Crankshaft for opposed twin 5cc CI aero engine

pat_pending · Jan 21, 2023

Hi people way more knowledgeable than I. I have made several single and one inline twin aero CI engines but would really like to have a go at an opposed twin or simultaneous firing V-twin.

I could use some advice (PICTURES!!) on making the crankshaft.

My dilemma is this: I can easily make the crankshaft one-piece in the lathe. No probs. Trouble is then I need to make the conrods 2-piece and, at that small size, any screwing together at the bottom-end is likely to be weak and and require enough material for screws etc making them heavy (tapping an m1 screw hole in aluminium anyone? Anyone?). I saw the FigTree Pocket engine from the MotorBoys which does just that but not totally convinced as the rods are so big and heavy.

The other option is to make the conrods 1-piece and the crank 2-piece (like on commercial 2-stroke engines). I prefer this option but have no idea how to make jigs to keep the whole thing perfectly aligned and get the tolerances right for a strong press-fit that wont self-destruct at 13K RPM plus. This option is great as I could use super hardened chrome steel roller bearing rollers as the crankpins which would last for ages.

Anyone out there with some ideas?

Thanks,
Patrick

Jasonb · Jan 21, 2023

One option here on MEN

http://modelenginenews.org/cardfile/gm12.html

pat_pending · Jan 21, 2023

Jasonb said:
One option here on MEN

http://modelenginenews.org/cardfile/gm12.html

Thanks . This might be worth a try. Just trying to get my head around the machining sequence!!!

Peter Twissell · Jan 22, 2023

I made the crank for my 15cc sidevalve boxer as a pressed assembly, using hard dowel pins for the big end journals.
I'm the end, I had to lock the assembly together by TIG welding.
There is a build thread on this site.
I will be making a smaller, 10cc version of the engine. For that, I will be making a one piece crank with split rods, just for a bit of variety.
Pete

pat_pending · Jan 23, 2023

Peter Twissell said:
I made the crank for my 15cc sidevalve boxer as a pressed assembly, using hard dowel pins for the big end journals.
I'm the end, I had to lock the assembly together by TIG welding.
There is a build thread on this site.
I will be making a smaller, 10cc version of the engine. For that, I will be making a one piece crank with split rods, just for a bit of variety.
Pete

Thanks Pete. Did you have any warping issues with the TIG weld?

Peter Twissell · Jan 23, 2023

No distortion issues, but I did take the precaution of leaving the main journals oversize and finishing them after welding.

Niels Abildgaard · 2024-04-18T16:33:20-0400

pat_pending said:
Hi people way more knowledgeable than I. I have made several single and one inline twin aero CI engines but would really like to have a go at an opposed twin or simultaneous firing V-twin.

I could use some advice (PICTURES!!) on making the crankshaft.

My dilemma is this: I can easily make the crankshaft one-piece in the lathe. No probs. Trouble is then I need to make the conrods 2-piece and, at that small size, any screwing together at the bottom-end is likely to be weak and and require enough material for screws etc making them heavy (tapping an m1 screw hole in aluminium anyone? Anyone?). I saw the FigTree Pocket engine from the MotorBoys which does just that but not totally convinced as the rods are so big and heavy.

The other option is to make the conrods 1-piece and the crank 2-piece (like on commercial 2-stroke engines). I prefer this option but have no idea how to make jigs to keep the whole thing perfectly aligned and get the tolerances right for a strong press-fit that wont self-destruct at 13K RPM plus. This option is great as I could use super hardened chrome steel roller bearing rollers as the crankpins which would last for ages.

Anyone out there with some ideas?

Thanks,
Patrick

Hello Patrick

Are You still interested in seing and diskussing V twin crankshaft proposals?
I have speculated a lot many years ago and like to explain.

Niels

pat_pending · 2024-04-18T17:37:18-0400

Niels Abildgaard said:
Hello Patrick

Are You still interested in seing and diskussing V twin crankshaft proposals?
I have speculated a lot many years ago and like to explain.

Niels

Hi Niels! Yes very much so.

I did an inline twin which was was basically two singles popped together. A v twin or boxer requiring an assembled crankshaft is still something very much on my bucket list.

Thanks

Patrick

Joe · 2024-04-18T19:51:22-0400

pat_pending said:
Thanks Pete. Did you have any warping issues with the TIG weld?

If TIG is an option would a 2 piece rod TIGed together work possibly ?

Niels Abildgaard · 2024-04-18T23:50:16-0400

pat_pending said:
Hi Niels! Yes very much so. I did an inline twin which was was basically two singles popped together. A v twin or boxer requiring an assembled crankshaft is still something very much on my bucket list.

Thanks

Patrick

OK.
Let us agree on some numbers.
Bore and strokes in mm please.
The two pistons need not have same diameter,but same mass
Big end bearing size(Crankpin dia)
Conrod mass
Conrod hole distance
Piston mass.

pat_pending · 2024-04-19T01:46:33-0400

I will model something up on cad at the weekend. I’m thinking the same as a Nalon Viper (x 2). I think the dimensions are on the model engine news site. Oddly enough there is an opposed twin in the MotorBoys book (Fig tree pocket) but I really don’t like the screws in the bottom end. Commercial cranks are press fitted for 2strokes normally.

Thanks
Patrick

Niels Abildgaard · 2024-04-19T10:22:07-0400

pat_pending said:
I will model something up on cad at the weekend.

Thanks
Patrick

Have started on dimensions from Holly Buddy.
Significant change is 6mm instead 5mm crankpin diameter but only 8 long instead of 10mm.

Are there someone that has indicated a model Diesel (that is not a diesel)?
What is max pressure?

View attachment 155444

Niels Abildgaard · 2024-04-19T13:38:38-0400

Anoter picture of screwed crank for V2 engine

GRAUBELE · 2024-04-19T15:49:26-0400

Hello Pat

In the early 1980s I built a 2-cylinder 2-stroke boxer engine based on the piston and liner of an ENYA 19 glow-ignition engine.
The crankshaft is case hardened and ground inside and out.
The crank pins are also case-hardened, ground and pressed in.
The crankshaft was then pinned to prevent rotation.
During assembly, the crankshaft was aligned on a ground mandrel.
After assembly, the hole for the mandrel in the middle crank web was closed again with a turned part with a press fit.
I hope the pictures explain the construction of the crankshaft.
The engine has a displacement of approx. 6.5 cc and runs wonderfully.
The crankshaft runs in bronze plain bearings.

Dieter