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Member Videos / OKMO Harley-Davidson Panhead V-Twin Engine Model R30
« Last post by Jim Dobson on June 05, 2025, 09:34:15 PM »
OKMO Harley-Davidson Panhead V-Twin Engine Model R30

An absolute delightful (gasoline) petrol engine from OKMO Microcosm the level of detail in the engine is incredible for the size,

This is a miniaturised version of the Harley - Davidson Panhead engine, it is scaled down by 1:5, and the details of each part are truly representative.

The engine is 120MM long, 90MM wide, 110MM high and 1.1kg in weight and the displacement is 4.2ML.

The engine cylinder diameters are 13MM, and the stroke is 16MM. The fuel is petrol with oil added (gasoline). The igniter for the spark plugs uses three 1.5V small batteries (4.5V) for power supply.

This is a rather complex model, which is made of aluminum alloy, brass and stainless steel CNC.

This one of the most complex engines that OKMO Microcosm produce and manufacture. This is a very representative V-type dual-cylinder engine with an included angle of 45 degrees.

Panhead (1948~1965) The 1948 FL was unveiled with a new "Bontou" engine and the nickname Panhead which comes from the engine cylinder head (rocker covers) resembled cooking pans.

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I've experimented a bit with rebuilding meshes in Blender, as imported ones are not necessarily that easy to edit.  For example, selecting vertex/edge/face loops or rings is usually impossible with such objects.

Comparison of imported mesh object (2800 vertices) vs rebuilt one (360 vertices):

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Imported objects appear to have their origin points at 0,0,0 location (yellow dot on the left):

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One way to set object's origin or pivot point to a desired location is to select a set of vertices/edges/faces in edit mode.  3D cursor (which acts as a temporary reference point) is then moved to the center/middle of those elements by Shift S → cursor to selected.

Next in the object mode right-click → set origin → origin to 3D cursor:

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Some parts of imported mesh may not have so regular structure, though:

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In this case, another way is to use a circular primitive object like UV sphere, which is scaled and moved to find approximate center point:

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93
Project Logs / Re: The Return of No. 83, a Hot Air Engine
« Last post by vtsteam on June 02, 2025, 08:05:49 PM »
Have thought a lot the past day about the Scotch yoke. I like it okay on its own, but what is giving me pause is the need for some kind of sliding support to keep it from flopping sideways.

It seems there are twenty animated examples of simplified Scotch yokes online, to every one photo of an actual physical working version. In practically all of the animations, the authors haven't bothered to restrain the yoke with a guide. So almost none are practical.

I think the guide is where the real friction is going to be. I've come up with a limited number of choices for guides:

1.) the crank pin, or pin and roller, bearing would need side rims to keep the yoke trapped between them.
2.) the yoke is grooved and a pin bearing rides in the groove
3.) the yoke is constrained between rubbing guides on either side, or a grooved guide at the bottom.
4.) the pushrod is flattened or square sectioned with a bushing to fit
5.) an additional pushrod above or below the active one is used as a second guide
6.) a pivot at the bottom of the yoke and a pivoting connection to the pushrod restrains the yoke (actually, this is technically a quick return linkage, not a Scotch yoke.) It is non linear in stroke speed. Hence the name.

I don't know. The #6 appeals to me because of the elimination sliding guides, but would the non-linear stroke be an advantage or disadvantage? And should it be favor the slower motion toward the hot or cold end of the displacer cylinder?
94
Metal Stuff / Re: How Not to Hate Casting Aluminum
« Last post by vtsteam on June 01, 2025, 03:21:49 PM »
I did a little research online, and I believe that the extrusion which created the worst muffin ingots was called an 8020 1515 linear rail, and it seems these are typically 6063 -T6 alloy. Good to know in general.

My guess at the alloy from the bell housing that made the best looking, very low shrinkage muffins is either AlSi12, aka A413 (US)and D6 (GB), also used for pistons. Or AlSi18 aka A390, which has the lowest shrinkage I can find, of the aluminum casting alloys.

6063 has a thermal elongation coefficient of 23, A413 is 21, and A390 is 18. (I'm leaving out units here, but you get the idea.

This is all of interest to me presently because I'm considering trying a cast iron piston in an aluminum cylinder, and thermal behavior is what I think will make or break that combo.

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Project Logs / Re: The Return of No. 83, a Hot Air Engine
« Last post by vtsteam on June 01, 2025, 03:09:29 PM »
I'm seeing some wear again in the displacer rod/bushing. I feel that the bushing is too short, compared with what I'd now like. Or at least with the side forces on it, considering the shortish connecting rod I now have for its throw.

However, increasing the bushing length isn't possible without major reconfiguration of the engine. I'd have to move the crankshaft, supports, bearings, and flywheel aft, and remake all of the rods. I also don't really want to increase the length of the engine for other reasons.

Instead, I'm considering a few more compact linkage systems to reduce the side forces on the displacer pushrod. Also making a Teflon displacer bushing. Among possible alternative linkages used on hot air engines are the Scotch yoke, the pivoted Scotch yoke, and the Ross linkage. I can probably add at least one of these without moving the crankshaft, and they might also allow a longer bushing and pushrod as well as reduced side forces.

The penalty is more friction and more moving mass. But I think it's a trade-off I want to make, because in future I'd like to try an axial regenerator in the displacer cylinder. That would require very accurate linear motion. Though presently working fine, I think there's too much play now in the bushing to fit anything in the small clearance space around the displacer without interference.
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The Design Shop / Re: Cast Iron Piston in an Aluminum Cylinder?
« Last post by vtsteam on May 31, 2025, 10:46:51 PM »
There is a difference though.

Aluminum expansion with temperature is several times as much as cast iron's. When an aluminum piston works in a cast iron cylinder the piston clearance reduces with temperature. When a cast iron piston works in an aluminum cylinder, the piston clearance increases with temperature.

I would expect that in any case, the aluminum alloy used for a piston has a relatively low thermal expansion coefficient -- for aluminum, so it would still be a good idea to try to use that material as a cylinder in this experiment.

Also, the use I have in mind probably means the piston is operating at higher temperature than the cylinder, since the cylinder is a cooling heat exchanger for a hot air engine. So that might offset to some extent their different material rates of expansion.


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The Design Shop / Re: Cast Iron Piston in an Aluminum Cylinder?
« Last post by vtsteam on May 31, 2025, 10:59:17 AM »
Thanks Bill, good suggestion.  :beer: I have automotive pistons I can use as the melting stock.
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New from Old / Re: Eurospark H425 Die Sinker EDM reborn
« Last post by BillTodd on May 31, 2025, 05:09:15 AM »
Just resurrecting this to say that hthey are scrapping a couple of Ingersoll centre 400s at my old works and wondering of there are any parts that might be useful for  other sparkers ?


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The Design Shop / Re: Cast Iron Piston in an Aluminum Cylinder?
« Last post by BillTodd on May 30, 2025, 12:44:10 PM »
I can't see why it wouldn't  work . There are plenty of aluminium pistons running in cast iron barrels.

The piston thinks the barrel  is moving most of the time .

Your barrel would probably best be made of a suitable piston alloy .
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The Design Shop / Cast Iron Piston in an Aluminum Cylinder?
« Last post by vtsteam on May 30, 2025, 10:37:08 AM »
Has anyone used or heard of a cast iron piston running in an aluminum cylinder? No, not Nikasil or chrome plated.

This would be for a model sized hot air engine (say 1", 25mm  bore), run dry, not oiled. Depending only on the natural aluminum oxide formation to harden the cylinder interior, and the graphite of the cast iron for lubrication. High degree of polish for both to start with. No rings, of course.

Natural inclination would be to guess issues might be galling, and differential expansion due to heat opening up too much clearance.

But maybe not.......?

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