A new look at Opposed Twin I.C. Engine

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Brian Rupnow

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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.
 

ShopShoe

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I'm glad you're revisiting this engine, Brian. I followed the original build and I really liked the whole concept. I'll be looking forward to seeing how it turns out.

--ShopShoe
 

Gordon

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This is interesting because I built the Upshur version of this engine and never got it to run properly. I just got it back off the shelf last week to try again. I believe that the biggest problem with this version of the engine is the valve timing. It uses only two cams so that the intake of cylinder #1 and the exhaust of cylinder #2 uses one cam and the other cam runs #2 intake and #1 exhaust. The problem is that if you adjust the intake on one cylinder it changes the exhaust on the other cylinder. There is really not enough room to use four cams. I will have to take a closer look at your original design but a quick glance it looks like you used two cam shafts with an idler gear.
 

coulsea

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This is interesting because I built the Upshur version of this engine and never got it to run properly. I just got it back off the shelf last week to try again. I believe that the biggest problem with this version of the engine is the valve timing. It uses only two cams so that the intake of cylinder #1 and the exhaust of cylinder #2 uses one cam and the other cam runs #2 intake and #1 exhaust. The problem is that if you adjust the intake on one cylinder it changes the exhaust on the other cylinder. There is really not enough room to use four cams. I will have to take a closer look at your original design but a quick glance it looks like you used two cam shafts with an idler gear.
My Upshur twin uses one cam lobe for both intake valves and the other lobe for both exhaust. plans are dated 2001.
 

Brian Rupnow

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Yesterday I was prowling thru the drawings and models of this build, and I discovered something good. I have been thinking all along that I would probably have to build a new crankcase, and probably new cam and crankshaft.---However---Taking a much closer look, I may be able to modify the crankcase to take sealed ball bearings on both the crankshaft and the camshaft. If so, that would be a real bonus.
 

Brian Rupnow

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This is what some research showed me. At the bottom of this model you see the original crankcase "as is" with a round blue insert on one side to hold the crankshaft bushing and a bushing on the opposite side to hold the the other end of the crankshaft. You can also see a strange shaped red insert on the near side that was the camshaft bushing, with a plain bushing on the far side to hold the other end of the camshaft. The round yellow cylinder represents the outer diameter of the crankshaft bearing, and the green cylinder represents the outer diameter of the camshaft ball bearing. At this point, I believe that I can rework the crankcase to accept ball bearings for the crankshaft and for the camshaft. This would let me keep the original crankshaft and camshaft, which would be a tremendous bonus.
 

Brian Rupnow

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So yes, it can be done. I have been holding off on this project, wanting to completely finish the T-Head engine before wading into this. The T-Head is essentially finished, and my new bandsaw is installed and working, so I think I will be making chips with this rework of my opposed twin engine.
 

Brian Rupnow

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I'm having a quiet day, tearing down the old engine. I gave myself a terrible flash yesterday with my old A.C. welder, and my eyes are howling about it. The new helmet I got when I bought my tig welder has a sensitivity dial on the side of the helmet, and somehow the dial had been turned to '0", so when I started to weld the lens didn't automatically darken. As I begin removing pieces from this engine, I am impressed by how many machined parts there are, and beginning t wonder if I should just buy some angle of the correct size and make a new crankcase.---Not sure yet.
 

Brian Rupnow

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So, here we are, right down to the nitty gritty. I guess the most surprising thing I have seen is how incredibly dirty everything was. Lots of grease and oil and sludge. I don't know how long ago it was that I designed and built this engine, but it's been a few years. I see a few simple things that I would change a bit if I was starting to build this engine from scratch now, but really not much. I can bore straight thru the large top-hat that supports one end of the crankshaft and out thru the other side of the engine in one set-up. The engine is 2.75" out to out, and the stroke on my mill quill is 3.00" The cam bore will be a little more difficult because the tappet guides are loctited in place and there is only 0.632" between the inner ends. The ball bearings for the camshaft are 0.750" o.d.---I can probably put a bit of heat on the tappet bushings to release the loctite and take them out until the camshaft bearing bore is completed and then loctite the bushings back into place. Actually. the bore for the camshaft bushings is only 0.75" so I would probably just run a 3/4" reamer thru there.
 
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