A 15cc sidevalve opposed twin

[Peter Twissell]

With the valve cages fitted, the ports are cut through the block and into the cage. The ports are counterbored for a small high temperature O ring which will seal against the manifold face. The M3 tapped hole below the ports will hold a clamp to retain both inlet and exhaust manifolds.
With the valves in place and open, there is a clear path for gas flow.

 

[Peter Twissell]

After Ken kindly suggested electroforming, I have looked into it and decided it is the way ahead for the manifolds.
I will be 3D printing the formers in water soluble PVA and painting them with graphite paint before plating.
Since this process affords considerable freedom of design, I have taken the opportunity to relocate the carburettor to behind the cases, where it sits neatly inside the engine mount while still being able to access all the adjusters, fuel feed etc.
I have also re-routed the exhaust pipes, so that the installation in the model's cowling is neater.
The intake manifold will require a couple of features which cannot practically be electroformed, but those parts can be machined in brass and silver soldered into place.
I will finish the electroformed copper parts with a few microns of electroless nickel, just because I can!

 

[Ken I]

Peter, Very cool - hope it all goes well and will be looking forward to the results.
(Somewhat anxiously, having steered you in that direction - it had better work.)
It sounds like you have been doing your homework.
IIRC when starting with a conductive paint - the initial deposition rate (current) is deliberately low and increased as the plate thickness increases - hence more current carrying capacity.
Regards, Ken

 

[Peter Twissell]

Thanks Ken. I am planning to plate at a relatively low current anyway, in order to get a good finish and even thickness.

 

[Ken I]

When you plate irregular, long or sharp cornered objects (the end of a tube is a sharp corner at the end of a long object - thus a double whammy) it is common practice to jig the part with "thief" electrodes - like a wire ring off the end of the tube - or an extension beyond the end of the tube.
This to have the overplate at the corner (caused by increased current density at that point) reduced or "stolen" by the "thief" - or have it occur on an extension portion to be cut off and/or discarded..
But I am sure you will stumble over this in your research.
Whilst you will probably find published guidelines - it is something of a black art - you would do well to talk it over with an experienced electroplater.
Regards, Ken

 

[Peter Twissell]

Thanks for the advice, Ken.
I have modelled the first parts with an extra 5mm length on each end for exactly the reason you state.

 

[Ken I]

Hmmm.... I should have known better than to teach you to suck eggs - I would just hate to see it go pear shaped for something I might have cautioned you against.
That said - this forum isn't just for us - hopefully others get to take something else away from this.

Regards, Ken

 

[Peter Twissell]

You are only telling me what I just learnt 5 minutes ago!
I shall detail the process for the benefit of everyone on the forum.

 

[propclock]

Thanks to both of you , electroplating is a new skill I would like to learn.

 

[Peter Twissell]

Crankshaft.
The crankshaft is first turned as a 'blank' with the main journals oversize and with excess length.

 

[Peter Twissell]

I then drilled an reamed the holes for the offset journals and machined the counterbalance forms.

 

[Peter Twissell]

Connecting rods are fairly straightforward, much of the machining done on the rotary table.

 

[Peter Twissell]

The spaces between the webs are partially machined, sufficient to allow assembly of the rods, but leaving the crankshaft as one piece.
The offset journals are hard dowel pins.
Here the assembly is shown with over-length pins pressed into place before trimming to length with an abrasive wheel in a Dremel.

 

[Peter Twissell]

Since this engine is intended to be used in a flying model, it is inevitable that at some point the prop will strike the ground, causing shock loading of the crankshaft.
With this in mind, I have decided to bolster the pressed up crankshaft assembly with a few tacks of TIG welding to lock the offset journal pins in place.
If I were building the engine just for running on a test stand, I would be confident of the pressed assembly without welding.

As an additional measure, I have decided that rather than the prop shaft being an integral part of the crankshaft, I will cut the crankshaft down to just ahead of the front bearing and drill and tap it for a separate prop shaft, which can be replaced if it is bent or broken.

 

[a41capt]

Nice work Pete. I love the design, and can’t wait to hear your first run!

John W

 

[Peter Twissell]

Crankshaft assembly:

 

[Peter Twissell]

The front cover of the engine supports two ballraces.
In the first operation on the cover, I turned the register which fits into the main bore of the pair of blocks and bored the through hole and one bearing housing.
To ensure that the bores are aligned, I made a fixture which locates on that first bearing housing with an expanding 'collet' to grip in the through bore.
The other end of the fixture (in the chuck here) is sized to fit in the bore of my rotary table, so it can also be used for the milling operations.

 

[Peter Twissell]

The turned blank for the front cover is seen here mounted on the fixture, with the second bearing housing bored and the external profile machined. There is a slight taper on the 'nose' diameter, which was generated by rotating the cross slide. My Drummond Roundbed lathe does not have a top slide, but the cross slide can be rotated, providing 3" of taper turning travel.

 

[Peter Twissell]

The parts so far temporarily assembled.
It is at this point that it became apparent that I need to machine some small clearances into the blocks to allow for the swing of the rods.
I'm not sure how I missed this in the CAD model!
Fortunately, the clearance required is only small and easily achieved without impacting any other aspects of the design.

 

[Peter Twissell]

Rear cover / timing gear housing:
This part took me the best part of 2 days, mostly on the rotary table, and my entire vocabulary of expletives.
At the first attempt, I milled straight across one of the small lugs at the bottom. The part was otherwise 90% complete at that point.
The small holes are tapped M1.6.