Halfa, a DOHC inline Twin

[RReid]

The next piece of unfinished business to knock off the list for this engine is a flywheel. The maximum diameter that will fit without hanging below table top level is 2”. So a short chunk of 1 3/4” 12L14 steel that had nothing better to do was chucked up in the 4-jaw on the lathe to be faced and the center drilled through and reamed 5/16”, to fit the crankshaft.

Then I moved the part, still on the chuck, over to the rotary table on the mill for the next step. At the end of my v-twin build I made this simple starter drive dog for mounting in a hand drill.


The starter hub on that engine just had simple drilled holes to accept the drive pins. While this worked fine for spinning the engine, I found I couldn't really tell if the engine was started or not. In fact, the first actual start (that I knew of) happened when I pulled the starter out thinking nothing was happening to find, much to my surprise, that the engine was running! Since I had also set it up for rope pull starting, I used that from then on.

For this engine I want to go back to drill starter, with the flywheel acting also as the starter hub. But I also want exit ramps for the drive pins this time. So first step on the mill was to drill two holes 1/4” deep to match the starter pins, and centered on the crankshaft hole. Then, using the rotary table and cranking it and the Z-axis together, a crank in each hand, I worked in as smooth a ramp as I could. The end result in actually much better than I expected, so I'm well pleased.




I think I'm going to try electronic ignition with a hall effect trigger this time, so after I part the flywheel off to final length on the lathe, it can be reversed in the chuck and come back to the mill to make a little pocket for the magnet.

[RReid]

While I wait for the TIM-6 ignition module kit I ordered to arrive, I've been continuing to catch up on minor “supporting bits”. One was the flywheel shown earlier and now finished except for the magnet pockets and I want to add a key slot to it and the crankshaft. I've also made a pair of proper hose nipples for the coolant in/out ports.


Next, I used some old business cards to help “design” patterns for the sheet stock front belt cover. This consists of two side pieces screwed to the block, and the actual cover that gets screwed to those. The side bits are brass, but the cover I cut from an aluminum reflector salvaged from a broken lamp. With the silvered overlay removed and rubbed up with some 220 sandpaper, it matches up well with the other ali surfaces.




This final cover cover is far more simple and plain in appearance than the rather complicated cast aluminum front cover of the actual Giulietta engines. To add interest to the bare flat surface, I've toyed with the idea of adding a traditional Alfa Quadrifoglio emblem there.

[Kim]

That's a great looking belt cover!  Sheet metal work can be pretty challenging to make everything come out where you want it to.  It looks really good!

And the little emblem looks great too!

Kim

[RReid]

Thanks Kim!

[RReid]

All of my previous engines have relied only on a set-screw to fix the flywheel in place. But I found with the v-twin that this wasn't always satisfactory when using the drill starter, allowing a bit of walk even with a set-screw flat. Since this engine will be using the flywheel to carry the ignition triggering magnets, I want its position to be known, repeatable, and solidly fixed. Which means using a key in addition to the set-screw.

Slotting the crankshaft for a 1/8” key was easy enough, a simple job with an end mill. The matching slot in the flywheel was a bit more involved, since I don't have a broach and didn't really want to buy one, plus a suitable press, right now. Instead, I set the crank/flywheel assembly up vertically on the mill and carefully aligned the center of the ˝ finished keyway under the spindle. Then a 1/8” end mill was run down, cutting a semi-circle in the flywheel and cutting only air in the crankshaft.


Because of the width of the flywheel, I had to flip in over and repeat from the other side. The set-screw flat was good enough to yield a good alignment when doing this. With that done, I removed the flywheel and attacked that semi-circle with a square needle file. This was the laborious part, but it really wasn't too bad. Apart for the bit of bell mouth that shows in the picture below, the fit is nicely snug and very solid.


With the flywheel fixed securely to the crankshaft, it makes sense to also use it as a degree wheel for setting the valve and ignition timing. To do this, I mounted the assembly into the 4-jaw, clamping on the outer edge of the flywheel. This was then mounted onto the rotary table. After carefully finding TDC, I used a small center drill to make little lines of dimples there and at 5-10-15-20-30-40-50 degrees to either side. The same thing was done for BDC as well.




The “T” makes it clear which set is corresponds to TDC, while the set-screw is in the middle of the BDC set. The 10 degree marks got 3 dimples, while those at 5 degrees got two more closely spaced dimples. To finish off I brushed on some black paint, then immediately wiped it off the surface, leaving the dimples dark.

[Roger B]

Cardboard Aided Design (CAD) is always good

That's quite a chunky flywheel, the timing marks will be helpful. I usually print a paper degree wheel and stick it on but the lifespan is definitely limited once oil and fuel start spraying around during the first trials 

[RReid]

Hi Roger, and thanks for checking in.

We've just gotten back from a few days camping in the Sierra Nevada foothills, and my ignition module kit was in the mail waiting for me.

Cardboard Aided Design (CAD) is always good 

As is the closely related BCAD (Business Card Aided Design).

That's quite a chunky flywheel,

I agree. I think chunky is good for first pop trials. I have an even bigger (larger diameter) one I can fall back on if needed. Once running is proven I can think about a trick, lightened, race version if desired.

A printed degree wheel was my original intention, then I had a second thought...

[Hugh Currin]

Ron:

Great progress. The timing marks are genius and I expect from prior experience. Nice.

For the keyway, I've made similar on a lathe. I took a length of shaft, sliding fit to the flywheel hole, and cut a blind keyway into it. Then put a set screw through the shaft at the leading edge of the keyway. A square ended HSS bit fits into the keyway for the cutting edge. Mount this shaft in the lathe tailstock and the flywheel in the chuck. Using the setscrew extend the bit out from the shaft a small amount and drive it thru the flywheel using the tailstock feed. The slight negative rake of the bit keeps it from digging into the cut.

This was a 14" lathe, I don't know if there are rigidity limits. I know your lathe is smaller. But the tool would work also on a press. Yes, I also need to get an arbor press for the shop. May also work on a mill.

Thanks for the updates.

[RReid]

Thanks Hugh!

I like your tip for cutting a keyway. The Taig tailstock is lever feed, fine for drilling within its capacity, but doesn't generate a great deal of force. Perhaps I could try it on the drill press or the mill though.

[vtsteam]

The Halfa is coming along great, Ron! 

[RReid]

Thanks Steve!

I got the ignition module soldered up today. Bench testing it, the LED lit up when the magnet passed over the Hall sensor, and a spark plug sparked when the magnet moved away. No smoke escaped. All good signs.

[Vixen]

Hello Ron,

Which ignition module did you go for?
The smoke free ones are always the best. 

Mike

[RReid]

Hi Mike. I'm using the well regarded TIM-6 module, available from https://www.model-engine-ignition.com/model-engine-ignition. This site is run by the son of the late Jerry Howell, as well as a separate site selling his engine plans. They also offer a fancier, microprocessor based module for a bit more money. That one was tempting, but I decided for my needs simpler is better.

[Vixen]

Hello Ron,

The TIM-6 ignition module is always a safe bet, provided you use a suitable coil to go with it. I see that Jerry Howell offers a wide range of 'proven' coils to match the TIM modules.  They get a bit pricey when you ship them across the pond. What with, shipping costs, import taxes etc can double or treble the US price.

Mike

[Charles Lamont]

I will secong Mike's advice about a 'suitable' coil. I fried a TIM-6 with a Minimag coil.