My riff on the Upshur Twin

[rklopp]

I got the first run on one of six engines I have been building based on Hamilton Upshur's Twin published in Model Engine Builder in 2006.

[youtube1]https://youtu.be/6a1YGoKc_Ww[/youtube1]

The engine ran briefly on a prime yesterday before I hooked up the fuel tank, and pretty much fired right up today. That's a testament to Hamilton's design, careful building, and the S/S Machine & Engineering CDI ignitions.

I departed significantly from the magazine plans. Differences include:

(1) The crankcase is hogged from solid rather than built up from plates.
(2) The cam gear cover is integral with the crankcase cover.
(3) The crankshaft is machined from solid with counterweights.
(4) The camshaft is machined from solid, including the gear.
(5) The cylinders are cast iron with shrunk-on aluminum cooling fins.
(6) The heads are larger, and the rocker post attachment is beefed up considerably.
(7) The carburetor is a 3/4-scale version of Jerry Howell's V-twin carburetor.
(8 ) The flywheel is hogged from solid cast iron, including integral fan blades.
(9) The crankcase breather is based on Jerry Howell's V-twin plans.
(10) The intake manifold is a fancy fabrication instead of hoses per the plans.

One issue that arose is that one of the aluminum fin assemblies did not have enough shrink interference, and moved relative to the iron cylinder liner when the engine got hot. That's a problem, because the head is attached to the fin assembly, so the head drifted away from the engine centerline, carrying the valves and rockers with it. I wondered why the valve lash kept getting looser and looser on that one cylinder. I need to figure out how to re-secure the fin to the liner, perhaps with high-temperature lock-tite, but more likely a few radial pins or "dutch" setscrews.

I have five more of these in nearly the same state of building. The cost of sparkplugs at $25 apiece and ignition modules at $85 apiece is slowing me down! There are two ignition modules per engine.

[Jasonb]

Nice

Can't you buy one of S/S ignitions for twin cylinder engines, I used one on a horizontal twin and have another for my Hoglet.

[rklopp]

Jason
Would that be configured to fire both sparkplugs simultaneously? ("Waste spark"?) Do you use one Hall sensor and two magnets? For some reason I did not think of using just one CDI module. I don't want a distributor.

Thanks & Happy New Year

RKlopp

[Jasonb]

The opposed cylinder one uses a single hall sensor and single magnet and uses lost spark, it has two coils and two plug leads that are fired together.

For the Hoglet that uses two magnets and a single sensor as the cylinders are not 180degrees apart

[Admiral_dk]

There is another version of this system with a single coil, where each end of the secondary coil is connected to a sparkplug - all twin and Four cylinder Japanese motorcycles from the seventies and eighties had that kind of coils - I heard that Harley Davidson used it for some years too and yes, it is called "waste spark"