Original Green Twin Oscillator Drawings by PatJ

Home Model Engine Machinist Forum

Help Support Home Model Engine Machinist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Well-Known Member
Staff member
HMEM Supporting Member
Global Moderator
Jul 2, 2021
Reaction score
MidSouth, USA
I had a close friend get taken out by Covid last year.
Working with him one day, gone the next.
Super brilliant individual with an EE degree, and a masters in Business degree.
Incredibly nice guy too.
These drawings are posted in memory and honor of L.L., nicest guy I ever met.

Don't take life for granted.
We are all just part timers here.
Share what you got.

The story behind this engine is that a buddy of mine in Canada showed me three photos of the original green twin in England from the 1800's, and he was trying to 3D model it, but not making much progress.
I decided to work with him on a lark basically, so as to better learn 3D modeling, never really seriously considering that we would complete the 3D model, much less build two engines.

I got the 3D model done, and it turned out really well.
I had never actually built a complete engine, and so bar stock construction was what we initially considered.

The it was like "What if we build a foundry?", "What if we cast it in aluminum?", What if we cast it in gray IRON?".
"What about publishing this engine in a model magazine?".

Honestly at the time it seemed like bold and crazy ideas, and totally out of touch with reality.
I had no foundry knowledge at all, no foundry, never published anything, and limited machining experience at the time.

"Damn the torpedoes" we said, so I built a foundry, cast two sets of engine parts in aluminum 356 and gray iron, and we built two engines.
My green twin got published in Live Steam and Outdoor Railroading, in a six part series.

Last month, someone else built a green twin oscillator (but not green) using the bar stock method, and that engine too was just published in Live Steam Magazine.

I never really dreamed I would pull it off, but here we are.

My first and only engine build.
I consider this the proverbial "warmup", or proof-of-concept so to speak, so as to learn the multitude of processes that it took to produce this engine and these drawings and 3D models.

I have many engine currently under design, and I can't wait to get casting again.

The photo below on the left is the original green twin engine from England.
The screencapture on the right is the 3D model I created for this engine.
Last edited:
And here is my green twin finished engine.
Against all odds, it got cast, and completed.

Three photos of the original engine from England was all I had to create everything for this engine.

You can do far more than you imagine if you set your mind to it.

All the nuts for this engine were custom made, so as to look old-school like those seen on old steam engines.
The oilers on the rod ends were purchased because I ran short on time.


Last edited:
I see the photos and something deep inside of me says "draw it, 3D print the moulds, Cast it". Work like this is an inspiration, well done to all involved.
Thanks much lee.
I had a great time casting these engines, but it was pretty hard work.

I will consider uploading .STP files here.

I used quite a few 3D printed patterns for this engine.

3D printing has been a game changer for the hobby I think, as far as being able to create patterns from a 3D model.

It should be noted that there are some additional steps between creating 3D models for an engine, and creating usable 3D printed patterns. I learned this the hard way.

Most (but not all) patterns need to have some amount of draft angle, and generally I use 3 degrees as a minimun, and 5 degrees as a maximum.

Any surface that has to be machined needs additional material added to the 3D model on those surfaces.

The corners of patterns need to have fillets added to them, to help the pattern release from the sand, to avoid casting defects at sharp intersections of planes, and to avoid high stress points at sharp intersections in castings.

3D printed patterns must be scaled up slightly to allow for shrinkage, and I generally do this in the slicer program.
I think I generally use a multiplier of 1.015 on my 3D printed patterns.

If you derive 2D drawings from the 3D model, they should be created before you add fillets, and before you add machining allowances or scale for shrinkage, otherwise you will have some very congested and incorrect 2D drawings.

The 2D drawings reflect the "as-machined" dimensions of the castings.

The fillets, and machining allowances are "features" in the 3D model that can normally be toggled of when created 2D drawings, and toggled on when 3D printed patterns.

Last edited:
Agreed. I was a carpenter for most of my working life, but I know for a fact, I couldn't make in wood what I can 3D print.
I actually made the original base pattern for the green twin using balsa wood.
I think I 3D printed the ribs.

This was a rookie mistake (I had no foundry experience at the time), and after a few uses, the balsa pattern broke up.

I rebuilt the base pattern in steel, and that held up well.

This is the first base pattern, in balsa and some 3D printed parts.
You can mix and match wood or metal and 3D printed parts.

More base patterns.

In my ignorance, I made a follower board (the plywood with the cutout).
Someone was kind enough to mention that a follower board is not needed for patterns with a flat side.

Very nice work, and a lot of it! I 3D printed a pattern to make an aluminium flask. When I started to ram the greensand I realised that the pattern was flexing in the middle. I epoxied a thin steel plate to the pattern and that fixed it.
Thanks lee-
There is a bit of an art to pattern making, mold making, and casting metal.

I learned through trial and LOTS of errors.
No pain, no gain, as they say.

Once you get a feel for foundry work, it is not bad really, but it is not something learned overnight.

Having one's own iron foundry does open up a huge number of possibilities in this hobby.

Thanks Jim-

It was quite a "trip" to say the least (perhaps wild rollercoaster ride is a better description).

I fully expected the entire build and foundry thing to fail; it all seemed several bridges too far for me.
There were quite a few days when I had pretty much decided to give it up.
In the end, I got too deep and too involved to give it up; I had too much time invested, and so it became "finish the engine or die trying".

My Canadian buddy never stopped encouraging me, and never gave up on me, so that helped a lot.

I think the green twin engine has a great look to it, and with a 6" flywheel, it is not too big for most hobby folks to build.
The original green twin from England has a 10" flywheel, so this is a 60% scale model.

I hope to see others attempt this engine.
I can provide the machining and assembly instructions too, if anybody gets ready to build one.


Latest posts