Lynx 15 - Single cylinder, 4-stroke, Air Cooled, Over Head Cam ICE

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Eccentric

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I’ve been looking for a simpler engine to build and I came across Malcolm Stride’s Lynx, a 4-stroke air cooled engine that uses a timing belt instead of gears to turn the camshaft. So I started noodling around with a CAD model with the intention of building my own version. I am looking to design an engine that would be suitable for a second 4-stroke engine build for someone who has built a Webster and wants to take their machining to the next level.

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I want to use easy to obtain parts so I selected the standard GT2 timing belt and pulleys used in everyday 3D printers. The crankshaft to camshaft spacing works out pretty well with a 320mm belt with a 24 and 48 tooth pulley. The bore will be 1" and 25mm. I live in California so I use the imperial measurement system, but wanted to try to design using Metric. I set off laying out the engine in Metric and soon found that my mind does not work in whole numbers, only in fractions with denominators of 16, and 32. So I reverted to designing in imperial. But I am determined to expand my brain and try to work in metric so I designed the engine in imperial, then designed another variant in metric. Not a metric conversion, but truly metric throughout. So I ended up with two sets of plans, one with a bore of 1" and another with a bore of 25mm. I am calling them the Lynx 90 (.90 cubic inches) and the Lynx 15 ( 15 cubic centimeters) in homage to the late Malcolm Stride. The valve train seemed the trickiest so once I figured out the Crankshaft to camshaft spacing I looked at the valve train. Something like this:

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This is what I have ended up with:

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I'll start by building the Crankcase Assembly.




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I bore the large crankshaft hole in the block:
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then square the block up on the mill zeroing the mill to this hole:


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Drill and tap the holes, then machine corners off:
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that is the crankcase done:

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And now it is on to the front crankcase:

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Next will be the cylinder sleeve and cylinder............................................
 
Continued work on the Front Crankcase for the Lynx15. I parted off the the work piece after completing the lathe work on the inside and outside. I loaded it back into the lathe reversing ends using a 4 jaw chuck. I used an indicator on the flange right next to the chuck jaws to center the work piece, then moved to indicate the nose. Both needed to be centered to the lathe axis to insure the bore was co linear with the lathe axis. It required a few solid taps with a hammer to get both ends lined up properly, then facing the front and finishing the pcoket for the front bearing was quick work.

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Then over to the mill to drill the 8 mounting holes in the flange. I fooled around with several clamping methods and ended up with this. The one thing I would do different is to set the work piece on parallels instead of using the two blocks to insure proper vertical positioning.

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After mounting the crankcase front to the crankcase it was back into the mill to machine the slot to clear the timing belt.


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Crankcase and Crankcase Front Complete:

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Then to the cylinder sleeve. I used 1144, I am liking it better than cast iron, less messy and pretty good finish.

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I machined this cylinder sleeve from the bottom becasue I need a flange to clear the conrod. Normally I like to machine from the top becasue my lathe setup gives me a slight flare to the sleeve and I like the widest at the bottom. It just takes more spring passes to work this out. Also I should have turned the outside last instead of first so I would have a much more ridgid part for the boring operation.

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I put the hone, one used for brake cylinders, in the tail stock and used a coarse, then fine grit compund.

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Cylinder Sleeve Done,

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Now its back to the lathe to work on the Cylinder..............................
 
Boy another hot day in the machine shop, but I am not going to even go into it as Terry Mayhugh has the record workshop temps record all sewn up. I am working on the crankcase assembly, specifically the cylinder.

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I load up a round of aluminum in the lathe, face the end and use progressively larger drill bits to get close the final bore required to hold the cylinder sleeve.

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Then I use the boring bar to get a snug fit with the cylinder sleeve.

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Unfortunately, I press the cylinder sleeve in and then can't get it back out easily. I don't want to mess up my lathe set up so I leave it where it is while I turn the outside of the cylinder. but I will need to figure out a way to remove it without hurting anything before I part the cylinder off. Later I use a razor blade under the lip and then a flat head screw driver to slide it back out. Now it is on to machining the cooling fins.

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I use a brand new carbide parting tool to machine the fins. Most of the heat the engine generates is at the top of the cylinder and in the cylinder head. For this reason I can taper the fins down toward the base, plus, I think it looks cool.

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Once I clean up the fins with a file, I part it off with .5mm extra material, then turn it around in the lathe, recentering with a dial indicator and face the bottom of the cylinder bringing the cylinder to the proper height dimension. I use some packing so the jaws don't mar the fins of the cylinder.

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And that completes the lathe work on the cylinder.

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I still need to drill and tap the cylinder head mounting holes in the top of the cylinder and the clearance holes allowing it to be bolted to the crankcase. These two sets of holes need to be perfectly aligned to insure the camshaft that rides over the cylinder head and the crankshaft that spins below it, are colinear. I need to think about the fixture I am going to use to insure this. Wow, getting close to completing the Crankcase Assembly. I do need to make a run to the hardware store and pickup some 4mm socket head cap screws.........
 

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Last edited:
No special fixture needed for drilling, do the flange first and you can simply screw cylinder to crankcase and set the crankcase true.

Purely on an aesthetic point, couple of flats on the side of the flange to stop it overhanging the edge would look better. Bonus would be the two flats could be used as another way to line the top and bottom holes up.
 
You have a very sharp eye, Jasonb. I cut the chamfers at the top of the crankcase too deep and as a result the cylinder flange hangs over the edges. I did as you suggested and machined a flat to hide the error. Below I am checking to see if I hit the proper width between the two machined flats.

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I also used the crankcase as the "fixture" to machine the cylinder head mounting screw holes. Today I rumaged through the scrap drawer and found the material I need to start on the crank shaft. Yeaa, that will be fun.

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