, the block is really quite small.
The block cover only required machining from one side so was a relatively easy part to fabricate. I used a roughing pass with a 1/4″ flat end mill, a horizontal finishing pass with the same end mill and finally a roughing and finishing pass with a 1/16″ flat end mill. this last operation milled the fins and all of the holes.
As mentioned earlier I decided to match drill the holes for the 2-56 cap head screws mounting the block cover to the block. This was done to make sure the cover plate does not interfere with the mate to the crankcase or the cylinder head. I used two .006″ shims against a second set of parallels. Since I had previously spot drilled the block and did not want the drill to find these and follow them, I first ran a clearance drill through the hole to create a new spot for the drill to center on the hole before drilling for the tap.
As a rule I don’t like match drilling, I believe in building to print. But there are cases where the required precision is too tight to call out on a print and not realistic to call out for fab in my home shop. Other parts where I will match drill include the main bearing housings to the crankcase and sump.
Here I am using the mill to tap the 2-56 holes in the block. A spring loaded tap guide is loaded in the mill chuck. I had to replace the spring in this guide with one much softer as the original provided too great a force for a tiny 2-56 tap. I used the DRO to locate the holes for drilling and tapping.
The block cover in place. I think the block looks really cool, I got the idea for this design from Terry Mayhugh’s build of Ron Colona’s Offy.
The engine so far.