Building a twin cylinder inline i.c. engine

[Brian Rupnow]

I have built single, opposed twin, and v-twin engines. I want to build an inline twin engine, and I have the drawings for Malcolm Stride's Bobcat engine. I build in the imperial inch system, and I will be upgrading the Bobcat engine to a 1" bore and using imperial inch measurements as close to the original drawings as possible, but keeping the same original stroke. This past year has been a very challenging one for me, and I have three engines designed and built that either didn't run well or didn't run at all. This has shaken my faith in myself, so I'm going back to an engine design that has proven itself to work well. I am just in the very beginning stages of this engine, so if you are interested, stick around. ---Brian

[crueby]

Watching along!

[Roger B]

I have built a 12 cc twin (approx 3/4 bore 3/4 stroke) that has run with 180° and 360° crankshafts as well as a CD (SU type) carb and fuel injection. The thread is here:

https://www.modelenginemaker.com/index.php/topic,7883.0.html

This is some load trials with fuel injection:

<a href="https://www.youtube.com/watch?v=7RVp_fwLo0o" target="_blank">http://www.youtube.com/watch?v=7RVp_fwLo0o</a>

I have attached my electronic 'back of an envelope drawings'

[Jasonb]

Can I suggest you also scale the stroke to the same proportion as you intend to scale the bore, same goes for everything.

1mm on the original drawings scaled to 0.046" on yours would be the ideal way to treat everything just adjusting for stock material and fixing sizes.

[Brian Rupnow]

Everything has to start somewhere, and seeing as the cylinder bore has changed a bit, and the piston stroke has changed a bit, then the cylinder will be the first candidate.-Because---the change in cylinder outside diameter will affect the center to center spacing of the two cylinders and this affects the main crankcase and the sump and the crankshaft and the camshaft. The original drawings asked for an inner sleeve of cast iron and an outer finned housing from aluminum. I find it a lot simpler to just make the whole cylinder out of cast iron.

[Brian Rupnow]

Yes, Jason, I will do that. The original had a 22 mm bore and a 20mm stroke. I will upgrade that to 25.4mm bore and 22 mm stroke.

[Brian Rupnow]

Because the outer diameter of the cylinders grew in size, the crankcase has to "grow" a corresponding amount. The crankcase model isn't finished yet, I have to add more pieces and see what interferes and what doesn't. Right now the crankcase is about 10 mm wider and 23 mm longer than the original crankcase design. Tomorrow I will get to add some more pieces and the holes to mount them. The trick here is deciding which dimensions change from the original drawings and which ones can remain the same.

[Jasonb]

Well already you are deviating from a working design and missing points that may be what made it work.

The liner on Malcoms original sits proud of the jacket/fins and presents a small area for the head to seal against which is quite common practice on small IC engines. Yours will need a wide gasket and could be a source of leaks and therefore compression loss.

You also have the 5 hole pattern the wrong way round.

I've altered my scale factor to 0.046" to 1mm as I was thinking the engine was 20mm bore

[Brian Rupnow]

Jason--that "no head gasket" thing has never worked for me. I will be modifying the top of the cylinder to take a 1/16" diameter Viton o-ring in a groove 0.030" deep. That works. I will look at the five hole pattern today when I do the cylinder heads.

[Vixen]

...............snip ................
 This past year has been a very challenging one for me, and I have three engines designed and built that either didn't run well or didn't run at all. This has shaken my faith in myself, so I'm going back to an engine design that has proven itself to work well. I am just in the very beginning stages of this engine, so if you are interested, stick around. ---Brian

Brian,

Good luck with all the changes you are proposing to make to "an engine design that has proven itself to work well".

Mike

[Jasonb]

Jason--that "no head gasket" thing has never worked for me.

really, you had a lip on the single that you built based on this design 10yrs ago and that ran as did the opposed twin.

Your cylinder

[Laurentic]

Brian.  You have obtained the plans for the Bobcat, which I assume are in mm.  You wish to work in imperial - i can understand that as I work in imperial, it's what I grew up with and was trained in, thous mean something to me, bits of a mm don't. 

As you will be machinig all parts from stock, not castings, am just wondering why you need to redesign in imperial with upgraded - or larger - dimensions.  Why not go through and do a direct conversion of units mm to imperial and work from that?  There may be some that need rounding up a bit or down a bit, but nothing that would cause a lot of upset, and would keep the design close to original.   Things that need to be obtained at a specific size, like say a 6mm machine screw, can just be converted to a local easily available imperial alternative which will be very close, certainly close enough to make no odds. 

Why complicate things?  If it ain't broke don't fix it as the saying goes. 

Don't lose sight of your aim to build "an engine of a design that has proven itself to work well".

Just suggesting.

Chris.     

[Art K]

Brian cought this start to a new build, I'm watching along.
Art

[Brian Rupnow]

And now you know how I spent my Sunday. Actually, the model is coming along very nicely. The cylinder head shape is going to change a bit. I haven't even thought very much about the camshaft yet. The position of the cam centerline is probably going to move a little bit, as I cut my own gears, and the pitch circle of the gears is what will determine the camshaft position---maybe.

[Brian Rupnow]

Although I can work totally in metric (If forced), my metal stock is imperial. My fasteners are imperial, my measuring instruments are in Imperial, and I'm just a whole lot more comfortable working in Imperial. There is a bit of magic in the software I use--I can start with a metric template and do everything in metric, or I can start with an imperial template and this allows me by default to use Imperial dimensions.---But--If I punch in a dimension and then hit mm on my keyboard, it automatically converts true metric to Imperial for me.