Building the Lynx, a 15 cc four-stroke engine

[Wolfgang]

The Lynx is a 15 cc SOHC engine, plans and instructions were published in "Best of Model Engineer" in 2013. I started building it 8 months ago, an now it is almost completed. A few simple things still to be made, and then we shall see if it runs!



But now, from the beginning...

As recommended by the instructions I started with the cylinder liner. A piece of cast iron was cut to length, faced, and drilled with my largest drill bit. The material cut nicely, but what a mess! I had the ways covered with paper towels, that made cleaning a lot easier.

All my boring bars were either to short or to skinny, so I used a largish endmill to bore the hole out to slightly below the final 25 mm diameter. I find that endmills can make nice boring bars when held at an angle in a prism and rotated to provide a little bit of top rake. They seem to produce far less taper then my "real" boring bars. On the other hand they don't leave a shiny surface, but this can be corrected by honing much easier than a tapered bore.

The bore came out straight, with no measurable difference in diameter at both ends.

The outer side is a simple turning job, and then the part was reversed in the chuck and faced to length, and here is the finished liner apart from the honing:

I used a homemade lap to bring the bore to size and to remove the machining marks. I held the lap in the collect chuck because it was easier to protect than the 3 jaw, and then: it has no jaws to bite into my fingers!

I took the picture before the start of the operation, you would not want to see the end of it! Honing cast iron makes even more of a mess than drilling it. The surface has no visible machining marks, but still looks a bit dull. Maybe my lapping compound was a bit on the coarse side.
The honing went reasonably quick, but cleaning the lathe afterwards took a long time. Luckily I did not know I would have to do it all again later for the piston rings.

Cheers, Wolfgang

[wheeltapper]

Nice one Wolfgang, best of luck with the rest of the build. having built this myself, I know how satisfying it is when it bursts into life.

I'll follow your build with interest.

Roy.

[Wolfgang]

Thank you for the encouragement, Roy. Which version did you build, glow or spark, and how does it run?

Next part is the cylinder jacket. I used a fairly tough grade of aluminium (3.1354 or EN 2024), which gave me trouble during the parting operations.

Turned to size, then a locating cut for the fins


The cuts between the fins are fairly deep at 10 mm, I cut them with a 1.5 mm parting blade


which snapped off at the 7th of 10 fins. I am just not good at parting operations.


The broken tool was just large enough to grind a new one out of it which held up long enough  for the rest of the fins.


I then used a wider parting tool to cut the large cut-out below the fins, but got too much scatter and had to finish with two small HSS tools, swapping between left-handed and right-handed. Maybe the work stuck out a bit too far, but my plan was to part off the cylinder jacket and to use the part remaining in the chuck for the cylinder head in the same setup. So far, the plan…


Next was drilling and boring the cylinder jacket, I used the same technique as for the liner and it went ok.


Parting off was the last operation on the lathe, and it failed. The tool jammed, the work moved in the chuck, and alignment was lost. Seems I am really not good at parting...

I said a nasty word, took the work out of the chuck, and cut it off with a hacksaw. Then I reversed it in the chuck and fiddled around with paper shims for an hour or so until the needle stopped moving before I faced it to length.


Then over to the mill to drill the crankcase mounting bolt holes. Normally a job for the rotary table, but I find I can hold better accuracy using my mill's DRO:


and that is the cylinder jacket finished for now:


The holes for the cylinder head mounting bolts will be drilled later with the crankcase mounted as a reference surface, that should make it easier to get crankshaft and camshaft parallel.

Wolfgang

[wheeltapper]

Hi. I built the glow version.
here's a link to the build, http://www.homemodelenginemachinist.com/showthread.php?t=14425

there's a video near the end.

cheers
Roy.

[Wolfgang]

Nice build log, Roy, and a good runner.

Liked the videos, too. The band-aid on your finger was a nice touch  !

[Wolfgang]

The crankcase started as a -slightly oversized- cut-off from a 70 mm square aluminum bar, and I was happy to have invested in a small bandsaw some weeks before. Useful tools are these 4x6 bandsaws, cutting this lump with a hacksaw would have been quite a job. Unfortunately my small basement shop is packed full with machinery, and so the poor thing has to live in the firewood shed.


I squared two neighboring faces in the mill vice, these will become the references for the bores for front bearing and cylinder liner, leaving the other faces slightly oversize. The centers of both bores were marked with a center drill, and then these marks were used to center the work in the four jaw.


Starting with the hole for the front bearing the block was  drilled in steps as far as I could get with my drills and endmills


then bored to size.


Same procedure for the cylinder liner bore:


Then over to the mill to drill and tap all the mounting holes.


My longest endmill was just long enough to cut the square opening at the rear, a lot of material to remove!


Looks like a piece of swiss cheese now. I had misplaced some of the mounting holes for the bearers and had to push in little plugs cut from the same material, they should become invisible when the outside is brought to final dimensions.


Which I did with my largest face mill, using the bores as references. Measuring against the bores is tricky without special equipment. I used an 8 mm steel ball held in my micrometer with a bit of grease, a trick I found in another build log, thanks! -Wolfgang

[Ye-Ole Steam Dude]

Looking good

[Roger B]

Good progress    I'm following in the background 

[fumopuc]

Hi Wolfgang, thanks for the tip with the ball for the measurment.

[Wolfgang]

Ok, so the new PB policy has reached me, too. All pictures gone.
I'll try to fix this before I continue the build log.

Wolfgang