Engines > Your Own Design

A new look at Opposed Twin i.c. Engine

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Brian Rupnow:
A few years ago, I built my version of an opposed twin i.c. engine. It ran very well, and I was very pleased with it. There was always a problem with the configuration of this engine, because the sparkplugs were at the very bottom of the horizontal cylinders, and if you happened to flood it while starting it, you could crank all day and it wouldn't clear itself. However, it did run, and I made some good videos of it running. The crankshaft was riding on bronze bushings, and the crank wasn't "dead nuts" straight. I was using my standard 1975 Chrysler ignition points, and a cam running on the crankshaft. As time went by, with more and more hours running time accumulated on the engine, the bronze crankshaft bushings began to wear, cause by the crank that wasn't truly perfect. It wasn't a wet sump engine, so that didn't really matter a lot  ---BUT--as it wore, the ignition cam, attached to the crankshaft began to move around with the sloppy crankshaft. Finally, the ignition cam on the less than perfect crankshaft was moving around enough to seriously affect the timing and spark of this engine. This took me the longest time to figure out what was going on, and when I did figure out what was happening the engine went "Up on the shelf". I promised myself at that time that eventually I would redesign this engine, using ball bearings on the crankshaft, a crankshaft that was truly straight, and a configuration which put the sparkplugs at the top of the horizonal cylinders. I would be able to re-use the cylinders, cylinder heads, overhead valve mechanisms and valves and cams, while making a new central crankcase and fan assembly. This is early days yet, and I still have some unanswered questions about the T head engine I have currently been building, but I think this will be the direction I move in next.

steamer:
Cool!   

Dave

Roger B:
It look's like you could turn the whole crankcase upside down. The exhausts could be moved next to the flywheel with a bend pointing outwards and the inlet ports moved to the side of the head with an additional bend in the inlet manifold.

Jasonb:
The biggest problem with reusing the heads to get the plugs at the top is you either have to run an open crankcase or suffer oil leaking out of the tappets when they are below the oil level.

If you want a closed wet sump then the cam at the top is the better option, I would then hand the heads so both inlet valves close to the flywheel, exhausts coming out horizontally and space the rockers further apart so the plug can fit between them, some repositioning of the cam lobes and angling the pushrods outwards may be needed. take a look at the various configurations of the Wall Wizard, some have the plugs at the top of the head, others at the side these were changed from the original lower plug layout.

Brian Rupnow:
Thanks Jason. I haven't really thought this thru in any way. I didn't run a closed crankcase on the first horizontal twin--it had a lift off lid on the crankcase which was attached magnetically to allow lifting it off easily and giving a few squirts of oil to the con rods and cams. I did like the flywheel/fan arrangement I originally had, and would try to re-use it. I actually did design and machine a second set of heads, which moved the sparkplug higher in the cylinder, but left it horribly tight to bet a standard sparkplug in there without the high tension lead shorting out to the rocker arms. I wonder if it is possible to get an insulated "extender" which attached to the top of a 1/4"-20 sparkplug and extends out about an inch to move the high tension lead away from the rocker arms. I've never seen such a thing, but then I have never seriously looked for one.

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