Wrote this a week ago, was waiting on some pics to post, guess i'll add them later...
Having the last two weeks off school has been nice, not quite as productive as it could have been, but thats the holidays for ya. I guess its supposed to be a vacation, but I've been working harder than usual to get some long-awaited projects finished up by the end of the year. Starting out with a very ambitious list in my little notebook, I'm happy to be able to say I checked off most of the boxes.
The Cranks hosted the third annual 'Fond-of-dudes' party with record attendance. Cool to see folks from Chicago, Madison, and Minneapolis join the merry-making this year. Good party with lots of food, but next year we'll do a better job of coordinating fondue pots.
I finally got around to machining the final few bits for the long-running stock Vespa project. I had to re-located the ring land pin, so i figured while i had the piston in the horizontal-axis rotary-index table-chuck-thingy in the mill, I'd use a 1/16" end mill to add some ring-land gas porting, or "BigBlockFiero Tech" as it is often called on the MA forums.
This is an idea seen on all sorts of pistons, mostly in 4 stroke applications, where the biggest advantage is better ring sealing. On a two stroke, the theory goes, that putting 'ports' into the upper face of the ring groove allows for gasses to escape around behind the ring on the down-stroke and deposit more oil on the piston and cylinder walls, cooling the piston. I've spent a lot of time mulling this over in my mind. Its a tricky thing, interpreting the theory and actual results, because tighter ring sealing is very important in piston cooling. You see, in a running engine you dont want any gasses between the piston and cylinder wall. The piston cools, primarily, through the cylinder wall, and oil conducts heat better than air. If you get any blow by you will run a lot hotter. In fact, i've seen a lot of piston seizures that show bad break-in or bad ring sealing and a lot of blow by, leading to carbon which further insulates the piston and cylinder wall, and gas insulating the piston and cylinder wall.
So for my money, I'd bet what we are really seeing with ring-land gas porting is a better piston ring seal. The down side to this is the greater pressure supposedly wears out the piston ring faster, although if done correctly I find it hard to believe it will make much difference. I've run the same rings for 10's of 1000's of miles with no problem on bikes before, so its a compromise i can live with if i burn out one ring at 2000 miles or so.
Anyhow, to make this all work you need to only run one ring. I was planning on doing that in the vespa anyhow because both the ring gaps landed right in the 3rd and 4th transfers I cut in. To replace the ring locating pin, i machined one out of brass. 2mm diameter, and pressed it into a 5/64" hole- which works out to 1.98mm and change. The pin went in with a little pounding so hopefully it will hold... it would be pretty depressing to shred another piston and cylinder after having put over 10 hrs of time into this setup. Especially considering that the last piston and cylinder lasted all of one trip around the block before I broke them trying to fix them. It would be pretty sad if i was also destroying $130 kits each time rather than free doorstops.
Everything finally got re-assembled on Friday. The vespa kicked right over and ran good once warmed up. I think i drilled out the atomizer tube too big on this one, because the low end has never been good, but now that i'm running 20 deg of blowdown and 158 deg of exhaust timing, its not expected to idle very well at low end. The clutch still grabs too early in my opinion, but hopefully once i get some more low-end back from tuning carb and ignition, it will have power to slip the clutch at lower speeds. The last time i had it together it was smoking the belt instead of slipping the clutch... stupid dry clutches. Once i get things dialed in i'll probably go to a larger diameter belt. I'm also looking at modifying the stock clutch by lightening it and adding some cooling fins, also machining out the belt groove so the wider belt will sit deeper and keep the dimensions the same, or even lower the gearing.
Also had some time to work on the Sachs, mechanically it is pretty much buttoned up. I had to add a small jumper chain for some reason... not sure why the chain got 6 links shorter from the 505/1C to the /2BX but thats how it goes sometimes. Got the chain on and squared away. Also finished up all the cable routing and such. The clutch and shifting is very smooth and accurate, but i can tell already that first gear is going to get annoying as it seems to require more articulation of the shift grip than neutral and 2nd. I wish the 1st/2nd positions were reversed. It was obviously designed with 2 as the 'main' gear, and 1 as a 'granny' gear for getting up hills and the like. 2 is the default position if the cable breaks or what not.
The way i have the gearing set up with a tomos 28 tooth in the rear and the standard 11 tooth up front gives it a standard-ish 1st gear, feels like maybe 16/40 puch gears. A big jump from the stock 11/43! This engine has so much butt-wrenching torque, i'm hoping to set this up with city/highway gears so i can shift comfortably at about 45 mph and pull up into the high 50's, low 60's without winding the piss out of the motor. Gonna be hard on the clutch, but being a manual I think smooth gentle shifting will prolong the life. When it does fry, i have a few tricks up my sleeve i'd like to try. The last straw being to completely drill out the output shaft on the crank and press in a splined 4130 shaft to adapt to a totally different(reinforced) clutch hub.
Since I'm building this to be a 'touring' bike- for long distance adventures and camping trips- i'm working on a slew of attachments starting with a large and very sturdy seat. To keep weight low I fabricated it from 3/16" sheet aluminum. Starting with a posterboard pattern, tracing and cutting from sheet metal, finally bending it up and welding the seams.
I'm getting much better at TIG on aluminum, but still not super happy with my results. Aluminum is a tricky son of a gun and demands perfect cleanliness and good fitment. Getting better with TIG I'm able to fill gaps better, but its hard to fill gaps and make it look good. Oh yeah, and make it strong, that is important too. Anyhow, the seat turned out pretty good. The bike is designed to have the weight on the seat post so I had to make a steel rib to transfer the weight onto the tube frame, and it should be strong enough to let me use the seat (probably tying in where the seat attaches to the luggage rack) to build a racking system. I would really like a retractable rack for carrying cases of PBR, and rails making it easy to attach and detach side bags.
I've also been thinking a lot lately of some sort of trailer hitch. Possibly using a piece of very stiff hydraulic hose to act like a ball hitch, but with a bit more control. Ideally I'd like to be able to tow a trailer that would be able to carry another moped like a 'tow dolly' so i can pick up broken down mopeds or transport them around without a car. Anyhow, maybe by next summer. Either way being able to fabricate aluminum will be a big help as it would be way too heavy to make any of this junk with steel, and aluminum doesn't rust. It is expensive if you buy it from the metal rack at home depot, but if you go to a good scrap yard, especially one that deals with industrial scrap, there is tons of it laying around they will sell you for the same scrap price they paid for it. I've found lots of good useful metal this way. Even built a copper still once from a scrap bin.
For Moped Factory work, I finished up a huge order over break and started work on some of my next round of projects. Another level of sophistication with some real top-notch performance parts. I'm trying to make headway on the moped factory blog as well, to try to get as much product information out there as possible.