A tiny 3 Cylinder Holt based engine

[gbritnell]

The rocker arms needed to be threaded for adjuster screws. The width of the boss at the valve end is only .094 so rather than go with 0-80 threads and leave only .015 around the thread I decided to use 1.2mm threads (.047 diam.) I used the same setup for drilling the adjuster end of the rocker arm as I did for the ball pocket at the pushrod end.
On my full sized Holt the pushrods are adjustable but given that these pushrods are only .062 diameter I didn't want to make the threaded joint so fragile that it might bend or break.
The rocker shaft were turned with a .094 diam. center area and .086 on the ends for the 2-56 retaining nuts.

[yogi]

Wonderful work, as always George! 
Thank you for sharing and inspiring! 

[b.lindsey]

Fabulous George. Gosh those rocker arms are teeny (even smaller than tiny). The flywheels look great too, too bad about the mismatch though but still probably easier than making them from bar stock.

Bill

[Art K]

George,
Looks great as usual.
Art

[jschoenly]

What a great engine!  Really enjoying watching the progress on this!!

[gbritnell]

Well it's down to the last 'big' pieces of the engine, the timer and distributor gear. This engine will have a Hall trigger ignition. Trying to scale it down and still make it workable and pleasing to look at took some doing on the CAD program. The secondary gear case has a hub (.211 dia.) that an adapter plate fits onto. It's held in place with a 1-72 set screw. The plate has a slot that will hold the Hall transistor.
Next on the .093 shaft is the timer disc. This is made of steel and is also held in place with a 1-72 set screw. Mounted to the adapter plate is the distributor base plate. The two plates are bolted together with 0-80 screws. This plate is counterbored to allow the timing disc to turn inside with .010 clearance. This plate also holds the .125 x .062 magnet.
In the first picture you are looking at the timing pieces from the front of the engine. The ear sticking out to the left is part of the adapter plate. It is held to the distributor plate with the 0-80 socket head screw. Sandwiched between you see the timing disc with one of the window cutouts at the top.
The second photo is taken from the rear of the engine and shows the counterbore that the distributor cap will fit into. The larger hole at the 4:00 position is the through hole for the magnet.
The third photo show the timing disc. To the right is the slot for the Hall transistor and to the left of the disc the hole you see is the tapped hole to hold the disc in place.

[gbritnell]

The next picture is a front 3/4 view showing the assembly a little better.
The last photo is of the engine with the parts assembled to the secondary gear case.
The distributor cap will be .75 diameter. I could have made it a little smaller but it's size was governed by how tight I could make the timer assembly.
gbritnell

[Ye-Ole Steam Dude]

Looking at the photos it is hard to get a perspective of the actual tiny size. Just beautiful workmanship.

[gbritnell]

I finished up the components for the distributor. I machined the brackets to give them a little more character. The rotor is complete.
I made the cap and rotor from black Delrin. I have used it in the past with good luck. It machines nice and seems to have good electrical qualities. I turned the cap then mounted it on the end of a fixture rod and set it up in the mill vise, indicating it to achieve true center. The center holes were drilled .062 followed by a .104 dia. drill for the plug wires. I then used my home-made coring tool to cut the terminal posts. The cutter was drilled out to the size of the terminals then bored to add 3 degrees of back draft to allow for clearance while it was cutting. The cutter was then fluted, hardened and the cutting edges honed with a diamond tool. The cap is held in place with 2 0-80 screws.

[BlueRock]

Looking fantastic George. I always love your work, you are a fine craftsman.

[fumopuc]

Hi George, a very interesting concept your disstributor with the half open running disc.

[gbritnell]

HI Achim,
On the full sized model Holt there are a set of points in that location. Not wanting to try to miniaturize the point assembly I went with the Hall setup. The open top is so I can adjust the timing disc without having to disassemble everything. The distributor base is mounted quite securely with three screws.
gbritnell

[cfellows]

Mighty pretty engine, George.  The old 3 cylinder industrial engines are among my favorites.  As usual, your work is meticulous and admirable.  Just curious, do you recall about how long it takes to whittle out something like the interior of the lower crankcase half?

Chuck

[Plani]

That's a gorgeous little engine 
Can't wait to see it running 

Plani

[gbritnell]

Hi Chuck,
To carve out the inside of the lower crankcase probably took about 3 hours. As you know my technique is to make a step-off chart for the shape so it's pretty much a matter of following the numbers. I don't mean to make it sound that simple but as long as you follow the numbers it goes quite fast. I'm guessing the whole lower crankcase took between 10-12 hours. (Machining) Finishing with files and such took another couple of hours.
gbritnell