Learning to Sprint the 100m before we can walk

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.

Andrew Pullin

Jun 26, 2019
Reaction score
Wodonga, Australia
Hi Guys,

My engineering School encourages us to get into our learning by getting our hands dirty
so as such I am putting my extremely limited knowledge to work.

I am designing a simple Wobbler Steam Engine - Bore 20mm, Stroke 40mm for a capacity
of about 12cc. There is about 10mm of overhead for inlet and exhaust ports. The Engine will
probably be made of Mild Steel with 15mm Cylinder Walls (12.5mm on Port Side) and end caps
10mm with a 1mm lip to help seal it. (If it ever ACTUALLY gets built - it is a paper design at present)

The Piston is to be made in two parts each 5mm thick and will be screwed into the Conrod at the
Little End. I have chosen this method so that in lieu of Piston rings I can use a Gasket instead. This
method is similar to Gerry's Beam Engine and the Gasket material is dependent upon Steam or
Compressed Air operation. The Big End drives a Fly Wheel via an eccentric Crank.

Ok, so my question is about the Tolerances for the Piston. If I run the Piston on Compressed Air then
I should not have any thermal expansion to worry about. If however I run it on Steam then I WILL
have to worry about it. The Steam Ports and Exhaust Ports are both 5mm diameter and I am using
a Double Action where Steam will drive the Piston down AND up.

What Tolerances should my Cylinder be turned to? E.g. 20.00mm to 20.05mm
What Tolerances should my Piston be turned to? E.g. 19.90mm to 19.95.00mm
What should my Gasket be made of for Steam or Compressed Air operation?
How big should I cut the Gasket? E.g. exactly 20.00mm
How much and what type of Lubrication will I need?
Since I am designing a Double Action engine, what Tolerance will I need on my Piston Rod
and the Hole in the bottom of the Cylinder Cap? Rod is 5mm in diameter.
Will I need Gaskets on the Cylinder End-Caps? If yes - made of what?

Any general advice.

At present all I have is a Piston and a Fly Wheel design, so no mounting hardware yet.
I have calculated TDC, BDC and the two Mid-Stroke positions and my Pivot point is at
the Mid-Stroke position. My Ports will be fully open and a Deflection of 9 degrees at the Mid position,
and a maximum Deflection of 10 degrees. This puts my Big-End centre tracing out a nice symetrical
eccentric circle that is just off-centre of the Fly-Wheel axle. My Ports should also always be BOTH
Closed or ONE Open to prevent cross-flow leaking.

While calculating I discovered that if I Pivot anywhere else then I get a nice elliptical trace. I am not
sure how I would or if I could connect the Big-End if the Trace is an ellipse. If I can then how would I
calculate where to put the connection? Does it connect to one of the Ellipse Focus Points or the centre
between the two?

Well I hope you guys have enough information to help me out. They say a little Knowledge is
Dangerous so right now I am pretty much Lethal!


In general you can take 1/000 of the diameter as a reasonable running clearance. Half that requires precision and twice as much is sloppy.

A gasket is a thin sheet seal between two stationary parts. The term is not used for a seal where there is movement. You could use O-rings for the piston and rod. Use Viton or equivalent for steam, and make the grooves deeper radially than the design manual says, as you only want a very light pinch for a little oscillating engine. Alternatively, the traditional piston rod seal is a 'gland' or 'stuffing box' in which a graphite impregnated cord 'packing' is compressed by a 'gland nut'.

You are over-thinking the geometry. The crankpin will be going round in a circular path, whatever you may think you want. Start from there and come back when you have had another think. Clue: tangents.
Last edited:
Hi Charles,

So if my Cylinder is 20mm then 20/1000 = 0.02mm. Is that clearance in general for a smooth fit or does it take into account thermal expansion?

I suggested that 0.05mm might work so a 20.00mm Cylinder should be toleranced at between 20.00mm and 20.05mm. I also suggested that my Piston should be a tiny bit smaller at 19.90mm to 19.95mm. At worse that would put the maximum difference at 0.15mm. Is this too much or does it have to be tighter? Using 0.02mm would be a maximum of 0.06mm - a little over 35% of my suggested worst figure.

I know a Gasket fits between two stationary parts but I am still learning the Jargon and I didn't know what to call a sheet of material between two halves of a Piston. I almost might have got it right as the only bit that moves is the Piston in the Cylinder, the Piston Halves do NOT move. I got the idea from Gerry's Beam Engine where the Piston is similarly in two halves with material between them. I may have got the reason that material was there a bit mixed up - I thought it was in lieu of a Piston Ring and that it helped seal the Piston, and also soaked up a little lubricant and so slid along the Cylinder walls. If I am wrong about this then what IS it for? And do I need it at all? My other question was do I need Gaskets on the Cylinder Caps if I have a 1mm "lip" inside the Cylinder?

In terms of the Geometry - it was a little bit trial and error to see what happened and a small amount of cheating with AutoCAD. Initially I drew the points I knew (TDC, BDC and then lined up the centre of my Ports) and I selected a Conrod length that seemed about right. I then used the Ellipse Tool on AutoCAD to look at what shape I would get if I connected three of those points at the Big End of the Conrod. When I used the exact centre of my Ports and the Mid-Point of the Stroke I got a perfect circle from the Tool. I had noticed that the line from Mid through the Port was ALMOST a Tangent and at maximum deflection it WAS a Tangent on the circle the CAD Tool produced. I think I fluked the answer initially but now I can see why it works. The Ellipse Tool was the wrong tool to use and only worked because I picked the right points for it to make a circle. I should have used the Circle Tool when the Points I picked were uneven to give me the correct answer, instead of being misled.

So to Clarify - the centre of the circle I get from TDC, BDC and the tangent point at Mid should be where the Axis of Rotation of my Fly-Wheel is, and the circle traced will be the path of the Crank Pin?

Thanks for your feedback.


I would still say your tolerances are too lax. If the piston and cylinder were made of materials with a large difference in their rates of thermal expansion, then that would need to be taken into account. Are you designing as an exercise for an notional production run or as a one-off? In the case of a one-off the parts can be made to fit rather than to specified tolerances.

I have not seen Gerry's Beam Engine so cannot comment. Gaskets on the cylinder covers would the normal thing to do.

You are still over-thinking the geometry but I am not sure why or exactly how. I think you may be trying to start in the wrong place. You want 40mm stoke, so the crank 'throw' is 20mm. That radius, taken with distance between the crankshaft axis and the cylinder pivot axis determines the swing angle. The swing angle, and the radial distance of the ports from the cylinder pivot axis, together determine the port sizes. As you have said, the gap between the ports in the port block should be equal to the diameter of the port in the cylinder. In practically laying out the engine, the stroke, the thickness of the piston, cover, and gland, and the dimensions of the big end will tell you how close you can get the cylinder pivot axis to the crankshaft axis.
Right now the design is a can I do it and could it actually be fabricated - a prototype is on the cards but beyond that I don't know. The plan was to make the Cylinder and Piston out of MS. Are you saying my suggested Tolerances of 0.05 are too lax or the recalculated ones of 0.02. The first figure was just a guess - it sounded good to me at the time. While I don't have any plans for the design in the future I am studying to be an Engineer so designing it so that it COULD be produced is the goal.

If you search this site for Gerry's Beam Engine and Metric there is a set of Production Drawings that I transcribed from the original A0 size AutoCAD files (with permission) if you are interested. Would I use Viton as you suggested for my Gaskets on the cylinder covers?

Hmm. Still getting in a tangle with the Geometry.
I have selected a Conrod length of 120mm. At TDC and BDC the end of the Conrod must be on the centre line and the two points are 40mm apart since there is a 40mm Stroke. I have drawn a circle with a centre half way between these points and a radius of 20mm. I have chosen to pivot my Cylinder at the Mid point, so at 20mm of Stroke I draw a line from this point to the circle. Here is where the trouble starts.

If I make this line a tangent to the circle then the end point of the line is OUTSIDE the circle. If I simply join the end of my line to the circle it is slightly before the top point of the circle and NOT a tangent as the line crosses the circle. The angle of this second line to the TDC-BDC line is 10 degrees.

<Logic Check> (What I think I know)
The end of the Conrod should trace a circle.
The circle I have drawn should be the circle the Conrod should trace?

The tangent point of a line from 20mm Stroke to this circle should be the maximum deflection from Zero.
I should centre my Ports on this line?

My Ports should be of a size such that they are never BOTH open at the same time to prevent cross feed.

The centre of my circle should be placed on the Axis of Rotation because the Pivot point is the Mid point of the Stroke and so everything is symetrical.
If my Pivot was NOT at this point I would still have a circular trace but this circle would be eccentric to the Axis of Rotation of my Fly-Wheel?

This should work but I am still not 100% sure why.

<End Logic Check>

This is pretty much where I am right now. I need to solve this so that I can move on to designing the Fly-Wheel and mounting hardware.



Latest posts