A tiny 3 Cylinder Holt based engine

[gbritnell]

Gentlemen,
I have engines in all cylinder configurations from 1 to 8 but I don't have any with 3 cylinders so I thought I would make one. I really like the Fairbanks 3 cylinder engine but I think is would be just a little too complex in this reduced size so I am designing and building my own based on the Holt drawings. It will be 1/2 size so the bore will be .500 and the stroke .625. I plan on the ignition being triggered by a Hall device and the cylinders will have an outer jacket so that if coolant is needed it will be in place.
As my construction technique is machining from solid this will be built that way.
I started with the bottom half of the crankcase. When I made my full sized Holt from scratch I split the crankcase for simplicity and ease of machining. This one is being built the same way.

Picture 1. The block of aluminum squared up and the screw holes drilled. I also have 2 .046 diameter holes in opposite corners for tiny dowel pins to keep the two halves aligned.
Picture 2. The inside of the part was roughed to shape including the oil sump.
Pictures 3&4. Using my Cad program I generated a step-off program to whittle out the inside.

[gbritnell]

Picture 1. The lower crankcase was then flipped on the vise and the outer shape was roughed leaving .010 for the finish cutter. (.187 ball mill).
Picture 2. Another step-off chart was created to put the contour onto the crankcase. First half completed.
Picture 3&4. Stepping off completed and a radius was milled around the bottom edge of the oil sump.
The screw holes were counterbored with a .125 endmill going deep enough to put the screw heads below the surface so they won't be seen.

[gbritnell]

While the piece was solid I drilled and reamed the cam hole. Trying to do this with the crankcase pockets milled is virtually impossible.
Picture 1. The upper half crankcase was milled to the overall dimensions then the holes were drilled for the mounting screws (1-72) The cylinder holes were drilled then bored to size.
Picture 2. The slots for the bearing caps were milled in the center 2 ribs then the side walls were stepped down. First pocket shown. Being as each side of the crankcase has a different angle I finished one side at a time so I wouldn't have so many numbers to step to.

[gbritnell]

The two halves were screwed together and put back into the mill vise.
The side walls of the crankcase are angled so I quickly set it with a protractor then used a DTI to get the angle exact.

Picture 1. Using a flycutter I milled the side wall.
Picture 2. A .025 raised area was created where the cover plate will be mounted and the inspection windows were milled. On the original engine the windows were actually for installing and bolting up the connecting rods as the crankcase was one piece. This was one of the reasons why I went with a 2 piece crankcase.
Picture 3. The windows finished.

[gbritnell]

These 3 pictures show the machining finished on the crankcase.

[b.lindsey]

Great!!! I just love your small projects George and this one will be another beautiful example no doubt! Just please tell me you didn't get all this done in one day 

Bill

[gbritnell]

Now it's time to take all those little ridges off and make a smooth surface.
Picture 1. the surface was painted with an ink marker.
Picture 2. The start of the finishing process. By inking the surface you can see when the high spots are being worked down. On this part a variety of tools was used, riffler files, 1/8 grinding points in my Dremel grinder and emery paper and home-made emery sticks.
Pictures 3&4. About 1-1/2 hours of hand work.

[gbritnell]

Not to worry Bill! I have about 3 days in it so far. About 7-8 hours.
gbritnell

[gbritnell]

A couple of finish pictures. I'll probably start on the gear cases as that's all I have designed to this point.
Being as it's a 3 cylinder engine I will have to develop a cam and figure out how the intake exhaust configuration will be.
There is nothing unique about the firing order, 1-2-3.

[Admiral_dk]

It looks like you're starting on another master piece George - I'll certainly follow along as usual 

There is nothing unique about the firing order, 1-2-3.

This still allows for at least two different configurations .... Assuming that you will use a 120 degree crank, it can be 120, 120, 120, coasting 360 or 240, 240, 240 firing interval.

Best wishes

Per

[Art K]

George,
Its always good to see you start a new engine. I always find it a learning experience. My smart car has a triple in it but that's about all I know about the internal workings.
Art

[jeff l]

As always outstanding !

[90LX_Notch]

George it is always an education following your build threads.  I am looking forward to watching the build as it progresses.

-Bob

[Roger B]

Another interesting build to follow along     

[Nick_G]

.
That is uber impressive of the highest order. 

Nick